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[bug#45409,v3,3/3] guix: Split (guix substitutes) from (guix scripts substitute).

Message ID 20210104211927.14959-3-mail@cbaines.net
State New
Headers show
Series [bug#45409,v3,1/3] substitute: Untangle skipping authentication from valid-narinfo?. | expand

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Commit Message

Christopher Baines Jan. 4, 2021, 9:19 p.m. UTC
This means there's a module for working with substitutes, rather than all the
code sitting in the script. The need for this can be seen with the weather and
challenge scripts, that now don't have to use code from the substitute script,
but can instead use the substitute module.

The separation here between the actual functionality of the substitute script
and the underlying functionality used both there and elsewhere should make
maintenance easier moving forward.

This commit just moves code, none of the code should have been changed
significantly.

* guix/scripts/substitute.scm (%narinfo-cache-directory, %narinfo-ttl,
%narinfo-negative-ttl, %narinfo-transient-error-ttl, %unreachable-hosts,
%max-cached-connections): Move variables to (guix substitutes).
(narinfo-cache-file, cached-narinfo, cache-narinfo!, narinfo-request, at-most,
http-multiple-get, read-to-eof, narinfo-from-file,
open-connection-for-uri/maybe, fetch-narinfos, lookup-narinfos,
lookup-narinfos/diverse, open-connection-for-uri/cached,
call-with-cached-connection): Move procedures to (guix substitutes).
(with-cached-connection): Move syntax rule to (guix substitutes).
* guix/substitutes.scm: New file.
* Makefile.am (MODULES): Add it.
* po/guix/POTFILES.in: Add 'guix/substitutes.scm'.
---
 Makefile.am                 |   1 +
 guix/scripts/challenge.scm  |   2 +-
 guix/scripts/substitute.scm | 486 +--------------------------------
 guix/scripts/weather.scm    |   2 +-
 guix/substitutes.scm        | 531 ++++++++++++++++++++++++++++++++++++
 po/guix/POTFILES.in         |   1 +
 6 files changed, 540 insertions(+), 483 deletions(-)
 create mode 100644 guix/substitutes.scm

Comments

Ludovic Courtès Jan. 5, 2021, 10:03 p.m. UTC | #1
Christopher Baines <mail@cbaines.net> skribis:

> This means there's a module for working with substitutes, rather than all the
> code sitting in the script. The need for this can be seen with the weather and
> challenge scripts, that now don't have to use code from the substitute script,
> but can instead use the substitute module.
>
> The separation here between the actual functionality of the substitute script
> and the underlying functionality used both there and elsewhere should make
> maintenance easier moving forward.
>
> This commit just moves code, none of the code should have been changed
> significantly.
>
> * guix/scripts/substitute.scm (%narinfo-cache-directory, %narinfo-ttl,
> %narinfo-negative-ttl, %narinfo-transient-error-ttl, %unreachable-hosts,
> %max-cached-connections): Move variables to (guix substitutes).
> (narinfo-cache-file, cached-narinfo, cache-narinfo!, narinfo-request, at-most,
> http-multiple-get, read-to-eof, narinfo-from-file,
> open-connection-for-uri/maybe, fetch-narinfos, lookup-narinfos,
> lookup-narinfos/diverse, open-connection-for-uri/cached,
> call-with-cached-connection): Move procedures to (guix substitutes).
> (with-cached-connection): Move syntax rule to (guix substitutes).
> * guix/substitutes.scm: New file.
> * Makefile.am (MODULES): Add it.
> * po/guix/POTFILES.in: Add 'guix/substitutes.scm'.

I’m reluctant starting this new module while it still contains
single-short-lived-process assumptions (connection caching & co.).

How about proceeding like this:

  1. Move ‘http-multiple-get’ to (guix http-client).

  2. Postpone the (guix substitute) bit to a separate patch series to
     leave us the time to polish things a bit and removes the
     single-process assumptions, or just move fewer things to (guix
     substitutes).

You could push (guix narinfo) in the meantime since I think that one is
almost ready.

How does that sound?  Am I being too cautious?

Thanks again! :-)

Ludo’.
Christopher Baines Jan. 7, 2021, 10:29 p.m. UTC | #2
Ludovic Courtès <ludo@gnu.org> writes:

> Christopher Baines <mail@cbaines.net> skribis:
>
>> This means there's a module for working with substitutes, rather than all the
>> code sitting in the script. The need for this can be seen with the weather and
>> challenge scripts, that now don't have to use code from the substitute script,
>> but can instead use the substitute module.
>>
>> The separation here between the actual functionality of the substitute script
>> and the underlying functionality used both there and elsewhere should make
>> maintenance easier moving forward.
>>
>> This commit just moves code, none of the code should have been changed
>> significantly.
>>
>> * guix/scripts/substitute.scm (%narinfo-cache-directory, %narinfo-ttl,
>> %narinfo-negative-ttl, %narinfo-transient-error-ttl, %unreachable-hosts,
>> %max-cached-connections): Move variables to (guix substitutes).
>> (narinfo-cache-file, cached-narinfo, cache-narinfo!, narinfo-request, at-most,
>> http-multiple-get, read-to-eof, narinfo-from-file,
>> open-connection-for-uri/maybe, fetch-narinfos, lookup-narinfos,
>> lookup-narinfos/diverse, open-connection-for-uri/cached,
>> call-with-cached-connection): Move procedures to (guix substitutes).
>> (with-cached-connection): Move syntax rule to (guix substitutes).
>> * guix/substitutes.scm: New file.
>> * Makefile.am (MODULES): Add it.
>> * po/guix/POTFILES.in: Add 'guix/substitutes.scm'.
>
> I’m reluctant starting this new module while it still contains
> single-short-lived-process assumptions (connection caching & co.).
>
> How about proceeding like this:
>
>   1. Move ‘http-multiple-get’ to (guix http-client).
>
>   2. Postpone the (guix substitute) bit to a separate patch series to
>      leave us the time to polish things a bit and removes the
>      single-process assumptions, or just move fewer things to (guix
>      substitutes).
>
> You could push (guix narinfo) in the meantime since I think that one is
> almost ready.
>
> How does that sound?  Am I being too cautious?

Well, separating out the connection caching might be helpful for
cleaning things up in the Guix Build Coordinator, I had to add a mutex
currently as I'm guessing the caching isn't thread safe.

I think it's possible to separate it out if some of the error handling
is pushed down in to the http procedures, and if when they get an error
indicating the connection is unusable, they close the port.

I've pushed some rough commits for this to this branch:

  https://git.cbaines.net/guix/log/?h=prepare-to-move-guix-scripts-substitute-code

I'm still struggling with the tests, currently make check hangs, I think
on the challenge tests, and I don't currently have a plan to work out
why the test is hanging.
Ludovic Courtès Jan. 11, 2021, 1:26 p.m. UTC | #3
Hi,

Christopher Baines <mail@cbaines.net> skribis:

>> I’m reluctant starting this new module while it still contains
>> single-short-lived-process assumptions (connection caching & co.).
>>
>> How about proceeding like this:
>>
>>   1. Move ‘http-multiple-get’ to (guix http-client).
>>
>>   2. Postpone the (guix substitute) bit to a separate patch series to
>>      leave us the time to polish things a bit and removes the
>>      single-process assumptions, or just move fewer things to (guix
>>      substitutes).
>>
>> You could push (guix narinfo) in the meantime since I think that one is
>> almost ready.
>>
>> How does that sound?  Am I being too cautious?
>
> Well, separating out the connection caching might be helpful for
> cleaning things up in the Guix Build Coordinator, I had to add a mutex
> currently as I'm guessing the caching isn't thread safe.

Precisely, that’s the kind of reason why it’s currently buried in (guix
scripts substitute) rather than exposed as a reusable library.  :-)

> I think it's possible to separate it out if some of the error handling
> is pushed down in to the http procedures, and if when they get an error
> indicating the connection is unusable, they close the port.

Ideally the (web …) modules would do that (that’s what you mean, right?)
but then we’d have to wait for Guile proper to implement these things.

> I've pushed some rough commits for this to this branch:
>
>   https://git.cbaines.net/guix/log/?h=prepare-to-move-guix-scripts-substitute-code
>
> I'm still struggling with the tests, currently make check hangs, I think
> on the challenge tests, and I don't currently have a plan to work out
> why the test is hanging.

Overall the approach LGTM.

How about first getting (guix narinfo) in ‘master’ (the bits we agreed
on), and then tackling the rest so that it’s less daunting?

Also, I’d like to get the zstd patches in.  :-)

Thanks,
Ludo’.
diff mbox series

Patch

diff --git a/Makefile.am b/Makefile.am
index 69166a2ea1..fe39eae53c 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -89,6 +89,7 @@  MODULES =					\
   guix/memoization.scm				\
   guix/utils.scm				\
   guix/sets.scm					\
+  guix/substitutes.scm				\
   guix/modules.scm				\
   guix/download.scm				\
   guix/discovery.scm				\
diff --git a/guix/scripts/challenge.scm b/guix/scripts/challenge.scm
index cc9cbe6f27..74cf163937 100644
--- a/guix/scripts/challenge.scm
+++ b/guix/scripts/challenge.scm
@@ -27,8 +27,8 @@ 
   #:use-module (guix packages)
   #:use-module ((guix progress) #:hide (dump-port*))
   #:use-module (guix serialization)
-  #:use-module (guix scripts substitute)
   #:use-module (guix narinfo)
+  #:use-module (guix substitutes)
   #:use-module (rnrs bytevectors)
   #:autoload   (guix http-client) (http-fetch)
   #:use-module ((guix build syscalls) #:select (terminal-columns))
diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index f9bcead045..45c07b1038 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -23,39 +23,30 @@ 
   #:use-module (guix ui)
   #:use-module (guix scripts)
   #:use-module (guix narinfo)
+  #:use-module (guix substitutes)
   #:use-module (guix store)
   #:use-module (guix utils)
-  #:use-module (guix combinators)
-  #:use-module (guix config)
-  #:use-module (guix records)
-  #:use-module (guix diagnostics)
   #:use-module (guix i18n)
   #:use-module ((guix serialization) #:select (restore-file dump-file))
   #:autoload   (guix store deduplication) (dump-file/deduplicate)
   #:autoload   (guix scripts discover) (read-substitute-urls)
   #:use-module (gcrypt hash)
   #:use-module (guix base32)
-  #:use-module (guix base64)
   #:use-module (guix cache)
   #:use-module (gcrypt pk-crypto)
   #:use-module (guix pki)
-  #:use-module ((guix build utils) #:select (mkdir-p dump-port))
+  #:use-module ((guix build utils) #:select (mkdir-p))
   #:use-module ((guix build download)
-                #:select (uri-abbreviation nar-uri-abbreviation
+                #:select (nar-uri-abbreviation
                           (open-connection-for-uri
-                           . guix:open-connection-for-uri)
-                          store-path-abbreviation byte-count->string))
-  #:autoload   (gnutls) (error/invalid-session)
+                           . guix:open-connection-for-uri)))
   #:use-module (guix progress)
   #:use-module ((guix build syscalls)
                 #:select (set-thread-name))
   #:use-module (ice-9 rdelim)
-  #:use-module (ice-9 regex)
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
   #:use-module (ice-9 ftw)
-  #:use-module (ice-9 binary-ports)
-  #:use-module (ice-9 vlist)
   #:use-module (rnrs bytevectors)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
@@ -69,10 +60,7 @@ 
   #:use-module (web request)
   #:use-module (web response)
   #:use-module (guix http-client)
-  #:export (lookup-narinfos
-            lookup-narinfos/diverse
-
-            %allow-unauthenticated-substitutes?
+  #:export (%allow-unauthenticated-substitutes?
             %error-to-file-descriptor-4?
 
             substitute-urls
@@ -89,17 +77,6 @@ 
 ;;;
 ;;; Code:
 
-(define %narinfo-cache-directory
-  ;; A local cache of narinfos, to avoid going to the network.  Most of the
-  ;; time, 'guix substitute' is called by guix-daemon as root and stores its
-  ;; cached data in /var/guix/….  However, when invoked from 'guix challenge'
-  ;; as a user, it stores its cache in ~/.cache.
-  (if (zero? (getuid))
-      (or (and=> (getenv "XDG_CACHE_HOME")
-                 (cut string-append <> "/guix/substitute"))
-          (string-append %state-directory "/substitute/cache"))
-      (string-append (cache-directory #:ensure? #f) "/substitute")))
-
 (define (warn-about-missing-authentication)
   (warning (G_ "authentication and authorization of substitutes \
 disabled!~%"))
@@ -112,20 +89,6 @@  disabled!~%"))
    (and=> (getenv "GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES")
           (cut string-ci=? <> "yes"))))
 
-(define %narinfo-ttl
-  ;; Number of seconds during which cached narinfo lookups are considered
-  ;; valid for substitute servers that do not advertise a TTL via the
-  ;; 'Cache-Control' response header.
-  (* 36 3600))
-
-(define %narinfo-negative-ttl
-  ;; Likewise, but for negative lookups---i.e., cached lookup failures (404).
-  (* 1 3600))
-
-(define %narinfo-transient-error-ttl
-  ;; Likewise, but for transient errors such as 504 ("Gateway timeout").
-  (* 10 60))
-
 (define %narinfo-expired-cache-entry-removal-delay
   ;; How often we want to remove files corresponding to expired cache entries.
   (* 7 24 3600))
@@ -213,369 +176,6 @@  connection (typically PORT) is kept open once data has been fetched from URI."
      (leave (G_ "unsupported substitute URI scheme: ~a~%")
             (uri->string uri)))))
 
-(define (narinfo-cache-file cache-url path)
-  "Return the name of the local file that contains an entry for PATH.  The
-entry is stored in a sub-directory specific to CACHE-URL."
-  ;; The daemon does not sanitize its input, so PATH could be something like
-  ;; "/gnu/store/foo".  Gracefully handle that.
-  (match (store-path-hash-part path)
-    (#f
-     (leave (G_ "'~a' does not name a store item~%") path))
-    ((? string? hash-part)
-     (string-append %narinfo-cache-directory "/"
-                    (bytevector->base32-string (sha256 (string->utf8 cache-url)))
-                    "/" hash-part))))
-
-(define (cached-narinfo cache-url path)
-  "Check locally if we have valid info about PATH coming from CACHE-URL.
-Return two values: a Boolean indicating whether we have valid cached info, and
-that info, which may be either #f (when PATH is unavailable) or the narinfo
-for PATH."
-  (define now
-    (current-time time-monotonic))
-
-  (define cache-file
-    (narinfo-cache-file cache-url path))
-
-  (catch 'system-error
-    (lambda ()
-      (call-with-input-file cache-file
-        (lambda (p)
-          (match (read p)
-            (('narinfo ('version 2)
-                       ('cache-uri cache-uri)
-                       ('date date) ('ttl ttl) ('value #f))
-             ;; A cached negative lookup.
-             (if (obsolete? date now ttl)
-                 (values #f #f)
-                 (values #t #f)))
-            (('narinfo ('version 2)
-                       ('cache-uri cache-uri)
-                       ('date date) ('ttl ttl) ('value value))
-             ;; A cached positive lookup
-             (if (obsolete? date now ttl)
-                 (values #f #f)
-                 (values #t (string->narinfo value cache-uri))))
-            (('narinfo ('version v) _ ...)
-             (values #f #f))))))
-    (lambda _
-      (values #f #f))))
-
-(define (cache-narinfo! cache-url path narinfo ttl)
-  "Cache locally NARNIFO for PATH, which originates from CACHE-URL, with the
-given TTL (a number of seconds or #f).  NARINFO may be #f, in which case it
-indicates that PATH is unavailable at CACHE-URL."
-  (define now
-    (current-time time-monotonic))
-
-  (define (cache-entry cache-uri narinfo)
-    `(narinfo (version 2)
-              (cache-uri ,cache-uri)
-              (date ,(time-second now))
-              (ttl ,(or ttl
-                        (if narinfo %narinfo-ttl %narinfo-negative-ttl)))
-              (value ,(and=> narinfo narinfo->string))))
-
-  (let ((file (narinfo-cache-file cache-url path)))
-    (mkdir-p (dirname file))
-    (with-atomic-file-output file
-      (lambda (out)
-        (write (cache-entry cache-url narinfo) out))))
-
-  narinfo)
-
-(define (narinfo-request cache-url path)
-  "Return an HTTP request for the narinfo of PATH at CACHE-URL."
-  (let ((url (string-append cache-url "/" (store-path-hash-part path)
-                            ".narinfo"))
-        (headers '((User-Agent . "GNU Guile"))))
-    (build-request (string->uri url) #:method 'GET #:headers headers)))
-
-(define (at-most max-length lst)
-  "If LST is shorter than MAX-LENGTH, return it and the empty list; otherwise
-return its MAX-LENGTH first elements and its tail."
-  (let loop ((len 0)
-             (lst lst)
-             (result '()))
-    (match lst
-      (()
-       (values (reverse result) '()))
-      ((head . tail)
-       (if (>= len max-length)
-           (values (reverse result) lst)
-           (loop (+ 1 len) tail (cons head result)))))))
-
-(define* (http-multiple-get base-uri proc seed requests
-                            #:key port (verify-certificate? #t)
-                            (open-connection guix:open-connection-for-uri)
-                            (keep-alive? #t)
-                            (batch-size 1000))
-  "Send all of REQUESTS to the server at BASE-URI.  Call PROC for each
-response, passing it the request object, the response, a port from which to
-read the response body, and the previous result, starting with SEED, à la
-'fold'.  Return the final result.
-
-When PORT is specified, use it as the initial connection on which HTTP
-requests are sent; otherwise call OPEN-CONNECTION to open a new connection for
-a URI.  When KEEP-ALIVE? is false, close the connection port before
-returning."
-  (let connect ((port     port)
-                (requests requests)
-                (result   seed))
-    (define batch
-      (at-most batch-size requests))
-
-    ;; (format (current-error-port) "connecting (~a requests left)..."
-    ;;         (length requests))
-    (let ((p (or port (open-connection base-uri
-                                       #:verify-certificate?
-                                       verify-certificate?))))
-      ;; For HTTPS, P is not a file port and does not support 'setvbuf'.
-      (when (file-port? p)
-        (setvbuf p 'block (expt 2 16)))
-
-      ;; Send BATCH in a row.
-      ;; XXX: Do our own caching to work around inefficiencies when
-      ;; communicating over TLS: <http://bugs.gnu.org/22966>.
-      (let-values (((buffer get) (open-bytevector-output-port)))
-        ;; Inherit the HTTP proxying property from P.
-        (set-http-proxy-port?! buffer (http-proxy-port? p))
-
-        (for-each (cut write-request <> buffer)
-                  batch)
-        (put-bytevector p (get))
-        (force-output p))
-
-      ;; Now start processing responses.
-      (let loop ((sent      batch)
-                 (processed 0)
-                 (result    result))
-        (match sent
-          (()
-           (match (drop requests processed)
-             (()
-              (unless keep-alive?
-                (close-port p))
-              (reverse result))
-             (remainder
-              (connect p remainder result))))
-          ((head tail ...)
-           (let* ((resp   (read-response p))
-                  (body   (response-body-port resp))
-                  (result (proc head resp body result)))
-             ;; The server can choose to stop responding at any time, in which
-             ;; case we have to try again.  Check whether that is the case.
-             ;; Note that even upon "Connection: close", we can read from BODY.
-             (match (assq 'connection (response-headers resp))
-               (('connection 'close)
-                (close-port p)
-                (connect #f                       ;try again
-                         (drop requests (+ 1 processed))
-                         result))
-               (_
-                (loop tail (+ 1 processed) result)))))))))) ;keep going
-
-(define (read-to-eof port)
-  "Read from PORT until EOF is reached.  The data are discarded."
-  (dump-port port (%make-void-port "w")))
-
-(define (narinfo-from-file file url)
-  "Attempt to read a narinfo from FILE, using URL as the cache URL.  Return #f
-if file doesn't exist, and the narinfo otherwise."
-  (catch 'system-error
-    (lambda ()
-      (call-with-input-file file
-        (cut read-narinfo <> url)))
-    (lambda args
-      (if (= ENOENT (system-error-errno args))
-          #f
-          (apply throw args)))))
-
-(define %unreachable-hosts
-  ;; Set of names of unreachable hosts.
-  (make-hash-table))
-
-(define* (open-connection-for-uri/maybe uri
-                                        #:key
-                                        fresh?
-                                        (time %fetch-timeout))
-  "Open a connection to URI via 'open-connection-for-uri/cached' and return a
-port to it, or, if connection failed, print a warning and return #f.  Pass
-#:fresh? to 'open-connection-for-uri/cached'."
-  (define host
-    (uri-host uri))
-
-  (catch #t
-    (lambda ()
-      (open-connection-for-uri/cached uri #:timeout time
-                                      #:fresh? fresh?))
-    (match-lambda*
-      (('getaddrinfo-error error)
-       (unless (hash-ref %unreachable-hosts host)
-         (hash-set! %unreachable-hosts host #t)   ;warn only once
-         (warning (G_ "~a: host not found: ~a~%")
-                  host (gai-strerror error)))
-       #f)
-      (('system-error . args)
-       (unless (hash-ref %unreachable-hosts host)
-         (hash-set! %unreachable-hosts host #t)
-         (warning (G_ "~a: connection failed: ~a~%") host
-                  (strerror
-                   (system-error-errno `(system-error ,@args)))))
-       #f)
-      (args
-       (apply throw args)))))
-
-(define (fetch-narinfos url paths)
-  "Retrieve all the narinfos for PATHS from the cache at URL and return them."
-  (define update-progress!
-    (let ((done 0)
-          (total (length paths)))
-      (lambda ()
-        (display "\r\x1b[K" (current-error-port)) ;erase current line
-        (force-output (current-error-port))
-        (format (current-error-port)
-                (G_ "updating substitutes from '~a'... ~5,1f%")
-                url (* 100. (/ done total)))
-        (set! done (+ 1 done)))))
-
-  (define hash-part->path
-    (let ((mapping (fold (lambda (path result)
-                           (vhash-cons (store-path-hash-part path) path
-                                       result))
-                         vlist-null
-                         paths)))
-      (lambda (hash)
-        (match (vhash-assoc hash mapping)
-          (#f #f)
-          ((_ . path) path)))))
-
-  (define (handle-narinfo-response request response port result)
-    (let* ((code   (response-code response))
-           (len    (response-content-length response))
-           (cache  (response-cache-control response))
-           (ttl    (and cache (assoc-ref cache 'max-age))))
-      (update-progress!)
-
-      ;; Make sure to read no more than LEN bytes since subsequent bytes may
-      ;; belong to the next response.
-      (if (= code 200)                            ; hit
-          (let ((narinfo (read-narinfo port url #:size len)))
-            (if (string=? (dirname (narinfo-path narinfo))
-                          (%store-prefix))
-                (begin
-                  (cache-narinfo! url (narinfo-path narinfo) narinfo ttl)
-                  (cons narinfo result))
-                result))
-          (let* ((path      (uri-path (request-uri request)))
-                 (hash-part (basename
-                             (string-drop-right path 8)))) ;drop ".narinfo"
-            (if len
-                (get-bytevector-n port len)
-                (read-to-eof port))
-            (cache-narinfo! url (hash-part->path hash-part) #f
-                            (if (or (= 404 code) (= 202 code))
-                                ttl
-                                %narinfo-transient-error-ttl))
-            result))))
-
-  (define (do-fetch uri)
-    (case (and=> uri uri-scheme)
-      ((http https)
-       ;; Note: Do not check HTTPS server certificates to avoid depending
-       ;; on the X.509 PKI.  We can do it because we authenticate
-       ;; narinfos, which provides a much stronger guarantee.
-       (let* ((requests (map (cut narinfo-request url <>) paths))
-              (result   (call-with-cached-connection uri
-                          (lambda (port)
-                            (if port
-                                (begin
-                                  (update-progress!)
-                                  (http-multiple-get uri
-                                                     handle-narinfo-response '()
-                                                     requests
-                                                     #:open-connection
-                                                     open-connection-for-uri/cached
-                                                     #:verify-certificate? #f
-                                                     #:port port))
-                                '()))
-                          open-connection-for-uri/maybe)))
-         (newline (current-error-port))
-         result))
-      ((file #f)
-       (let* ((base  (string-append (uri-path uri) "/"))
-              (files (map (compose (cut string-append base <> ".narinfo")
-                                   store-path-hash-part)
-                          paths)))
-         (filter-map (cut narinfo-from-file <> url) files)))
-      (else
-       (leave (G_ "~s: unsupported server URI scheme~%")
-              (if uri (uri-scheme uri) url)))))
-
-  (do-fetch (string->uri url)))
-
-(define (lookup-narinfos cache paths)
-  "Return the narinfos for PATHS, invoking the server at CACHE when no
-information is available locally."
-  (let-values (((cached missing)
-                (fold2 (lambda (path cached missing)
-                         (let-values (((valid? value)
-                                       (cached-narinfo cache path)))
-                           (if valid?
-                               (if value
-                                   (values (cons value cached) missing)
-                                   (values cached missing))
-                               (values cached (cons path missing)))))
-                       '()
-                       '()
-                       paths)))
-    (if (null? missing)
-        cached
-        (let ((missing (fetch-narinfos cache missing)))
-          (append cached (or missing '()))))))
-
-(define (lookup-narinfos/diverse caches paths authorized?)
-  "Look up narinfos for PATHS on all of CACHES, a list of URLS, in that order.
-That is, when a cache lacks an AUTHORIZED? narinfo, look it up in the next
-cache, and so on.
-
-Return a list of narinfos for PATHS or a subset thereof.  The returned
-narinfos are either AUTHORIZED?, or they claim a hash that matches an
-AUTHORIZED? narinfo."
-  (define (select-hit result)
-    (lambda (path)
-      (match (vhash-fold* cons '() path result)
-        ((one)
-         one)
-        ((several ..1)
-         (let ((authorized (find authorized? (reverse several))))
-           (and authorized
-                (find (cut equivalent-narinfo? <> authorized)
-                      several)))))))
-
-  (let loop ((caches caches)
-             (paths  paths)
-             (result vlist-null)                  ;path->narinfo vhash
-             (hits   '()))                        ;paths
-    (match paths
-      (()                                         ;we're done
-       ;; Now iterate on all the HITS, and return exactly one match for each
-       ;; hit: the first narinfo that is authorized, or that has the same hash
-       ;; as an authorized narinfo, in the order of CACHES.
-       (filter-map (select-hit result) hits))
-      (_
-       (match caches
-         ((cache rest ...)
-          (let* ((narinfos (lookup-narinfos cache paths))
-                 (definite (map narinfo-path (filter authorized? narinfos)))
-                 (missing  (lset-difference string=? paths definite))) ;XXX: perf
-            (loop rest missing
-                  (fold vhash-cons result
-                        (map narinfo-path narinfos) narinfos)
-                  (append definite hits))))
-         (()                                      ;that's it
-          (filter-map (select-hit result) hits)))))))
-
 (define (lookup-narinfo caches path authorized?)
   "Return the narinfo for PATH in CACHES, or #f when no substitute for PATH
 was found."
@@ -719,82 +319,6 @@  authorized substitutes."
     (wtf
      (error "unknown `--query' command" wtf))))
 
-(define %max-cached-connections
-  ;; Maximum number of connections kept in cache by
-  ;; 'open-connection-for-uri/cached'.
-  16)
-
-(define open-connection-for-uri/cached
-  (let ((cache '()))
-    (lambda* (uri #:key fresh? timeout verify-certificate?)
-      "Return a connection for URI, possibly reusing a cached connection.
-When FRESH? is true, delete any cached connections for URI and open a new one.
-Return #f if URI's scheme is 'file' or #f.
-
-When true, TIMEOUT is the maximum number of milliseconds to wait for
-connection establishment.  When VERIFY-CERTIFICATE? is true, verify HTTPS
-server certificates."
-      (define host (uri-host uri))
-      (define scheme (uri-scheme uri))
-      (define key (list host scheme (uri-port uri)))
-
-      (and (not (memq scheme '(file #f)))
-           (match (assoc-ref cache key)
-             (#f
-              ;; Open a new connection to URI and evict old entries from
-              ;; CACHE, if any.
-              (let-values (((socket)
-                            (guix:open-connection-for-uri
-                             uri
-                             #:verify-certificate? verify-certificate?
-                             #:timeout timeout))
-                           ((new-cache evicted)
-                            (at-most (- %max-cached-connections 1) cache)))
-                (for-each (match-lambda
-                            ((_ . port)
-                             (false-if-exception (close-port port))))
-                          evicted)
-                (set! cache (alist-cons key socket new-cache))
-                socket))
-             (socket
-              (if (or fresh? (port-closed? socket))
-                  (begin
-                    (false-if-exception (close-port socket))
-                    (set! cache (alist-delete key cache))
-                    (open-connection-for-uri/cached uri #:timeout timeout
-                                                    #:verify-certificate?
-                                                    verify-certificate?))
-                  (begin
-                    ;; Drain input left from the previous use.
-                    (drain-input socket)
-                    socket))))))))
-
-(define* (call-with-cached-connection uri proc
-                                      #:optional
-                                      (open-connection
-                                       open-connection-for-uri/cached))
-  (let ((port (open-connection uri)))
-    (catch #t
-      (lambda ()
-        (proc port))
-      (lambda (key . args)
-        ;; If PORT was cached and the server closed the connection in the
-        ;; meantime, we get EPIPE.  In that case, open a fresh connection and
-        ;; retry.  We might also get 'bad-response or a similar exception from
-        ;; (web response) later on, once we've sent the request, or a
-        ;; ERROR/INVALID-SESSION from GnuTLS.
-        (if (or (and (eq? key 'system-error)
-                     (= EPIPE (system-error-errno `(,key ,@args))))
-                (and (eq? key 'gnutls-error)
-                     (eq? (first args) error/invalid-session))
-                (memq key '(bad-response bad-header bad-header-component)))
-            (proc (open-connection uri #:fresh? #t))
-            (apply throw key args))))))
-
-(define-syntax-rule (with-cached-connection uri port exp ...)
-  "Bind PORT with EXP... to a socket connected to URI."
-  (call-with-cached-connection uri (lambda (port) exp ...)))
-
 (define* (process-substitution store-item destination
                                #:key cache-urls acl
                                deduplicate? print-build-trace?)
diff --git a/guix/scripts/weather.scm b/guix/scripts/weather.scm
index 97e4a73802..527a63560d 100644
--- a/guix/scripts/weather.scm
+++ b/guix/scripts/weather.scm
@@ -32,8 +32,8 @@ 
   #:use-module (guix gexp)
   #:use-module ((guix build syscalls) #:select (terminal-columns))
   #:use-module ((guix build utils) #:select (every*))
-  #:use-module (guix scripts substitute)
   #:use-module (guix narinfo)
+  #:use-module (guix substitutes)
   #:use-module (guix http-client)
   #:use-module (guix ci)
   #:use-module (guix sets)
diff --git a/guix/substitutes.scm b/guix/substitutes.scm
new file mode 100644
index 0000000000..7c2012d307
--- /dev/null
+++ b/guix/substitutes.scm
@@ -0,0 +1,531 @@ 
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014 Nikita Karetnikov <nikita@karetnikov.org>
+;;; Copyright © 2018 Kyle Meyer <kyle@kyleam.com>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix substitutes)
+  #:use-module (guix i18n)
+  #:use-module (guix cache)
+  #:use-module (guix store)
+  #:use-module (guix utils)
+  #:use-module (guix base32)
+  #:use-module (guix config)
+  #:use-module (guix narinfo)
+  #:use-module (guix combinators)
+  #:use-module (guix diagnostics)
+  #:use-module ((guix build utils) #:select (mkdir-p dump-port))
+  #:use-module ((guix build download)
+                #:select ((open-connection-for-uri
+                           . guix:open-connection-for-uri)))
+  #:autoload   (gnutls) (error/invalid-session)
+  #:use-module (gcrypt hash)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 vlist)
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 binary-ports)
+  #:use-module (rnrs bytevectors)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-19)
+  #:use-module (srfi srfi-26)
+  #:use-module (web uri)
+  #:use-module (web http)
+  #:use-module (web request)
+  #:use-module (web response)
+  #:export (%narinfo-cache-directory
+
+            with-cached-connection
+
+            lookup-narinfos
+            lookup-narinfos/diverse))
+
+(define %narinfo-cache-directory
+  ;; A local cache of narinfos, to avoid going to the network.  Most of the
+  ;; time, 'guix substitute' is called by guix-daemon as root and stores its
+  ;; cached data in /var/guix/….  However, when invoked from 'guix challenge'
+  ;; as a user, it stores its cache in ~/.cache.
+  (if (zero? (getuid))
+      (or (and=> (getenv "XDG_CACHE_HOME")
+                 (cut string-append <> "/guix/substitute"))
+          (string-append %state-directory "/substitute/cache"))
+      (string-append (cache-directory #:ensure? #f) "/substitute")))
+
+(define %narinfo-ttl
+  ;; Number of seconds during which cached narinfo lookups are considered
+  ;; valid for substitute servers that do not advertise a TTL via the
+  ;; 'Cache-Control' response header.
+  (* 36 3600))
+
+(define %narinfo-negative-ttl
+  ;; Likewise, but for negative lookups---i.e., cached lookup failures (404).
+  (* 1 3600))
+
+(define %narinfo-transient-error-ttl
+  ;; Likewise, but for transient errors such as 504 ("Gateway timeout").
+  (* 10 60))
+
+(define %fetch-timeout
+  ;; Number of seconds after which networking is considered "slow".
+  5)
+
+(define (narinfo-cache-file cache-url path)
+  "Return the name of the local file that contains an entry for PATH.  The
+entry is stored in a sub-directory specific to CACHE-URL."
+  ;; The daemon does not sanitize its input, so PATH could be something like
+  ;; "/gnu/store/foo".  Gracefully handle that.
+  (match (store-path-hash-part path)
+    (#f
+     (leave (G_ "'~a' does not name a store item~%") path))
+    ((? string? hash-part)
+     (string-append %narinfo-cache-directory "/"
+                    (bytevector->base32-string (sha256 (string->utf8 cache-url)))
+                    "/" hash-part))))
+
+(define (cache-narinfo! cache-url path narinfo ttl)
+  "Cache locally NARNIFO for PATH, which originates from CACHE-URL, with the
+given TTL (a number of seconds or #f).  NARINFO may be #f, in which case it
+indicates that PATH is unavailable at CACHE-URL."
+  (define now
+    (current-time time-monotonic))
+
+  (define (cache-entry cache-uri narinfo)
+    `(narinfo (version 2)
+              (cache-uri ,cache-uri)
+              (date ,(time-second now))
+              (ttl ,(or ttl
+                        (if narinfo %narinfo-ttl %narinfo-negative-ttl)))
+              (value ,(and=> narinfo narinfo->string))))
+
+  (let ((file (narinfo-cache-file cache-url path)))
+    (mkdir-p (dirname file))
+    (with-atomic-file-output file
+      (lambda (out)
+        (write (cache-entry cache-url narinfo) out))))
+
+  narinfo)
+
+(define %max-cached-connections
+  ;; Maximum number of connections kept in cache by
+  ;; 'open-connection-for-uri/cached'.
+  16)
+
+(define open-connection-for-uri/cached
+  (let ((cache '()))
+    (lambda* (uri #:key fresh? timeout verify-certificate?)
+      "Return a connection for URI, possibly reusing a cached connection.
+When FRESH? is true, delete any cached connections for URI and open a new one.
+Return #f if URI's scheme is 'file' or #f.
+
+When true, TIMEOUT is the maximum number of milliseconds to wait for
+connection establishment.  When VERIFY-CERTIFICATE? is true, verify HTTPS
+server certificates."
+      (define host (uri-host uri))
+      (define scheme (uri-scheme uri))
+      (define key (list host scheme (uri-port uri)))
+
+      (and (not (memq scheme '(file #f)))
+           (match (assoc-ref cache key)
+             (#f
+              ;; Open a new connection to URI and evict old entries from
+              ;; CACHE, if any.
+              (let-values (((socket)
+                            (guix:open-connection-for-uri
+                             uri
+                             #:verify-certificate? verify-certificate?
+                             #:timeout timeout))
+                           ((new-cache evicted)
+                            (at-most (- %max-cached-connections 1) cache)))
+                (for-each (match-lambda
+                            ((_ . port)
+                             (false-if-exception (close-port port))))
+                          evicted)
+                (set! cache (alist-cons key socket new-cache))
+                socket))
+             (socket
+              (if (or fresh? (port-closed? socket))
+                  (begin
+                    (false-if-exception (close-port socket))
+                    (set! cache (alist-delete key cache))
+                    (open-connection-for-uri/cached uri #:timeout timeout
+                                                    #:verify-certificate?
+                                                    verify-certificate?))
+                  (begin
+                    ;; Drain input left from the previous use.
+                    (drain-input socket)
+                    socket))))))))
+
+(define* (call-with-cached-connection uri proc
+                                      #:optional
+                                      (open-connection
+                                       open-connection-for-uri/cached))
+  (let ((port (open-connection uri)))
+    (catch #t
+      (lambda ()
+        (proc port))
+      (lambda (key . args)
+        ;; If PORT was cached and the server closed the connection in the
+        ;; meantime, we get EPIPE.  In that case, open a fresh connection and
+        ;; retry.  We might also get 'bad-response or a similar exception from
+        ;; (web response) later on, once we've sent the request, or a
+        ;; ERROR/INVALID-SESSION from GnuTLS.
+        (if (or (and (eq? key 'system-error)
+                     (= EPIPE (system-error-errno `(,key ,@args))))
+                (and (eq? key 'gnutls-error)
+                     (eq? (first args) error/invalid-session))
+                (memq key '(bad-response bad-header bad-header-component)))
+            (proc (open-connection uri #:fresh? #t))
+            (apply throw key args))))))
+
+(define-syntax-rule (with-cached-connection uri port exp ...)
+  "Bind PORT with EXP... to a socket connected to URI."
+  (call-with-cached-connection uri (lambda (port) exp ...)))
+
+(define (at-most max-length lst)
+  "If LST is shorter than MAX-LENGTH, return it and the empty list; otherwise
+return its MAX-LENGTH first elements and its tail."
+  (let loop ((len 0)
+             (lst lst)
+             (result '()))
+    (match lst
+      (()
+       (values (reverse result) '()))
+      ((head . tail)
+       (if (>= len max-length)
+           (values (reverse result) lst)
+           (loop (+ 1 len) tail (cons head result)))))))
+
+(define* (http-multiple-get base-uri proc seed requests
+                            #:key port (verify-certificate? #t)
+                            (open-connection guix:open-connection-for-uri)
+                            (keep-alive? #t)
+                            (batch-size 1000))
+  "Send all of REQUESTS to the server at BASE-URI.  Call PROC for each
+response, passing it the request object, the response, a port from which to
+read the response body, and the previous result, starting with SEED, à la
+'fold'.  Return the final result.
+
+When PORT is specified, use it as the initial connection on which HTTP
+requests are sent; otherwise call OPEN-CONNECTION to open a new connection for
+a URI.  When KEEP-ALIVE? is false, close the connection port before
+returning."
+  (let connect ((port     port)
+                (requests requests)
+                (result   seed))
+    (define batch
+      (at-most batch-size requests))
+
+    ;; (format (current-error-port) "connecting (~a requests left)..."
+    ;;         (length requests))
+    (let ((p (or port (open-connection base-uri
+                                       #:verify-certificate?
+                                       verify-certificate?))))
+      ;; For HTTPS, P is not a file port and does not support 'setvbuf'.
+      (when (file-port? p)
+        (setvbuf p 'block (expt 2 16)))
+
+      ;; Send BATCH in a row.
+      ;; XXX: Do our own caching to work around inefficiencies when
+      ;; communicating over TLS: <http://bugs.gnu.org/22966>.
+      (let-values (((buffer get) (open-bytevector-output-port)))
+        ;; Inherit the HTTP proxying property from P.
+        (set-http-proxy-port?! buffer (http-proxy-port? p))
+
+        (for-each (cut write-request <> buffer)
+                  batch)
+        (put-bytevector p (get))
+        (force-output p))
+
+      ;; Now start processing responses.
+      (let loop ((sent      batch)
+                 (processed 0)
+                 (result    result))
+        (match sent
+          (()
+           (match (drop requests processed)
+             (()
+              (unless keep-alive?
+                (close-port p))
+              (reverse result))
+             (remainder
+              (connect p remainder result))))
+          ((head tail ...)
+           (let* ((resp   (read-response p))
+                  (body   (response-body-port resp))
+                  (result (proc head resp body result)))
+             ;; The server can choose to stop responding at any time, in which
+             ;; case we have to try again.  Check whether that is the case.
+             ;; Note that even upon "Connection: close", we can read from BODY.
+             (match (assq 'connection (response-headers resp))
+               (('connection 'close)
+                (close-port p)
+                (connect #f                       ;try again
+                         (drop requests (+ 1 processed))
+                         result))
+               (_
+                (loop tail (+ 1 processed) result)))))))))) ;keep going
+
+(define %unreachable-hosts
+  ;; Set of names of unreachable hosts.
+  (make-hash-table))
+
+(define* (open-connection-for-uri/maybe uri
+                                        #:key
+                                        fresh?
+                                        (time %fetch-timeout))
+  "Open a connection to URI via 'open-connection-for-uri/cached' and return a
+port to it, or, if connection failed, print a warning and return #f.  Pass
+#:fresh? to 'open-connection-for-uri/cached'."
+  (define host
+    (uri-host uri))
+
+  (catch #t
+    (lambda ()
+      (open-connection-for-uri/cached uri #:timeout time
+                                      #:fresh? fresh?))
+    (match-lambda*
+      (('getaddrinfo-error error)
+       (unless (hash-ref %unreachable-hosts host)
+         (hash-set! %unreachable-hosts host #t)   ;warn only once
+         (warning (G_ "~a: host not found: ~a~%")
+                  host (gai-strerror error)))
+       #f)
+      (('system-error . args)
+       (unless (hash-ref %unreachable-hosts host)
+         (hash-set! %unreachable-hosts host #t)
+         (warning (G_ "~a: connection failed: ~a~%") host
+                  (strerror
+                   (system-error-errno `(system-error ,@args)))))
+       #f)
+      (args
+       (apply throw args)))))
+
+(define (read-to-eof port)
+  "Read from PORT until EOF is reached.  The data are discarded."
+  (dump-port port (%make-void-port "w")))
+
+(define (narinfo-request cache-url path)
+  "Return an HTTP request for the narinfo of PATH at CACHE-URL."
+  (let ((url (string-append cache-url "/" (store-path-hash-part path)
+                            ".narinfo"))
+        (headers '((User-Agent . "GNU Guile"))))
+    (build-request (string->uri url) #:method 'GET #:headers headers)))
+
+(define (narinfo-from-file file url)
+  "Attempt to read a narinfo from FILE, using URL as the cache URL.  Return #f
+if file doesn't exist, and the narinfo otherwise."
+  (catch 'system-error
+    (lambda ()
+      (call-with-input-file file
+        (cut read-narinfo <> url)))
+    (lambda args
+      (if (= ENOENT (system-error-errno args))
+          #f
+          (apply throw args)))))
+
+(define (fetch-narinfos url paths)
+  "Retrieve all the narinfos for PATHS from the cache at URL and return them."
+  (define update-progress!
+    (let ((done 0)
+          (total (length paths)))
+      (lambda ()
+        (display "\r\x1b[K" (current-error-port)) ;erase current line
+        (force-output (current-error-port))
+        (format (current-error-port)
+                (G_ "updating substitutes from '~a'... ~5,1f%")
+                url (* 100. (/ done total)))
+        (set! done (+ 1 done)))))
+
+  (define hash-part->path
+    (let ((mapping (fold (lambda (path result)
+                           (vhash-cons (store-path-hash-part path) path
+                                       result))
+                         vlist-null
+                         paths)))
+      (lambda (hash)
+        (match (vhash-assoc hash mapping)
+          (#f #f)
+          ((_ . path) path)))))
+
+  (define (handle-narinfo-response request response port result)
+    (let* ((code   (response-code response))
+           (len    (response-content-length response))
+           (cache  (response-cache-control response))
+           (ttl    (and cache (assoc-ref cache 'max-age))))
+      (update-progress!)
+
+      ;; Make sure to read no more than LEN bytes since subsequent bytes may
+      ;; belong to the next response.
+      (if (= code 200)                            ; hit
+          (let ((narinfo (read-narinfo port url #:size len)))
+            (if (string=? (dirname (narinfo-path narinfo))
+                          (%store-prefix))
+                (begin
+                  (cache-narinfo! url (narinfo-path narinfo) narinfo ttl)
+                  (cons narinfo result))
+                result))
+          (let* ((path      (uri-path (request-uri request)))
+                 (hash-part (basename
+                             (string-drop-right path 8)))) ;drop ".narinfo"
+            (if len
+                (get-bytevector-n port len)
+                (read-to-eof port))
+            (cache-narinfo! url (hash-part->path hash-part) #f
+                            (if (or (= 404 code) (= 202 code))
+                                ttl
+                                %narinfo-transient-error-ttl))
+            result))))
+
+  (define (do-fetch uri)
+    (case (and=> uri uri-scheme)
+      ((http https)
+       ;; Note: Do not check HTTPS server certificates to avoid depending
+       ;; on the X.509 PKI.  We can do it because we authenticate
+       ;; narinfos, which provides a much stronger guarantee.
+       (let* ((requests (map (cut narinfo-request url <>) paths))
+              (result   (call-with-cached-connection uri
+                          (lambda (port)
+                            (if port
+                                (begin
+                                  (update-progress!)
+                                  (http-multiple-get uri
+                                                     handle-narinfo-response '()
+                                                     requests
+                                                     #:open-connection
+                                                     open-connection-for-uri/cached
+                                                     #:verify-certificate? #f
+                                                     #:port port))
+                                '()))
+                          open-connection-for-uri/maybe)))
+         (newline (current-error-port))
+         result))
+      ((file #f)
+       (let* ((base  (string-append (uri-path uri) "/"))
+              (files (map (compose (cut string-append base <> ".narinfo")
+                                   store-path-hash-part)
+                          paths)))
+         (filter-map (cut narinfo-from-file <> url) files)))
+      (else
+       (leave (G_ "~s: unsupported server URI scheme~%")
+              (if uri (uri-scheme uri) url)))))
+
+  (do-fetch (string->uri url)))
+
+(define (cached-narinfo cache-url path)
+  "Check locally if we have valid info about PATH coming from CACHE-URL.
+Return two values: a Boolean indicating whether we have valid cached info, and
+that info, which may be either #f (when PATH is unavailable) or the narinfo
+for PATH."
+  (define now
+    (current-time time-monotonic))
+
+  (define cache-file
+    (narinfo-cache-file cache-url path))
+
+  (catch 'system-error
+    (lambda ()
+      (call-with-input-file cache-file
+        (lambda (p)
+          (match (read p)
+            (('narinfo ('version 2)
+                       ('cache-uri cache-uri)
+                       ('date date) ('ttl ttl) ('value #f))
+             ;; A cached negative lookup.
+             (if (obsolete? date now ttl)
+                 (values #f #f)
+                 (values #t #f)))
+            (('narinfo ('version 2)
+                       ('cache-uri cache-uri)
+                       ('date date) ('ttl ttl) ('value value))
+             ;; A cached positive lookup
+             (if (obsolete? date now ttl)
+                 (values #f #f)
+                 (values #t (string->narinfo value cache-uri))))
+            (('narinfo ('version v) _ ...)
+             (values #f #f))))))
+    (lambda _
+      (values #f #f))))
+
+(define (lookup-narinfos cache paths)
+  "Return the narinfos for PATHS, invoking the server at CACHE when no
+information is available locally."
+  (let-values (((cached missing)
+                (fold2 (lambda (path cached missing)
+                         (let-values (((valid? value)
+                                       (cached-narinfo cache path)))
+                           (if valid?
+                               (if value
+                                   (values (cons value cached) missing)
+                                   (values cached missing))
+                               (values cached (cons path missing)))))
+                       '()
+                       '()
+                       paths)))
+    (if (null? missing)
+        cached
+        (let ((missing (fetch-narinfos cache missing)))
+          (append cached (or missing '()))))))
+
+(define (lookup-narinfos/diverse caches paths authorized?)
+  "Look up narinfos for PATHS on all of CACHES, a list of URLS, in that order.
+That is, when a cache lacks an AUTHORIZED? narinfo, look it up in the next
+cache, and so on.
+
+Return a list of narinfos for PATHS or a subset thereof.  The returned
+narinfos are either AUTHORIZED?, or they claim a hash that matches an
+AUTHORIZED? narinfo."
+  (define (select-hit result)
+    (lambda (path)
+      (match (vhash-fold* cons '() path result)
+        ((one)
+         one)
+        ((several ..1)
+         (let ((authorized (find authorized? (reverse several))))
+           (and authorized
+                (find (cut equivalent-narinfo? <> authorized)
+                      several)))))))
+
+  (let loop ((caches caches)
+             (paths  paths)
+             (result vlist-null)                  ;path->narinfo vhash
+             (hits   '()))                        ;paths
+    (match paths
+      (()                                         ;we're done
+       ;; Now iterate on all the HITS, and return exactly one match for each
+       ;; hit: the first narinfo that is authorized, or that has the same hash
+       ;; as an authorized narinfo, in the order of CACHES.
+       (filter-map (select-hit result) hits))
+      (_
+       (match caches
+         ((cache rest ...)
+          (let* ((narinfos (lookup-narinfos cache paths))
+                 (definite (map narinfo-path (filter authorized? narinfos)))
+                 (missing  (lset-difference string=? paths definite))) ;XXX: perf
+            (loop rest missing
+                  (fold vhash-cons result
+                        (map narinfo-path narinfos) narinfos)
+                  (append definite hits))))
+         (()                                      ;that's it
+          (filter-map (select-hit result) hits)))))))
+
+;;; Local Variables:
+;;; eval: (put 'with-timeout 'scheme-indent-function 1)
+;;; eval: (put 'with-cached-connection 'scheme-indent-function 2)
+;;; eval: (put 'call-with-cached-connection 'scheme-indent-function 1)
+;;; End:
+
+;;; substitutes.scm ends here
diff --git a/po/guix/POTFILES.in b/po/guix/POTFILES.in
index 666e630adf..fa397d7969 100644
--- a/po/guix/POTFILES.in
+++ b/po/guix/POTFILES.in
@@ -88,6 +88,7 @@  guix/status.scm
 guix/http-client.scm
 guix/nar.scm
 guix/narinfo.scm
+guix/substitutes.scm
 guix/channels.scm
 guix/profiles.scm
 guix/git.scm