credential: new attribute password_expiry_utc

Some passwords have an expiry date known at generation. This may be
years away for a personal access token or hours for an OAuth access
token.

When multiple credential helpers are configured, `credential fill` tries
each helper in turn until it has a username and password, returning
early. If Git authentication succeeds, `credential approve`
stores the successful credential in all helpers. If authentication
fails, `credential reject` erases matching credentials in all helpers.
Helpers implement corresponding operations: get, store, erase.

The credential protocol has no expiry attribute, so helpers cannot
store expiry information. Even if a helper returned an improvised
expiry attribute, git credential discards unrecognised attributes
between operations and between helpers.

This is a particular issue when a storage helper and a
credential-generating helper are configured together:

	[credential]
		helper = storage  # eg. cache or osxkeychain
		helper = generate  # eg. oauth

`credential approve` stores the generated credential in both helpers
without expiry information. Later `credential fill` may return an
expired credential from storage. There is no workaround, no matter how
clever the second helper. The user sees authentication fail (a retry
will succeed).

Introduce a password expiry attribute. In `credential fill`, ignore
expired passwords and continue to query subsequent helpers.

In the example above, `credential fill` ignores the expired password
and a fresh credential is generated. If authentication succeeds,
`credential approve` replaces the expired password in storage.
If authentication fails, the expired credential is erased by
`credential reject`. It is unnecessary but harmless for storage
helpers to self prune expired credentials.

Add support for the new attribute to credential-cache.
Eventually, I hope to see support in other popular storage helpers.

Example usage in a credential-generating helper
https://github.com/hickford/git-credential-oauth/pull/16

Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Reviewed-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential.txt

@@ -144,6 +144,12 @@ Git understands the following attributes:
 
 	The credential's password, if we are asking it to be stored.
 
+`password_expiry_utc`::
+
+	Generated passwords such as an OAuth access token may have an expiry date.
+	When reading credentials from helpers, `git credential fill` ignores expired
+	passwords. Represented as Unix time UTC, seconds since 1970.
+
 `url`::
 
 	When this special attribute is read by `git credential`, the

Documentation/gitcredentials.txt

@@ -167,7 +167,7 @@ helper::
 If there are multiple instances of the `credential.helper` configuration
 variable, each helper will be tried in turn, and may provide a username,
 password, or nothing. Once Git has acquired both a username and a
-password, no more helpers will be tried.
+non-expired password, no more helpers will be tried.
 +
 If `credential.helper` is configured to the empty string, this resets
 the helper list to empty (so you may override a helper set by a

builtin/credential-cache--daemon.c

@@ -127,6 +127,9 @@ static void serve_one_client(FILE *in, FILE *out)
 		if (e) {
 			fprintf(out, "username=%s\n", e->item.username);
 			fprintf(out, "password=%s\n", e->item.password);
+			if (e->item.password_expiry_utc != TIME_MAX)
+				fprintf(out, "password_expiry_utc=%"PRItime"\n",
+					e->item.password_expiry_utc);
 		}
 	}
 	else if (!strcmp(action.buf, "exit")) {

credential.c

@@ -7,6 +7,7 @@
 #include "prompt.h"
 #include "sigchain.h"
 #include "urlmatch.h"
+#include "git-compat-util.h"
 
 void credential_init(struct credential *c)
 {
@@ -234,6 +235,11 @@ int credential_read(struct credential *c, FILE *fp)
 		} else if (!strcmp(key, "path")) {
 			free(c->path);
 			c->path = xstrdup(value);
+		} else if (!strcmp(key, "password_expiry_utc")) {
+			errno = 0;
+			c->password_expiry_utc = parse_timestamp(value, NULL, 10);
+			if (c->password_expiry_utc == 0 || errno == ERANGE)
+				c->password_expiry_utc = TIME_MAX;
 		} else if (!strcmp(key, "url")) {
 			credential_from_url(c, value);
 		} else if (!strcmp(key, "quit")) {
@@ -269,6 +275,11 @@ void credential_write(const struct credential *c, FILE *fp)
 	credential_write_item(fp, "path", c->path, 0);
 	credential_write_item(fp, "username", c->username, 0);
 	credential_write_item(fp, "password", c->password, 0);
+	if (c->password_expiry_utc != TIME_MAX) {
+		char *s = xstrfmt("%"PRItime, c->password_expiry_utc);
+		credential_write_item(fp, "password_expiry_utc", s, 0);
+		free(s);
+	}
 }
 
 static int run_credential_helper(struct credential *c,
@@ -342,6 +353,12 @@ void credential_fill(struct credential *c)
 
 	for (i = 0; i < c->helpers.nr; i++) {
 		credential_do(c, c->helpers.items[i].string, "get");
+		if (c->password_expiry_utc < time(NULL)) {
+			/* Discard expired password */
+			FREE_AND_NULL(c->password);
+			/* Reset expiry to maintain consistency */
+			c->password_expiry_utc = TIME_MAX;
+		}
 		if (c->username && c->password)
 			return;
 		if (c->quit)
@@ -360,7 +377,7 @@ void credential_approve(struct credential *c)
 
 	if (c->approved)
 		return;
-	if (!c->username || !c->password)
+	if (!c->username || !c->password || c->password_expiry_utc < time(NULL))
 		return;
 
 	credential_apply_config(c);
@@ -381,6 +398,7 @@ void credential_reject(struct credential *c)
 
 	FREE_AND_NULL(c->username);
 	FREE_AND_NULL(c->password);
+	c->password_expiry_utc = TIME_MAX;
 	c->approved = 0;
 }
 

credential.h

@@ -126,10 +126,12 @@ struct credential {
 	char *protocol;
 	char *host;
 	char *path;
+	timestamp_t password_expiry_utc;
 };
 
 #define CREDENTIAL_INIT { \
 	.helpers = STRING_LIST_INIT_DUP, \
+	.password_expiry_utc = TIME_MAX, \
 }
 
 /* Initialize a credential structure, setting all fields to empty. */

t/t0300-credentials.sh

@@ -35,6 +35,16 @@ test_expect_success 'setup helper scripts' '
 	test -z "$pass" || echo password=$pass
 	EOF
 
+	write_script git-credential-verbatim-with-expiry <<-\EOF &&
+	user=$1; shift
+	pass=$1; shift
+	pexpiry=$1; shift
+	. ./dump
+	test -z "$user" || echo username=$user
+	test -z "$pass" || echo password=$pass
+	test -z "$pexpiry" || echo password_expiry_utc=$pexpiry
+	EOF
+
 	PATH="$PWD:$PATH"
 '
 
@@ -109,6 +119,43 @@ test_expect_success 'credential_fill continues through partial response' '
 	EOF
 '
 
+test_expect_success 'credential_fill populates password_expiry_utc' '
+	check fill "verbatim-with-expiry one two 9999999999" <<-\EOF
+	protocol=http
+	host=example.com
+	--
+	protocol=http
+	host=example.com
+	username=one
+	password=two
+	password_expiry_utc=9999999999
+	--
+	verbatim-with-expiry: get
+	verbatim-with-expiry: protocol=http
+	verbatim-with-expiry: host=example.com
+	EOF
+'
+
+test_expect_success 'credential_fill ignores expired password' '
+	check fill "verbatim-with-expiry one two 5" "verbatim three four" <<-\EOF
+	protocol=http
+	host=example.com
+	--
+	protocol=http
+	host=example.com
+	username=three
+	password=four
+	--
+	verbatim-with-expiry: get
+	verbatim-with-expiry: protocol=http
+	verbatim-with-expiry: host=example.com
+	verbatim: get
+	verbatim: protocol=http
+	verbatim: host=example.com
+	verbatim: username=one
+	EOF
+'
+
 test_expect_success 'credential_fill passes along metadata' '
 	check fill "verbatim one two" <<-\EOF
 	protocol=ftp
@@ -149,6 +196,24 @@ test_expect_success 'credential_approve calls all helpers' '
 	EOF
 '
 
+test_expect_success 'credential_approve stores password expiry' '
+	check approve useless <<-\EOF
+	protocol=http
+	host=example.com
+	username=foo
+	password=bar
+	password_expiry_utc=9999999999
+	--
+	--
+	useless: store
+	useless: protocol=http
+	useless: host=example.com
+	useless: username=foo
+	useless: password=bar
+	useless: password_expiry_utc=9999999999
+	EOF
+'
+
 test_expect_success 'do not bother storing password-less credential' '
 	check approve useless <<-\EOF
 	protocol=http
@@ -159,6 +224,17 @@ test_expect_success 'do not bother storing password-less credential' '
 	EOF
 '
 
+test_expect_success 'credential_approve does not store expired password' '
+	check approve useless <<-\EOF
+	protocol=http
+	host=example.com
+	username=foo
+	password=bar
+	password_expiry_utc=5
+	--
+	--
+	EOF
+'
 
 test_expect_success 'credential_reject calls all helpers' '
 	check reject useless "verbatim one two" <<-\EOF
@@ -181,6 +257,24 @@ test_expect_success 'credential_reject calls all helpers' '
 	EOF
 '
 
+test_expect_success 'credential_reject erases credential regardless of expiry' '
+	check reject useless <<-\EOF
+	protocol=http
+	host=example.com
+	username=foo
+	password=bar
+	password_expiry_utc=5
+	--
+	--
+	useless: erase
+	useless: protocol=http
+	useless: host=example.com
+	useless: username=foo
+	useless: password=bar
+	useless: password_expiry_utc=5
+	EOF
+'
+
 test_expect_success 'usernames can be preserved' '
 	check fill "verbatim \"\" three" <<-\EOF
 	protocol=http