git-core: Support retrieving passwords with GIT_ASKPASS

git tries to read a password from the terminal in imap-send and
when talking to a http server that requires authentication.

When a GUI is driving git, however, the end user is not paying
attention to the terminal (there may not even be a terminal).
GUI would appear to hang forever.

Fix this problem by allowing a password-retrieving command
to be specified in GIT_ASKPASS

Signed-off-by: Frank Li <lznuaa@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -877,6 +877,7 @@ struct ref {
 extern struct ref *find_ref_by_name(const struct ref *list, const char *name);
 
 #define CONNECT_VERBOSE       (1u << 0)
+extern char *git_getpass(const char *prompt);
 extern struct child_process *git_connect(int fd[2], const char *url, const char *prog, int flags);
 extern int finish_connect(struct child_process *conn);
 extern int path_match(const char *path, int nr, char **match);

connect.c

@@ -647,3 +647,40 @@ int finish_connect(struct child_process *conn)
 	free(conn);
 	return code;
 }
+
+char *git_getpass(const char *prompt)
+{
+	char *askpass;
+	struct child_process pass;
+	const char *args[3];
+	static struct strbuf buffer = STRBUF_INIT;
+
+	askpass = getenv("GIT_ASKPASS");
+
+	if (!askpass || !(*askpass))
+		return getpass(prompt);
+
+	args[0] = askpass;
+	args[1]	= prompt;
+	args[2] = NULL;
+
+	memset(&pass, 0, sizeof(pass));
+	pass.argv = args;
+	pass.out = -1;
+
+	if (start_command(&pass))
+		exit(1);
+
+	strbuf_reset(&buffer);
+	if (strbuf_read(&buffer, pass.out, 20) < 0)
+		die("failed to read password from %s\n", askpass);
+
+	close(pass.out);
+
+	if (finish_command(&pass))
+		exit(1);
+
+	strbuf_setlen(&buffer, strcspn(buffer.buf, "\r\n"));
+
+	return buffer.buf;
+}

http.c

@@ -204,7 +204,7 @@ static void init_curl_http_auth(CURL *result)
 	if (user_name) {
 		struct strbuf up = STRBUF_INIT;
 		if (!user_pass)
-			user_pass = xstrdup(getpass("Password: "));
+			user_pass = xstrdup(git_getpass("Password: "));
 		strbuf_addf(&up, "%s:%s", user_name, user_pass);
 		curl_easy_setopt(result, CURLOPT_USERPWD,
 				 strbuf_detach(&up, NULL));
@@ -219,7 +219,7 @@ static int has_cert_password(void)
 		return 0;
 	/* Only prompt the user once. */
 	ssl_cert_password_required = -1;
-	ssl_cert_password = getpass("Certificate Password: ");
+	ssl_cert_password = git_getpass("Certificate Password: ");
 	if (ssl_cert_password != NULL) {
 		ssl_cert_password = xstrdup(ssl_cert_password);
 		return 1;

imap-send.c

@@ -1107,7 +1107,7 @@ static struct store *imap_open_store(struct imap_server_conf *srvc)
 		if (!srvc->pass) {
 			char prompt[80];
 			sprintf(prompt, "Password (%s@%s): ", srvc->user, srvc->host);
-			arg = getpass(prompt);
+			arg = git_getpass(prompt);
 			if (!arg) {
 				perror("getpass");
 				exit(1);