convert: split start_multi_file_filter() into two separate functions

To enable future reuse of the filter.<driver>.process infrastructure,
split start_multi_file_filter() into two separate parts.

start_multi_file_filter() will now only contain the generic logic to
manage the creation and tracking of the child process in a hashmap.

start_multi_file_filter_fn() is a protocol specific initialization
function that will negotiate the multi-file-filter interface version
and capabilities.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benpeart

convert.c

@@ -565,35 +565,14 @@ static void stop_multi_file_filter(struct child_process *process)
 	finish_command(process);
 }
 
-static struct cmd2process *start_multi_file_filter(struct hashmap *hashmap, const char *cmd)
+static int start_multi_file_filter_fn(struct cmd2process *entry)
 {
 	int err;
-	struct cmd2process *entry;
-	struct child_process *process;
-	const char *argv[] = { cmd, NULL };
 	struct string_list cap_list = STRING_LIST_INIT_NODUP;
 	char *cap_buf;
 	const char *cap_name;
-
-	entry = xmalloc(sizeof(*entry));
-	entry->cmd = cmd;
-	entry->supported_capabilities = 0;
-	process = &entry->process;
-
-	child_process_init(process);
-	process->argv = argv;
-	process->use_shell = 1;
-	process->in = -1;
-	process->out = -1;
-	process->clean_on_exit = 1;
-	process->clean_on_exit_handler = stop_multi_file_filter;
-
-	if (start_command(process)) {
-		error("cannot fork to run external filter '%s'", cmd);
-		return NULL;
-	}
-
-	hashmap_entry_init(entry, strhash(cmd));
+	struct child_process *process = &entry->process;
+	const char *cmd = entry->cmd;
 
 	sigchain_push(SIGPIPE, SIG_IGN);
 
@@ -642,6 +621,37 @@ static struct cmd2process *start_multi_file_filter(struct hashmap *hashmap, cons
 done:
 	sigchain_pop(SIGPIPE);
 
+	return err;
+}
+
+static struct cmd2process *start_multi_file_filter(struct hashmap *hashmap, const char *cmd)
+{
+	int err;
+	struct cmd2process *entry;
+	struct child_process *process;
+	const char *argv[] = { cmd, NULL };
+
+	entry = xmalloc(sizeof(*entry));
+	entry->cmd = cmd;
+	entry->supported_capabilities = 0;
+	process = &entry->process;
+
+	child_process_init(process);
+	process->argv = argv;
+	process->use_shell = 1;
+	process->in = -1;
+	process->out = -1;
+	process->clean_on_exit = 1;
+	process->clean_on_exit_handler = stop_multi_file_filter;
+
+	if (start_command(process)) {
+		error("cannot fork to run external filter '%s'", cmd);
+		return NULL;
+	}
+
+	hashmap_entry_init(entry, strhash(cmd));
+
+	err = start_multi_file_filter_fn(entry);
 	if (err) {
 		error("initialization for external filter '%s' failed", cmd);
 		kill_multi_file_filter(hashmap, entry);