pack: "fakechroot" engine always creates its store.

Previously it would silently fail to create the /gnu/store symlink when
the host has a read-only /gnu as is the case in these tests.

* gnu/packages/aux-files/run-in-namespace.c (exec_with_loader): Unlink
the ancestor of ORIGINAL_STORE under NEW_ROOT.  Check the return value
of 'symlink' when creating NEW_STORE.
* tests/guix-pack-relocatable.sh: Check the contents of the store as
seen by the wrapped executable, with all three engines, and with both
"/gnu" and "/gnu/store" erased.

1 files changed, 14 insertions, 1 deletions

diff --git a/gnu/packages/aux-files/run-in-namespace.c b/gnu/packages/aux-files/run-in-namespace.c
index 7f7e5c6885..436c0b6286 100644
--- a/gnu/packages/aux-files/run-in-namespace.c
+++ b/gnu/packages/aux-files/run-in-namespace.c
@@ -441,10 +441,23 @@ exec_with_loader (const char *store, int argc, char *argv[])
   char *new_root = mkdtemp (strdup ("/tmp/guix-exec-XXXXXX"));
   mirror_directory ("/", new_root, make_symlink);
 
+  /* 'mirror_directory' created a symlink for the ancestor of ORIGINAL_STORE,
+     typically "/gnu".  Remove that entry so we can create NEW_STORE
+     below.  */
+  const char *slash = strchr (original_store + 1, '/');
+  const char *top = slash != NULL
+    ? strndupa (original_store, slash - original_store)
+    : original_store;
+  char *new_store_top = concat (new_root, top);
+  unlink (new_store_top);
+
+  /* Now create the store under NEW_ROOT.  */
   char *new_store = concat (new_root, original_store);
   char *new_store_parent = dirname (strdup (new_store));
   mkdir_p (new_store_parent);
-  symlink (store, new_store);
+  err = symlink (store, new_store);
+  if (err < 0)
+    assert_perror (errno);
 
 #ifdef GCONV_DIRECTORY
   /* Tell libc where to find its gconv modules.  This is necessary because