substitute: Retry downloading when a nar is unavailable.

Fixes <https://issues.guix.gnu.org/57978>
Reported by Attila Lendvai <attila@lendvai.name>.

Previously, if a narinfo was available but its corresponding nar was
missing (for instance because the narinfo was cached and the server
became unreachable in the meantime), 'guix substitute --substitute'
would try to download the nar from its preferred location and abort when
that fails.  This change forces one retry with each of the URLs.

* guix/scripts/substitute.scm (download-nar): Do not catch
'http-get-error?' exceptions.
(system-error?, network-error?, process-substitution/fallback): New
procedures.
(process-substitution): Call 'process-substitution/fallback' upon
'network-error?'.
* tests/substitute.scm ("substitute, first URL has narinfo but lacks nar, second URL unauthorized")
("substitute, first URL has narinfo but nar is 404, both URLs authorized")
("substitute, first URL has narinfo but nar is 404, one URL authorized")
("substitute, narinfo is available but nar is missing"): New tests.

2 files changed, 203 insertions, 23 deletions

diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index e3b382d0d8..cf59db4315 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -460,25 +460,20 @@ STATUS-PORT."
        (let ((port (open-file (uri-path uri) "r0b")))
          (values port (stat:size (stat port)))))
       ((http https)
-       (guard (c ((http-get-error? c)
-                  (leave (G_ "download from '~a' failed: ~a, ~s~%")
-                         (uri->string (http-get-error-uri c))
-                         (http-get-error-code c)
-                         (http-get-error-reason c))))
-         ;; Test this with:
-         ;;   sudo tc qdisc add dev eth0 root netem delay 1500ms
-         ;; and then cancel with:
-         ;;   sudo tc qdisc del dev eth0 root
-         (with-timeout %fetch-timeout
-           (begin
-             (warning (G_ "while fetching ~a: server is somewhat slow~%")
-                      (uri->string uri))
-             (warning (G_ "try `--no-substitutes' if the problem persists~%")))
-           (with-cached-connection uri port
-             (http-fetch uri #:text? #f
-                         #:port port
-                         #:keep-alive? #t
-                         #:buffered? #f)))))
+       ;; Test this with:
+       ;;   sudo tc qdisc add dev eth0 root netem delay 1500ms
+       ;; and then cancel with:
+       ;;   sudo tc qdisc del dev eth0 root
+       (with-timeout %fetch-timeout
+         (begin
+           (warning (G_ "while fetching ~a: server is somewhat slow~%")
+                    (uri->string uri))
+           (warning (G_ "try `--no-substitutes' if the problem persists~%")))
+         (with-cached-connection uri port
+           (http-fetch uri #:text? #f
+                       #:port port
+                       #:keep-alive? #t
+                       #:buffered? #f))))
       (else
        (leave (G_ "unsupported substitute URI scheme: ~a~%")
               (uri->string uri)))))
@@ -572,6 +567,68 @@ STATUS-PORT."
                     (bytevector->nix-base32-string expected)
                     (bytevector->nix-base32-string actual)))))))
 
+(define system-error?
+  (let ((kind-and-args? (exception-predicate &exception-with-kind-and-args)))
+    (lambda (exception)
+      "Return true if EXCEPTION is a Guile 'system-error exception."
+      (and (kind-and-args? exception)
+           (eq? 'system-error (exception-kind exception))))))
+
+(define network-error?
+  (let ((kind-and-args? (exception-predicate &exception-with-kind-and-args)))
+    (lambda (exception)
+      "Return true if EXCEPTION denotes a networking error."
+      (or (and (system-error? exception)
+               (let ((errno (system-error-errno
+                             (cons 'system-error (exception-args exception)))))
+                 (memv errno (list ECONNRESET ECONNABORTED
+                                   ECONNREFUSED EHOSTUNREACH
+                                   ENOENT))))     ;for "file://"
+          (and (kind-and-args? exception)
+               (memq (exception-kind exception)
+                     '(gnutls-error getaddrinfo-error)))
+          (and (http-get-error? exception)
+               (begin
+                 (warning (G_ "download from '~a' failed: ~a, ~s~%")
+                          (uri->string (http-get-error-uri exception))
+                          (http-get-error-code exception)
+                          (http-get-error-reason exception))
+                 #t))))))
+
+(define* (process-substitution/fallback port narinfo destination
+                                        #:key cache-urls acl
+                                        deduplicate? print-build-trace?)
+  "Attempt to substitute NARINFO, which is assumed to be authorized or
+equivalent, by trying to download its nar from each entry in CACHE-URLS.
+
+This can be less efficient than 'lookup-narinfo', which stops at the first
+entry that provides a valid narinfo, but it makes sure we eventually find a
+way to download the nar."
+  ;; Note: Keep NARINFO's uri-base in CACHE-URLS: that lets us retry in case
+  ;; this was a transient issue.
+  (let loop ((cache-urls cache-urls))
+    (match cache-urls
+      (()
+       (leave (G_ "failed to find alternative substitute for '~a'~%")
+              (narinfo-path narinfo)))
+      ((cache-url rest ...)
+       (match (lookup-narinfos cache-url
+                               (list (narinfo-path narinfo))
+                               #:open-connection
+                               open-connection-for-uri/cached)
+         ((alternate)
+          (if (or (equivalent-narinfo? narinfo alternate)
+                  (valid-narinfo? alternate acl)
+                  (%allow-unauthenticated-substitutes?))
+              (guard (c ((network-error? c) (loop rest)))
+                (download-nar alternate destination
+                              #:status-port port
+                              #:deduplicate? deduplicate?
+                              #:print-build-trace? print-build-trace?))
+              (loop rest)))
+         (()
+          (loop rest)))))))
+
 (define* (process-substitution port store-item destination
                                #:key cache-urls acl
                                deduplicate? print-build-trace?)
@@ -590,10 +647,20 @@ PORT."
     (leave (G_ "no valid substitute for '~a'~%")
            store-item))
 
-  (download-nar narinfo destination
-                #:status-port port
-                #:deduplicate? deduplicate?
-                #:print-build-trace? print-build-trace?))
+  (guard (c ((network-error? c)
+             (format (current-error-port)
+                     (G_ "retrying download of '~a' with other substitute URLs...~%")
+                     store-item)
+             (process-substitution/fallback port narinfo destination
+                                            #:cache-urls cache-urls
+                                            #:acl acl
+                                            #:deduplicate? deduplicate?
+                                            #:print-build-trace?
+                                            print-build-trace?)))
+    (download-nar narinfo destination
+                  #:status-port port
+                  #:deduplicate? deduplicate?
+                  #:print-build-trace? print-build-trace?)))
 
 
 ;;;
diff --git a/tests/substitute.scm b/tests/substitute.scm
index 5315292987..9032a50268 100644
--- a/tests/substitute.scm
+++ b/tests/substitute.scm
@@ -523,6 +523,119 @@ System: mips64el-linux\n")))
         (lambda ()
           (false-if-exception (delete-file "substitute-retrieved")))))))
 
+(test-equal "substitute, first URL has narinfo but lacks nar, second URL unauthorized"
+  "Substitutable data."
+  (with-narinfo*
+      (string-append %narinfo "Signature: "
+                     (signature-field
+                      %narinfo
+                      #:public-key %wrong-public-key))
+      %alternate-substitute-directory
+
+    (with-narinfo* (string-append %narinfo "Signature: "
+                                  (signature-field %narinfo))
+        %main-substitute-directory
+
+      (dynamic-wind
+        (const #t)
+        (lambda ()
+          ;; Remove this file so that the substitute can only be retrieved
+          ;; from %ALTERNATE-SUBSTITUTE-DIRECTORY.
+          (delete-file (string-append %main-substitute-directory
+                                      "/example.nar"))
+
+          (parameterize ((substitute-urls
+                          (map (cut string-append "file://" <>)
+                               (list %main-substitute-directory
+                                     %alternate-substitute-directory))))
+            (request-substitution (string-append (%store-prefix)
+                                                 "/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-foo")
+                                  "substitute-retrieved"))
+          (call-with-input-file "substitute-retrieved" get-string-all))
+        (lambda ()
+          (false-if-exception (delete-file "substitute-retrieved")))))))
+
+(test-equal "substitute, first URL has narinfo but nar is 404, both URLs authorized"
+  "Substitutable data."
+  (with-narinfo*
+      (string-append %narinfo "Signature: "
+                     (signature-field %narinfo))
+      %main-substitute-directory
+
+    (with-http-server `((200 ,(string-append %narinfo "Signature: "
+                                             (signature-field %narinfo)))
+                        (404 "Sorry, nar is missing!"))
+      (dynamic-wind
+        (const #t)
+        (lambda ()
+          (parameterize ((substitute-urls
+                          (list (%local-url)
+                                (string-append "file://"
+                                               %main-substitute-directory))))
+            (request-substitution (string-append (%store-prefix)
+                                                 "/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-foo")
+                                  "substitute-retrieved"))
+          (call-with-input-file "substitute-retrieved" get-string-all))
+        (lambda ()
+          (false-if-exception (delete-file "substitute-retrieved")))))))
+
+(test-equal "substitute, first URL has narinfo but nar is 404, one URL authorized"
+  "Substitutable data."
+  (with-narinfo*
+      (string-append %narinfo "Signature: "
+                     (signature-field
+                      %narinfo
+                      #:public-key %wrong-public-key))
+      %main-substitute-directory
+
+    (with-http-server `((200 ,(string-append %narinfo "Signature: "
+                                             (signature-field
+                                              %narinfo
+                                              #:public-key %wrong-public-key)))
+                        (404 "Sorry, nar is missing!"))
+      (let ((url1 (%local-url)))
+        (parameterize ((%http-server-port 0))
+          (with-http-server `((200 ,(string-append %narinfo "Signature: "
+                                                   (signature-field %narinfo)))
+                              (404 "Sorry, nar is missing!"))
+            (let ((url2 (%local-url)))
+              (dynamic-wind
+                (const #t)
+                (lambda ()
+                  (parameterize ((substitute-urls
+                                  (list url1 url2
+                                        (string-append "file://"
+                                                       %main-substitute-directory))))
+                    (request-substitution (string-append (%store-prefix)
+                                                         "/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-foo")
+                                          "substitute-retrieved"))
+                  (call-with-input-file "substitute-retrieved" get-string-all))
+                (lambda ()
+                  (false-if-exception (delete-file "substitute-retrieved")))))))))))
+
+(test-quit "substitute, narinfo is available but nar is missing"
+    "failed to find alternative substitute"
+  (with-narinfo*
+      (string-append %narinfo "Signature: "
+                     (signature-field
+                      %narinfo
+                      #:public-key %wrong-public-key))
+      %main-substitute-directory
+
+    (with-http-server `((200 ,(string-append %narinfo "Signature: "
+                                             (signature-field %narinfo)))
+                        (404 "Sorry, nar is missing!"))
+      (parameterize ((substitute-urls
+                      (list (%local-url)
+                            (string-append "file://"
+                                           %main-substitute-directory))))
+        (delete-file (string-append %main-substitute-directory
+                                    "/example.nar"))
+        (request-substitution (string-append (%store-prefix)
+                                             "/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-foo")
+                              "substitute-retrieved")
+        (not (file-exists? "substitute-retrieved"))))))
+
 (test-equal "substitute, first narinfo is unsigned and has wrong hash"
   "Substitutable data."
   (with-narinfo* (regexp-substitute #f