git-remote-testgit: fix race when spawning fast-import

Test "pushing to local repo" in t5800-remote-helpers can hang
due to a race condition in git-remote-testgit.  Fix it by
setting stdin to unbuffered.

On the writer side, "git push" invokes push_refs_with_export(),
which sends to stdout the command "export\n" and immediately
starts up "git fast-export".  The latter writes its output stream
to the same stdout.

On the reader side, remote helper "git-remote-testgit" reads from
stdin to get its next command.  It uses getc() to read characters
from libc up until \n.  Libc has buffered a potentially much
larger chunk of stdin.  When it sees the "export\n" command, it
forks "git fast-import" to read the stream.

If fast-export finishes before git fast-import starts, the
fast-export output can end up in libc's buffer in
git-remote-testgit, rather than in git fast-import.  The latter
hangs indefinitely on a now-empty stdin.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5800-remote-helpers.sh

@@ -72,6 +72,19 @@ test_expect_success 'pushing to local repo' '
 	compare_refs localclone HEAD server HEAD
 '
 
+# Generally, skip this test.  It demonstrates a now-fixed race in
+# git-remote-testgit, but is too slow to leave in for general use.
+: test_expect_success 'racily pushing to local repo' '
+	test_when_finished "rm -rf server2 localclone2" &&
+	cp -a server server2 &&
+	git clone "testgit::${PWD}/server2" localclone2 &&
+	(cd localclone2 &&
+	echo content >>file &&
+	git commit -a -m three &&
+	GIT_REMOTE_TESTGIT_SLEEPY=2 git push) &&
+	compare_refs localclone2 HEAD server2 HEAD
+'
+
 test_expect_success 'synch with changes from localclone' '
 	(cd clone &&
 	 git pull)