tmp-objdir: quote paths we add to alternates

Commit 722ff7f (receive-pack: quarantine objects until
pre-receive accepts, 2016-10-03) regressed pushes to
repositories with colon (or semi-colon in Windows in them)
because it adds the repository's main object directory to
GIT_ALTERNATE_OBJECT_DIRECTORIES. The receiver interprets
the colon as a delimiter, not as part of the path, and
index-pack is unable to find objects which it needs to
resolve deltas.

The previous commit introduced a quoting mechanism for the
alternates list; let's use it here to cover this case. We'll
avoid quoting when we can, though. This alternate setup is
also used when calling hooks, so it's possible that the user
may call older git implementations which don't understand
the quoting mechanism. By quoting only when necessary, this
setup will continue to work unless the user _also_ has a
repository whose path contains the delimiter.

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

t/t5547-push-quarantine.sh

@@ -33,4 +33,23 @@ test_expect_success 'rejected objects are removed' '
 	test_cmp expect actual
 '
 
+# MINGW does not allow colons in pathnames in the first place
+test_expect_success !MINGW 'push to repo path with colon' '
+	# The interesting failure case here is when the
+	# receiving end cannot access its original object directory,
+	# so make it likely for us to generate a delta by having
+	# a non-trivial file with multiple versions.
+
+	test-genrandom foo 4096 >file.bin &&
+	git add file.bin &&
+	git commit -m bin &&
+	git clone --bare . xxx:yyy.git &&
+
+	echo change >>file.bin &&
+	git commit -am change &&
+	# Note that we have to use the full path here, or it gets confused
+	# with the ssh host:path syntax.
+	git push "$PWD/xxx:yyy.git" HEAD
+'
+
 test_done

tmp-objdir.c

@@ -5,6 +5,7 @@
 #include "string-list.h"
 #include "strbuf.h"
 #include "argv-array.h"
+#include "quote.h"
 
 struct tmp_objdir {
 	struct strbuf path;
@@ -79,12 +80,27 @@ static void remove_tmp_objdir_on_signal(int signo)
  */
 static void env_append(struct argv_array *env, const char *key, const char *val)
 {
-	const char *old = getenv(key);
+	struct strbuf quoted = STRBUF_INIT;
+	const char *old;
 
+	/*
+	 * Avoid quoting if it's not necessary, for maximum compatibility
+	 * with older parsers which don't understand the quoting.
+	 */
+	if (*val == '"' || strchr(val, PATH_SEP)) {
+		strbuf_addch(&quoted, '"');
+		quote_c_style(val, &quoted, NULL, 1);
+		strbuf_addch(&quoted, '"');
+		val = quoted.buf;
+	}
+
+	old = getenv(key);
 	if (!old)
 		argv_array_pushf(env, "%s=%s", key, val);
 	else
 		argv_array_pushf(env, "%s=%s%c%s", key, old, PATH_SEP, val);
+
+	strbuf_release(&quoted);
 }
 
 static void env_replace(struct argv_array *env, const char *key, const char *val)