gexp: 'local-file' resolves relative file names.

* guix/gexp.scm (<local-file>): Rename constructor to '%%local-file'.
Add 'absolute' field.
(%local-file, extract-directory, absolute-file-name): New procedures.
(current-source-directory): New macro.
(local-file): Adjust call to '%local-file'.
(local-file-absolute-file-name): New procedure.
(local-file-compiler): Force the 'absolute' field.
* tests/guix-system.sh: Test whether 'local-file' canonicalization
works.
* doc/guix.texi (G-Expressions): Adjust.

doc/guix.texi

@@ -3489,8 +3489,9 @@ content is directly passed as a string.
 @deffn {Scheme Procedure} local-file @var{file} [@var{name}] @
    [#:recursive? #t]
 Return an object representing local file @var{file} to add to the store; this
-object can be used in a gexp.  @var{file} will be added to the store under @var{name}--by
-default the base name of @var{file}.
+object can be used in a gexp.  If @var{file} is a relative file name, it is looked
+up relative to the source file where this form appears.  @var{file} will be added to
+the store under @var{name}--by default the base name of @var{file}.
 
 When @var{recursive?} is true, the contents of @var{file} are added recursively; if @var{file}
 designates a flat file and @var{recursive?} is true, its contents are added, and its

guix/gexp.scm

@@ -35,6 +35,7 @@
             local-file
             local-file?
             local-file-file
+            local-file-absolute-file-name
             local-file-name
             local-file-recursive?
 
@@ -182,35 +183,76 @@ cross-compiling.)"
 ;;; File declarations.
 ;;;
 
+;; A local file name.  FILE is the file name the user entered, which can be a
+;; relative file name, and ABSOLUTE is a promise that computes its canonical
+;; absolute file name.  We keep it in a promise to compute it lazily and avoid
+;; repeated 'stat' calls.
 (define-record-type <local-file>
-  (%local-file file name recursive?)
+  (%%local-file file absolute name recursive?)
   local-file?
   (file       local-file-file)                    ;string
+  (absolute   %local-file-absolute-file-name)     ;promise string
   (name       local-file-name)                    ;string
   (recursive? local-file-recursive?))             ;Boolean
 
-(define* (local-file file #:optional (name (basename file))
+(define* (%local-file file promise #:optional (name (basename file))
                       #:key recursive?)
+  ;; This intermediate procedure is part of our ABI, but the underlying
+  ;; %%LOCAL-FILE is not.
+  (%%local-file file promise name recursive?))
+
+(define (extract-directory properties)
+  "Extract the directory name from source location PROPERTIES."
+  (match (assq 'filename properties)
+    (('filename . (? string? file-name))
+     (dirname file-name))
+    (_
+     #f)))
+
+(define-syntax-rule (current-source-directory)
+  "Expand to the directory of the current source file or #f if it could not
+be determined."
+  (extract-directory (current-source-location)))
+
+(define (absolute-file-name file directory)
+  "Return the canonical absolute file name for FILE, which lives in the
+vicinity of DIRECTORY."
+  (canonicalize-path
+   (cond ((string-prefix? "/" file) file)
+         ((not directory) file)
+         ((string-prefix? "/" directory)
+          (string-append directory "/" file))
+         (else file))))
+
+(define-syntax-rule (local-file file rest ...)
   "Return an object representing local file FILE to add to the store; this
-object can be used in a gexp.  FILE will be added to the store under NAME--by
-default the base name of FILE.
+object can be used in a gexp.  If FILE is a relative file name, it is looked
+up relative to the source file where this form appears.  FILE will be added to
+the store under NAME--by default the base name of FILE.
 
 When RECURSIVE? is true, the contents of FILE are added recursively; if FILE
 designates a flat file and RECURSIVE? is true, its contents are added, and its
 permission bits are kept.
 
 This is the declarative counterpart of the 'interned-file' monadic procedure."
-  ;; Canonicalize FILE so that if it's a symlink, it is resolved.  Failing to
-  ;; do that, when RECURSIVE? is #t, we could end up creating a dangling
-  ;; symlink in the store, and when RECURSIVE? is #f 'add-to-store' would just
-  ;; throw an error, both of which are inconvenient.
-  (%local-file (canonicalize-path file) name recursive?))
+  (%local-file file
+               (delay (absolute-file-name file (current-source-directory)))
+               rest ...))
+
+(define (local-file-absolute-file-name file)
+  "Return the absolute file name for FILE, a <local-file> instance.  A
+'system-error' exception is raised if FILE could not be found."
+  (force (%local-file-absolute-file-name file)))
 
 (define-gexp-compiler (local-file-compiler (file local-file?) system target)
   ;; "Compile" FILE by adding it to the store.
   (match file
-    (($ <local-file> file name recursive?)
-     (interned-file file name #:recursive? recursive?))))
+    (($ <local-file> file (= force absolute) name recursive?)
+     ;; Canonicalize FILE so that if it's a symlink, it is resolved.  Failing
+     ;; to do that, when RECURSIVE? is #t, we could end up creating a dangling
+     ;; symlink in the store, and when RECURSIVE? is #f 'add-to-store' would
+     ;; just throw an error, both of which are inconvenient.
+     (interned-file absolute name #:recursive? recursive?))))
 
 (define-record-type <plain-file>
   (%plain-file name content references)

tests/guix-system.sh

@@ -17,7 +17,7 @@
 # along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 #
-# Test the daemon and its interaction with 'guix substitute'.
+# Test 'guix system', mostly error reporting.
 #
 
 set -e
@@ -26,7 +26,15 @@ guix system --version
 
 tmpfile="t-guix-system-$$"
 errorfile="t-guix-system-error-$$"
-trap 'rm -f "$tmpfile" "$errorfile"' EXIT
+
+# Note: This directory is chosen outside $builddir so that relative file name
+# canonicalization doesn't mess up with 'current-source-directory', used by
+# 'local-file' ('load' forces 'relative' for
+# %FILE-PORT-NAME-CANONICALIZATION.)
+tmpdir="${TMPDIR:-/tmp}/t-guix-system-$$"
+mkdir "$tmpdir"
+
+trap 'rm -f "$tmpfile" "$errorfile" "$tmpdir"/*; rmdir "$tmpdir"' EXIT
 
 # Reporting of syntax errors.
 
@@ -180,3 +188,23 @@ make_user_config "users" "group-that-does-not-exist"
 if guix system build "$tmpfile" -n 2> "$errorfile"
 then false
 else grep "supplementary group.*group-that-does-not-exist.*undeclared" "$errorfile"; fi
+
+# Try 'local-file' and relative file name resolution.
+
+cat > "$tmpdir/config.scm"<<EOF
+(use-modules (gnu))
+(use-service-modules networking)
+
+(operating-system
+  $OS_BASE
+  (services (cons (tor-service (local-file "my-torrc"))
+                  %base-services)))
+EOF
+
+cat > "$tmpdir/my-torrc"<<EOF
+# This is an example file.
+EOF
+
+# In both cases 'my-torrc' should be properly resolved.
+guix system build "$tmpdir/config.scm" -n
+(cd "$tmpdir"; guix system build "config.scm" -n)