add ability to script IMAP user query

It was already possible to retrieve passwords from arbitrary commands.
But this goes only half the way to allowing automated derivation of
login credentials, as some environments may also have different user
names based on the system. Therefore, add the UserCmd option to
complement PassCmd.

Based on a patch series by Patrick Steinhardt <ps@pks.im>

NEWS

@@ -11,6 +11,8 @@ Support for configuring a TLS cipher string was added.
 
 IMAP mailbox subscriptions are supported now.
 
+The IMAP user query can be scripted now.
+
 [1.3.0]
 
 Network timeout handling has been added.

src/drv_imap.c

@@ -50,6 +50,7 @@ typedef struct imap_server_conf {
 	char *name;
 	server_conf_t sconf;
 	char *user;
+	char *user_cmd;
 	char *pass;
 	char *pass_cmd;
 	int max_in_progress;
@@ -1936,22 +1937,9 @@ imap_open_store_authenticate_p3( imap_store_t *ctx, imap_cmd_t *cmd ATTR_UNUSED,
 }
 #endif
 
-static const char *
-ensure_user( imap_server_conf_t *srvc )
+static char *
+cred_from_cmd( const char *cred, const char *cmd, const char *srv_name )
 {
-	if (!srvc->user) {
-		error( "Skipping account %s, no user\n", srvc->name );
-		return NULL;
-	}
-	return srvc->user;
-}
-
-static const char *
-ensure_password( imap_server_conf_t *srvc )
-{
-	char *cmd = srvc->pass_cmd;
-
-	if (cmd) {
 	FILE *fp;
 	int ret;
 	char buffer[8192];  // Hopefully more than enough room for XOAUTH2, etc. tokens
@@ -1962,7 +1950,7 @@ ensure_password( imap_server_conf_t *srvc )
 	}
 	if (!(fp = popen( cmd, "r" ))) {
 	  pipeerr:
-			sys_error( "Skipping account %s, password command failed", srvc->name );
+		sys_error( "Skipping account %s, %s failed", srv_name, cred );
 		return NULL;
 	}
 	if (!fgets( buffer, sizeof(buffer), fp ))
@@ -1971,24 +1959,43 @@ ensure_password( imap_server_conf_t *srvc )
 		goto pipeerr;
 	if (ret) {
 		if (WIFSIGNALED( ret ))
-				error( "Skipping account %s, password command crashed\n", srvc->name );
+			error( "Skipping account %s, %s crashed\n", srv_name, cred );
 		else
-				error( "Skipping account %s, password command exited with status %d\n", srvc->name, WEXITSTATUS( ret ) );
+			error( "Skipping account %s, %s exited with status %d\n", srv_name, cred, WEXITSTATUS( ret ) );
 		return NULL;
 	}
 	if (!buffer[0]) {
-			error( "Skipping account %s, password command produced no output\n", srvc->name );
+		error( "Skipping account %s, %s produced no output\n", srv_name, cred );
 		return NULL;
 	}
 	buffer[strcspn( buffer, "\n" )] = 0; /* Strip trailing newline */
-		free( srvc->pass ); /* From previous runs */
-		srvc->pass = nfstrdup( buffer );
-	} else if (!srvc->pass) {
-		char *pass, prompt[80];
+	return nfstrdup( buffer );
+}
 
+static const char *
+ensure_user( imap_server_conf_t *srvc )
+{
+	if (!srvc->user) {
+		if (srvc->user_cmd) {
+			srvc->user = cred_from_cmd( "UserCmd", srvc->user_cmd, srvc->name );
+		} else {
+			error( "Skipping account %s, no user\n", srvc->name );
+		}
+	}
+	return srvc->user;
+}
+
+static const char *
+ensure_password( imap_server_conf_t *srvc )
+{
+	if (!srvc->pass) {
+		if (srvc->pass_cmd) {
+			srvc->pass = cred_from_cmd( "PassCmd", srvc->pass_cmd, srvc->name );
+		} else {
 			flushn();
+			char prompt[80];
 			sprintf( prompt, "Password (%s): ", srvc->name );
-		pass = getpass( prompt );
+			char *pass = getpass( prompt );
 			if (!pass) {
 				perror( "getpass" );
 				exit( 1 );
@@ -2000,6 +2007,7 @@ ensure_password( imap_server_conf_t *srvc )
 			/* getpass() returns a pointer to a static buffer. Make a copy for long term storage. */
 			srvc->pass = nfstrdup( pass );
 		}
+	}
 	return srvc->pass;
 }
 
@@ -2182,6 +2190,17 @@ imap_open_store_authenticate2( imap_store_t *ctx )
 	char saslmechs[1024], *saslend = saslmechs;
 #endif
 
+	// Ensure that there are no leftovers from previous runs. This is needed in case
+	// the credentials have a timing dependency or otherwise lose validity after use.
+	if (srvc->user_cmd) {
+		free( srvc->user );
+		srvc->user = NULL;
+	}
+	if (srvc->pass_cmd) {
+		free( srvc->pass );
+		srvc->pass = NULL;
+	}
+
 	info( "Logging in...\n" );
 	for (mech = srvc->auth_mechs; mech; mech = mech->next) {
 		int any = !strcmp( mech->string, "*" );
@@ -3268,6 +3287,8 @@ imap_parse_store( conffile_t *cfg, store_conf_t **storep )
 		}
 		else if (!strcasecmp( "User", cfg->cmd ))
 			server->user = nfstrdup( cfg->val );
+		else if (!strcasecmp( "UserCmd", cfg->cmd ))
+			server->user_cmd = nfstrdup( cfg->val );
 		else if (!strcasecmp( "Pass", cfg->cmd ))
 			server->pass = nfstrdup( cfg->val );
 		else if (!strcasecmp( "PassCmd", cfg->cmd ))
@@ -3431,6 +3452,11 @@ imap_parse_store( conffile_t *cfg, store_conf_t **storep )
 			cfg->err = 1;
 			return 1;
 		}
+		if (server->user && server->user_cmd) {
+			error( "%s '%s' has both User and UserCmd\n", type, name );
+			cfg->err = 1;
+			return 1;
+		}
 		if (server->pass && server->pass_cmd) {
 			error( "%s '%s' has both Pass and PassCmd\n", type, name );
 			cfg->err = 1;

src/mbsync.1

@@ -315,6 +315,15 @@ Zero means unlimited.
 Specify the login name on the IMAP server.
 .
 .TP
+\fBUserCmd\fR [\fB+\fR]\fIcommand\fR
+Specify a shell command to obtain a user rather than specifying a
+user directly. This allows you to script retrieving user names.
+The command must produce exactly one line on stdout; the trailing newline
+is optional.
+Prepend \fB+\fR to the command to indicate that it produces TTY output
+(e.g., a prompt); failure to do so will merely produce messier output.
+.
+.TP
 \fBPass\fR \fIpassword\fR
 Specify the password for \fIusername\fR on the IMAP server.
 Note that this option is \fInot\fR required.