Add IncludeCmd directive to config parser

Sometimes, UserCmd & PassCmd are not enough.

While it would be possible to call mbsync with a completely generated
config file, that seems mildly inelegant due to requiring a wrapper. On
the downside, making other options' values available to the executed
command would require some kind of "expando" feature.

Complex shell commands are better delegated to scripts, so we do not add
support for multi-line commands or try to avoid the need for multiple
levels of quoting/escaping.

A more conventional name for the directive would be "Eval", but it seems
preferable to be consistent with the existing *Cmd options. An "Include"
directive might be added later.

This patch has been fixed up somewhat by the maintainer.

AUTHORS

@@ -61,6 +61,9 @@ Oliver Runge <oliver.runge@gmail.com>
 Georgy Kibardin <georgy@kibardin.name>
 - Support for UTF-7 IMAP mailbox names
 
+Michiel van den Heuvel <michielvdnheuvel@gmail.com>
+- IncludeCmd option
+
 Honorary Contributors
 =====================
 

NEWS

@@ -1,3 +1,11 @@
+1.6.0 (TBD)
+==================
+
+New Features:
+
+- Improved support for dynamic configurations; option IncludeCmd
+
+
 1.5.1 (2025-03-11)
 ==================
 

src/config.c

@@ -61,12 +61,21 @@ expand_strdup( const char *s, const conffile_t *cfile )
 	}
 }
 
+static void
+conf_print_loc( const conffile_t *cfile )
+{
+	if (cfile->eval_fp)
+		fprintf( stderr, "%s:%d:included:%d: ", cfile->file, cfile->line, cfile->eval_line );
+	else
+		fprintf( stderr, "%s:%d: ", cfile->file, cfile->line );
+}
+
 void
 conf_error( conffile_t *cfile, const char *fmt, ... )
 {
 	va_list va;
 
-	fprintf( stderr, "%s:%d: ", cfile->file, cfile->line );
+	conf_print_loc( cfile );
 	va_start( va, fmt );
 	vfprintf( stderr, fmt, va );
 	va_end( va );
@@ -79,7 +88,7 @@ conf_sys_error( conffile_t *cfile, const char *fmt, ... )
 	va_list va;
 
 	int errno_bak = errno;
-	fprintf( stderr, "%s:%d: ", cfile->file, cfile->line );
+	conf_print_loc( cfile );
 	errno = errno_bak;
 	va_start( va, fmt );
 	vsys_error( fmt, va );
@@ -318,16 +327,78 @@ getopt_helper( conffile_t *cfile, int *cops, channel_conf_t *conf )
 	return 1;
 }
 
+static void
+eval_cmd_popen( conffile_t *cfile, const char *cmd )
+{
+	char *cd_cmd = xasprintf( "cd '%.*'s'; %s", cfile->path_len, cfile->file, cmd );
+
+	if (!(cfile->eval_fp = popen( cd_cmd, "r" ))) {
+		sys_error( "popen" );
+		cfile->err = 1;
+	} else {
+		cfile->eval_line = 0;
+		cfile->eval_command = nfstrdup( cmd );
+	}
+
+	free( cd_cmd );
+}
+
+static void
+eval_cmd_pclose( conffile_t *cfile )
+{
+	int ret = pclose( cfile->eval_fp );
+
+	// Do this here, so the exit code is not attributed to nested lines.
+	cfile->eval_fp = NULL;
+
+	if (ret) {
+		if (ret < 0) {
+			sys_error( "pclose" );
+			cfile->err = 1;
+		} else if (WIFSIGNALED( ret )) {
+			conf_error( cfile, "command \"%s\" crashed with signal %d\n",
+			            cfile->eval_command, WTERMSIG( ret ) );
+		} else {
+			conf_error( cfile, "command \"%s\" exited with status %d\n",
+			            cfile->eval_command, WEXITSTATUS( ret ) );
+		}
+	}
+
+	free( cfile->eval_command );
+	cfile->eval_command = NULL;
+}
+
+static int
+read_cline( conffile_t *cfile )
+{
+	if (cfile->eval_fp) {
+		cfile->eval_line++;
+		if ((cfile->rest = fgets( cfile->buf, cfile->bufl, cfile->eval_fp )) != NULL)
+			return 1;
+		eval_cmd_pclose( cfile );
+	}
+	cfile->line++;
+	return (cfile->rest = fgets( cfile->buf, cfile->bufl, cfile->fp )) != NULL;
+}
+
+static int
+check_excess_tokens( conffile_t *cfile )
+{
+	if (cfile->rest) {
+		char *arg = get_arg( cfile, ARG_OPTIONAL, NULL );
+		if (arg) {
+			conf_error( cfile, "excess token '%s'\n", arg );
+			return 1;
+		}
+	}
+	return 0;
+}
+
 int
 getcline( conffile_t *cfile )
 {
-	char *arg;
-
-	if (cfile->rest && (arg = get_arg( cfile, ARG_OPTIONAL, NULL )))
-		conf_error( cfile, "excess token '%s'\n", arg );
-	while (fgets( cfile->buf, cfile->bufl, cfile->fp )) {
-		cfile->line++;
-		cfile->rest = cfile->buf;
+	check_excess_tokens( cfile );
+	while (read_cline( cfile )) {
 		int comment = 0;
 		if (!(cfile->cmd = get_arg( cfile, ARG_OPTIONAL, &comment ))) {
 			if (comment)
@@ -336,6 +407,13 @@ getcline( conffile_t *cfile )
 		}
 		if (!(cfile->val = get_arg( cfile, ARG_REQUIRED, NULL )))
 			continue;
+		if (!strcasecmp( cfile->cmd, "IncludeCmd" )) {
+			if (cfile->eval_fp)
+				conf_error( cfile, "nested IncludeCmd\n" );
+			else if (!check_excess_tokens( cfile ))
+				eval_cmd_popen( cfile, cfile->val );
+			continue;
+		}
 		return 1;
 	}
 	return 0;
@@ -488,6 +566,7 @@ load_config( const char *where )
 		return 1;
 	}
 	buf[sizeof(buf) - 1] = 0;
+	cfile.eval_fp = NULL;
 	cfile.buf = buf;
 	cfile.bufl = sizeof(buf) - 1;
 	cfile.line = 0;
@@ -495,6 +574,7 @@ load_config( const char *where )
 	cfile.ms_warn = 0;
 	cfile.renew_warn = 0;
 	cfile.delete_warn = 0;
+	cfile.cmd = NULL;
 	cfile.rest = NULL;
 
 	gcops = 0;

src/config.h

@@ -12,10 +12,13 @@
 
 typedef struct {
 	const char *file;
+	char *eval_command;
 	FILE *fp;
+	FILE *eval_fp;
 	char *buf;
 	int bufl;
 	int line;
+	int eval_line;
 	int err;
 	int ms_warn, renew_warn, delete_warn;
 	int path_len;

src/mbsync.1.in

@@ -786,6 +786,15 @@ absolute limit, as even a single message can consume more memory than
 this.
 (Default: \fI10M\fR)
 .
+.SS Dynamic Configuration
+.TP
+\fBIncludeCmd\fR \fIcommand\fR
+Specify a shell command to obtain configuration file content from.
+The command is run in the configuration file's containing directory.
+This keyword can appear anywhere.
+The command's output will be interpreted as if it appeared inline;
+it can range from nothing at all to multiple sections.
+.
 .SH CONSOLE OUTPUT
 If \fBmbsync\fR's output is connected to a console, it will print progress
 counters by default. The output will look like this: