alternates: accept double-quoted paths

We read lists of alternates from objects/info/alternates
files (delimited by newline), as well as from the
GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable
(delimited by colon or semi-colon, depending on the
platform).

There's no mechanism for quoting the delimiters, so it's
impossible to specify an alternate path that contains a
colon in the environment, or one that contains a newline in
a file. We've lived with that restriction for ages because
both alternates and filenames with colons are relatively
rare, and it's only a problem when the two meet. But since
722ff7f87 (receive-pack: quarantine objects until
pre-receive accepts, 2016-10-03), which builds on the
alternates system, every push causes the receiver to set
GIT_ALTERNATE_OBJECT_DIRECTORIES internally.

It would be convenient to have some way to quote the
delimiter so that we can represent arbitrary paths.

The simplest thing would be an escape character before a
quoted delimiter (e.g., "\:" as a literal colon). But that
creates a backwards compatibility problem: any path which
uses that escape character is now broken, and we've just
shifted the problem. We could choose an unlikely escape
character (e.g., something from the non-printable ASCII
range), but that's awkward to use.

Instead, let's treat names as unquoted unless they begin
with a double-quote, in which case they are interpreted via
our usual C-stylke quoting rules. This also breaks
backwards-compatibility, but in a smaller way: it only
matters if your file has a double-quote as the very _first_
character in the path (whereas an escape character is a
problem anywhere in the path).  It's also consistent with
many other parts of git, which accept either a bare pathname
or a double-quoted one, and the sender can choose to quote
or not as required.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/t/t5615-alternate-env.sh b/t/t5615-alternate-env.sh
index 22d9d8178b..c33d089980 100755
--- a/t/t5615-alternate-env.sh
+++ b/t/t5615-alternate-env.sh
@@ -68,4 +68,22 @@ test_expect_success 'access alternate via relative path (subdir)' '
 	EOF
 '
 
+# set variables outside test to avoid quote insanity; the \057 is '/',
+# which doesn't need quoting, but just confirms that de-quoting
+# is working.
+quoted='"one.git\057objects"'
+unquoted='two.git/objects'
+test_expect_success 'mix of quoted and unquoted alternates' '
+	check_obj "$quoted:$unquoted" <<-EOF
+	$one blob
+	$two blob
+'
+
+test_expect_success 'broken quoting falls back to interpreting raw' '
+	mv one.git \"one.git &&
+	check_obj \"one.git/objects <<-EOF
+	$one blob
+	EOF
+'
+
 test_done