substitute: If a server's nar URL is 404, try the next one(s).

If a substitute server advertises in its narinfo, for example, both a
/zstd and a /lzip URL but the /zstd URL is unreachable, try the /lzip
URL.

Fixes <https://issues.guix.gnu.org/63634>.

* guix/narinfo.scm (narinfo-preferred-uris): New procedure.
(narinfo-best-uri): Rebase on top of it.
* guix/scripts/substitute.scm (download-nar)[try-fetch]: New procedure.
Use 'narinfo-preferred-uris' and 'try-fetch' to attempt all the URLs of
NARINFO.
* tests/substitute.scm (request-substitution): Remove 'parameterize'.
Delete DESTINATION.
("substitute, preferred nar URL is 404, other is 200"): New test.

2 files changed, 40 insertions, 16 deletions

diff --git a/guix/narinfo.scm b/guix/narinfo.scm
index 741c7ad406..a149d9a901 100644
--- a/guix/narinfo.scm
+++ b/guix/narinfo.scm
@@ -54,6 +54,7 @@
             narinfo-hash-algorithm+value
 
             narinfo-hash->sha256
+            narinfo-preferred-uris
             narinfo-best-uri
 
             valid-narinfo?
@@ -309,9 +310,11 @@ than COMPRESSION2."
     ("gzip" (string=? compression2 "lzip"))
     (_      #f)))
 
-(define* (narinfo-best-uri narinfo #:key fast-decompression?)
-  "Select the \"best\" URI to download NARINFO's nar, and return three values:
-the URI, its compression method (a string), and the compressed file size.
+(define* (narinfo-preferred-uris narinfo #:key fast-decompression?)
+  "Return the sorted list of \"preferred\" nar URIs from NARINFO (preferred
+comes first) where each entry is a tuple containing: the URI, its compression
+method (a string), and the compressed file size.
+
 When FAST-DECOMPRESSION? is true, prefer substitutes with faster
 decompression (typically zstd) rather than substitutes with a higher
 compression ratio (typically lzip)."
@@ -343,6 +346,16 @@ compression ratio (typically lzip)."
          ((uri2 compression2 . _)
           (decompresses-faster? compression2 compression1))))))
 
-  (match (sort choices (if fast-decompression? (negate speed<?) file-size<?))
+  (sort choices (if fast-decompression? (negate speed<?) file-size<?)))
+
+(define* (narinfo-best-uri narinfo #:key fast-decompression?)
+  "Select the \"best\" URI to download NARINFO's nar, and return three values:
+the URI, its compression method (a string), and the compressed file size.
+
+When FAST-DECOMPRESSION? is true, prefer substitutes with faster
+decompression (typically zstd) rather than substitutes with a higher
+compression ratio (typically lzip)."
+  (match (narinfo-preferred-uris narinfo
+                                 #:fast-decompression? fast-decompression?)
     (((uri compression file-size) _ ...)
      (values uri compression file-size))))
diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index 0b27ebb0fc..3626832dda 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -481,18 +481,29 @@ STATUS-PORT."
        (leave (G_ "unsupported substitute URI scheme: ~a~%")
               (uri->string uri)))))
 
-  (let ((uri compression file-size
-             (narinfo-best-uri narinfo
-                               #:fast-decompression?
-                               %prefer-fast-decompression?)))
-    (unless print-build-trace?
-      (format (current-error-port)
-              (G_ "Downloading ~a...~%") (uri->string uri)))
-
-    (let* ((raw download-size
-                ;; 'guix publish' without '--cache' doesn't specify a
-                ;; Content-Length, so DOWNLOAD-SIZE is #f in this case.
-                (fetch uri))
+  (define (try-fetch choices)
+    (match choices
+      (((uri compression file-size) rest ...)
+       (guard (c ((and (pair? rest) (http-get-error? c))
+                  (warning (G_ "download from '~a' failed, trying next URL~%")
+                           (uri->string uri))
+                  (try-fetch rest)))
+         (let ((port download-size (fetch uri)))
+           (unless print-build-trace?
+             (format (current-error-port)
+                     (G_ "Downloading ~a...~%") (uri->string uri)))
+           (values port uri compression download-size))))
+      (()
+       (leave (G_ "no valid nar URLs for ~a at ~a~%")
+              (narinfo-path narinfo)
+              (narinfo-uri-base narinfo)))))
+
+  (let ((choices (narinfo-preferred-uris narinfo
+                                         #:fast-decompression?
+                                         %prefer-fast-decompression?)))
+    ;; 'guix publish' without '--cache' doesn't specify a Content-Length, so
+    ;; DOWNLOAD-SIZE is #f in this case.
+    (let* ((raw uri compression download-size (try-fetch choices))
            (progress
             (let* ((dl-size  (or download-size
                                  (and (equal? compression "none")