Provide a mechanism to turn off symlink resolution in ceiling paths

Commit 1b77d83cab 'setup_git_directory_gently_1(): resolve symlinks
in ceiling paths' changed the setup code to resolve symlinks in the
entries in GIT_CEILING_DIRECTORIES.  Because those entries are
compared textually to the symlink-resolved current directory, an
entry in GIT_CEILING_DIRECTORIES that contained a symlink would have
no effect.  It was known that this could cause performance problems
if the symlink resolution *itself* touched slow filesystems, but it
was thought that such use cases would be unlikely.  The intention of
the earlier change was to deal with a case when the user has this:

	GIT_CEILING_DIRECTORIES=/home/gitster

but in reality, /home/gitster is a symbolic link to somewhere else,
e.g. /net/machine/home4/gitster. A textual comparison between the
specified value /home/gitster and the location getcwd(3) returns
would not help us, but readlink("/home/gitster") would still be
fast.

After this change was released, Anders Kaseorg <andersk@mit.edu>
reported:

> [...] my computer has been acting so slow when I’m not connected to
> the network.  I put various network filesystem paths in
> $GIT_CEILING_DIRECTORIES, such as
> /afs/athena.mit.edu/user/a/n/andersk (to avoid hitting its parents
> /afs/athena.mit.edu, /afs/athena.mit.edu/user/a, and
> /afs/athena.mit.edu/user/a/n which all live in different AFS
> volumes).  Now when I’m not connected to the network, every
> invocation of Git, including the __git_ps1 in my shell prompt, waits
> for AFS to timeout.

To allow users to work around this problem, give them a mechanism to
turn off symlink resolution in GIT_CEILING_DIRECTORIES entries.  All
the entries that follow an empty entry will not be checked for symbolic
links and used literally in comparison.  E.g. with these:

	GIT_CEILING_DIRECTORIES=:/foo/bar:/xyzzy or
	GIT_CEILING_DIRECTORIES=/foo/bar::/xyzzy

we will not readlink("/xyzzy") because it comes after an empty entry.

With the former (but not with the latter), "/foo/bar" comes after an
empty entry, and we will not readlink it, either.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/t/t1504-ceiling-dirs.sh b/t/t1504-ceiling-dirs.sh
index cce87a5ab5..3d51615e42 100755
--- a/t/t1504-ceiling-dirs.sh
+++ b/t/t1504-ceiling-dirs.sh
@@ -44,6 +44,10 @@ test_prefix ceil_at_sub ""
 GIT_CEILING_DIRECTORIES="$TRASH_ROOT/sub/"
 test_prefix ceil_at_sub_slash ""
 
+if test_have_prereq SYMLINKS
+then
+	ln -s sub top
+fi
 
 mkdir -p sub/dir || exit 1
 cd sub/dir || exit 1
@@ -68,6 +72,19 @@ test_fail subdir_ceil_at_sub
 GIT_CEILING_DIRECTORIES="$TRASH_ROOT/sub/"
 test_fail subdir_ceil_at_sub_slash
 
+if test_have_prereq SYMLINKS
+then
+	GIT_CEILING_DIRECTORIES="$TRASH_ROOT/top"
+	test_fail subdir_ceil_at_top
+	GIT_CEILING_DIRECTORIES="$TRASH_ROOT/top/"
+	test_fail subdir_ceil_at_top_slash
+
+	GIT_CEILING_DIRECTORIES=":$TRASH_ROOT/top"
+	test_prefix subdir_ceil_at_top_no_resolve "sub/dir/"
+	GIT_CEILING_DIRECTORIES=":$TRASH_ROOT/top/"
+	test_prefix subdir_ceil_at_top_slash_no_resolve "sub/dir/"
+fi
+
 GIT_CEILING_DIRECTORIES="$TRASH_ROOT/sub/dir"
 test_prefix subdir_ceil_at_subdir "sub/dir/"