credential: warn about git-credential-store [RFC]

git-credential-store saves secrets unencrypted on disk.

Warn the user before they type their password, suggesting alternative
credential helpers.

An alternative could be to warn in "credential-store store". A
disadvantage is that the user wouldn't see the warning until after they
typed their password, which is less helpful. The warning would appear
again every time the user authenticated, which feels too frequently.

Signed-off-by: M Hickford <mirth.hickford@gmail.com>

credential.c

@@ -285,9 +285,13 @@ static int credential_getpass(struct repository *r, struct credential *c)
 	if (!c->username)
 		c->username = credential_ask_one("Username", c,
 						 PROMPT_ASKPASS|PROMPT_ECHO);
-	if (!c->password)
+	if (!c->password) {
+		if (c->helpers.nr >= 1 && starts_with(c->helpers.items[0].string, "store"))
+			warning("git-credential-store saves passwords unencrypted on disk. For alternatives, see gitcredentials(7).");
+
 		c->password = credential_ask_one("Password", c,
 						 PROMPT_ASKPASS);
+	}
 	trace2_region_leave("credential", "interactive", r);
 
 	return 0;

t/lib-credential.sh

@@ -67,6 +67,8 @@ reject() {
 helper_test() {
 	HELPER=$1
 
+	# help wanted: expect warning "git-credential-store saves passwords
+	# unencrypted" when helper equals "store"
 	test_expect_success "helper ($HELPER) has no existing data" '
 		check fill $HELPER <<-\EOF
 		protocol=https

t/t0302-credential-store.sh

@@ -133,6 +133,7 @@ invalid_credential_test() {
 		password=askpass-password
 		--
 		askpass: Username for '\''https://example.com'\'':
+		warning: git-credential-store saves passwords unencrypted on disk. For alternatives, see gitcredentials(7) or https://git-scm.com/doc/credential-helpers.
 		askpass: Password for '\''https://askpass-username@example.com'\'':
 		--
 		EOF
@@ -155,6 +156,7 @@ test_expect_success 'get: credentials with DOS line endings are invalid' '
 	password=askpass-password
 	--
 	askpass: Username for '\''https://example.com'\'':
+	warning: git-credential-store saves passwords unencrypted on disk. For alternatives, see gitcredentials(7) or https://git-scm.com/doc/credential-helpers.
 	askpass: Password for '\''https://askpass-username@example.com'\'':
 	--
 	EOF
@@ -186,6 +188,7 @@ test_expect_success 'get: credentials with DOS line endings are invalid if path
 	password=askpass-password
 	--
 	askpass: Username for '\''https://example.com/repo.git'\'':
+	warning: git-credential-store saves passwords unencrypted on disk. For alternatives, see gitcredentials(7) or https://git-scm.com/doc/credential-helpers.
 	askpass: Password for '\''https://askpass-username@example.com/repo.git'\'':
 	--
 	EOF