profiles: Gracefully deal with packages containing an etc/ symlink.

This fixes a bug whereby 'guix package -i gcc-toolchain' would fail in
'build-profile'.  This is because in 'gcc-toolchain', etc/ is a symlink,
and so the 'scandir' call in 'unsymlink' would return #f instead of
returning a list.

Reported by Andreas Enge <andreas.enge@inria.fr>.

* guix/build/profiles.scm (ensure-writable-directory)[unsymlink]: Append
  "/" to TARGET before calling 'scandir'.
* tests/profiles.scm ("etc/profile when etc/ is a symlink"): New test.

guix/build/profiles.scm

@@ -94,7 +94,9 @@ (define (ensure-writable-directory directory)
 instead make DIRECTORY a \"real\" directory containing symlinks."
   (define (unsymlink link)
     (let* ((target (readlink link))
-           (files  (scandir target
+           ;; TARGET might itself be a symlink, so append "/" to make sure
+           ;; 'scandir' enters it.
+           (files  (scandir (string-append target "/")
                             (negate (cut member <> '("." ".."))))))
       (delete-file link)
       (mkdir link)

tests/profiles.scm

@@ -277,6 +277,34 @@ (define glibc
                                get-string-all)
                              "foo!"))))))
 
+(test-assertm "etc/profile when etc/ is a symlink"
+  ;; When etc/ is a symlink, the unsymlink code in 0.8.2 would fail
+  ;; gracelessly because 'scandir' would return #f.
+  (mlet* %store-monad
+      ((thing ->   (dummy-package "dummy"
+                     (build-system trivial-build-system)
+                     (arguments
+                      `(#:guile ,%bootstrap-guile
+                        #:builder
+                        (let ((out (assoc-ref %outputs "out")))
+                          (mkdir out)
+                          (mkdir (string-append out "/foo"))
+                          (symlink "foo" (string-append out "/etc"))
+                          (call-with-output-file (string-append out "/etc/bar")
+                            (lambda (port)
+                              (display "foo!" port))))))))
+       (entry ->   (package->manifest-entry thing))
+       (drv        (profile-derivation (manifest (list entry))
+                                       #:hooks '()))
+       (profile -> (derivation->output-path drv)))
+    (mbegin %store-monad
+      (built-derivations (list drv))
+      (return (and (file-exists? (string-append profile "/etc/profile"))
+                   (string=? (call-with-input-file
+                                 (string-append profile "/etc/bar")
+                               get-string-all)
+                             "foo!"))))))
+
 (test-end "profiles")