credential: let helpers tell us to quit

When we are trying to fill a credential, we loop over the
set of defined credential-helpers, then fall back to running
askpass, and then finally prompt on the terminal. Helpers
which cannot find a credential are free to tell us nothing,
but they cannot currently ask us to stop prompting.

This patch lets them provide a "quit" attribute, which asks
us to stop the process entirely (avoiding running more
helpers, as well as the askpass/terminal prompt).

This has a few possible uses:

  1. A helper which prompts the user itself (e.g., in a
     dialog) can provide a "cancel" button to the user to
     stop further prompts.

  2. Some helpers may know that prompting cannot possibly
     work. For example, if their role is to broker a ticket
     from an external auth system and that auth system
     cannot be contacted, there is no point in continuing
     (we need a ticket to authenticate, and the user cannot
     provide one by typing it in).

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

1 files changed, 9 insertions, 0 deletions

diff --git a/t/t0300-credentials.sh b/t/t0300-credentials.sh
index 57ea5a10c5..d7ef44b4a2 100755
--- a/t/t0300-credentials.sh
+++ b/t/t0300-credentials.sh
@@ -289,4 +289,13 @@ test_expect_success 'http paths can be part of context' '
 	EOF
 '
 
+test_expect_success 'helpers can abort the process' '
+	test_must_fail git \
+		-c credential.helper="!f() { echo quit=1; }; f" \
+		-c credential.helper="verbatim foo bar" \
+		credential fill >stdout &&
+	>expect &&
+	test_cmp expect stdout
+'
+
 test_done