lint: formatting: Gracefully handle relative file names.

Fixes <https://bugs.gnu.org/42543>.
Reported by Jack Hill <jackhill@jackhill.us>.

* guix/lint.scm (check-formatting): Always return a list (previously we
would return #f when 'search-path' returns #f).  Check whether
LOCATION's file is a relative file name.  Return a warning if not.
* tests/guix-lint.sh: Add test.

2 files changed, 25 insertions, 8 deletions

diff --git a/guix/lint.scm b/guix/lint.scm
index 4a6abe4275..ec43a4dcad 100644
--- a/guix/lint.scm
+++ b/guix/lint.scm
@@ -1355,12 +1355,20 @@ them for PACKAGE."
   "Check the formatting of the source code of PACKAGE."
   (let ((location (package-location package)))
     (if location
-        (and=> (search-path %load-path (location-file location))
-               (lambda (file)
-                 ;; Report issues starting from the line before the 'package'
-                 ;; form, which usually contains the 'define' form.
-                 (report-formatting-issues package file
-                                           (- (location-line location) 1))))
+        ;; Report issues starting from the line before the 'package'
+        ;; form, which usually contains the 'define' form.
+        (let ((line (- (location-line location) 1)))
+          (match (search-path %load-path (location-file location))
+            ((? string? file)
+             (report-formatting-issues package file line))
+            (#f
+             ;; It could be that LOCATION lists a "true" relative file
+             ;; name--i.e., not relative to an element of %LOAD-PATH.
+             (let ((file (location-file location)))
+               (if (file-exists? file)
+                   (report-formatting-issues package file line)
+                   (list (make-warning package
+                                       (G_ "source file not found"))))))))
         '())))
 
 
diff --git a/tests/guix-lint.sh b/tests/guix-lint.sh
index f0df1fda3a..ebe79efb84 100644
--- a/tests/guix-lint.sh
+++ b/tests/guix-lint.sh
@@ -22,8 +22,11 @@
 
 guix lint --version
 
-module_dir="t-guix-lint-$$"
-mkdir "$module_dir"
+# Choose a module directory not below any %LOAD-PATH component.  This is
+# necessary when testing '-L' with a relative file name.
+module_dir="$(mktemp -d)"
+
+mkdir -p "$module_dir"
 trap "rm -rf $module_dir" EXIT
 
 
@@ -87,3 +90,9 @@ then false; else true; fi
 
 # Make sure specifying multiple packages works.
 guix lint -L $module_dir -c inputs-should-be-native dummy dummy@42 dummy
+
+# Test '-L' with a relative file name.  'guix lint' will see "t-xyz/foo.scm"
+# (instead of "foo.scm") and will thus fail to find it in %LOAD-PATH.  Check
+# that it does find it anyway.  See <https://bugs.gnu.org/42543>.
+(cd "$module_dir"/.. ; guix lint -c formatting -L "$(basename "$module_dir")" dummy@42) 2>&1 > "$module_dir/out"
+test -z "$(cat "$module_dir/out")"