ui: 'load*' accepts /dev/fd/N files pointing to a pipe.

This allows users to write Bash commands like:

  guix time-machine -C <(echo %default-channels) -- ...

or:

  guix build -m <(echo '(specifications->manifest (list "guile"))')

Previously, on GNU/Linux, they would fail with:

  error: failed to load '/dev/fd/63': No such file or directory

* guix/ui.scm (try-canonicalize-path): New procedure.
(load*): Use it.
* tests/guix-build.sh: Test 'guix build -m' with a /dev/fd/N file.

2 files changed, 23 insertions, 2 deletions

diff --git a/guix/ui.scm b/guix/ui.scm
index 47a118364a..6f2d4fe245 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -200,6 +200,20 @@ information, or #f if it could not be found."
   (parameterize (((@ (system base compile) default-optimization-level) 1))
     exp))
 
+(define (try-canonicalize-path file)
+  "Like 'canonicalize-path', but return FILE as-is if 'canonicalize-path'
+throws.
+
+This is necessary for corner cases where 'canonicalize-path' fails.  One
+example is on Linux when a /dev/fd/N file denotes a pipe, represented as a
+symlink to a non-existent file like 'pipe:[1234]', as in this example:
+
+  sh -c 'stat $(readlink -f /dev/fd/1)' | cat"
+  (catch 'system-error
+    (lambda ()
+      (canonicalize-path file))
+    (const file)))
+
 (define* (load* file user-module
                 #:key (on-error 'nothing-special))
   "Load the user provided Scheme source code FILE."
@@ -230,7 +244,7 @@ information, or #f if it could not be found."
                ;; 'primitive-load', so that FILE is compiled, which then allows
                ;; us to provide better error reporting with source line numbers.
                (without-compiler-optimizations
-                (load (canonicalize-path file))))
+                (load (try-canonicalize-path file))))
              (const #f))))))
     (lambda _
       ;; XXX: Errors are reported from the pre-unwind handler below, but
diff --git a/tests/guix-build.sh b/tests/guix-build.sh
index 317c58ac42..4eab0e38b6 100644
--- a/tests/guix-build.sh
+++ b/tests/guix-build.sh
@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012-2014, 2016-2022 Ludovic Courtès <[email protected]>
+# Copyright © 2012-2014, 2016-2023 Ludovic Courtès <[email protected]>
 # Copyright © 2020 Marius Bakke <[email protected]>
 # Copyright © 2021 Chris Marusich <[email protected]>
 #
@@ -397,6 +397,13 @@ guix build -d -m "$module_dir/manifest.scm" \
 
 rm "$module_dir"/*.scm
 
+if [ -n "$BASH_VERSION" ]
+then
+    # Check whether we can load from a /dev/fd/N denoting a pipe, using this
+    # handy Bash-specific construct.
+    guix build -m <(echo '(specifications->manifest (list "guile"))') -n
+fi
+
 # Using 'GUIX_BUILD_OPTIONS'.
 GUIX_BUILD_OPTIONS="--dry-run --no-grafts"
 export GUIX_BUILD_OPTIONS