sub-process: move sub-process functions into separate files

Move the sub-proces functions into sub-process.h/c.  Add documentation
for the new module in Documentation/technical/api-sub-process.txt

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

sub-process.c

@@ -0,0 +1,102 @@
+/*
+ * Generic implementation of background process infrastructure.
+ */
+#include "sub-process.h"
+#include "sigchain.h"
+#include "pkt-line.h"
+
+int cmd2process_cmp(const struct subprocess_entry *e1,
+			   const struct subprocess_entry *e2,
+			   const void *unused)
+{
+	return strcmp(e1->cmd, e2->cmd);
+}
+
+struct subprocess_entry *subprocess_find_entry(struct hashmap *hashmap, const char *cmd)
+{
+	struct subprocess_entry key;
+
+	hashmap_entry_init(&key, strhash(cmd));
+	key.cmd = cmd;
+	return hashmap_get(hashmap, &key, NULL);
+}
+
+void subprocess_read_status(int fd, struct strbuf *status)
+{
+	struct strbuf **pair;
+	char *line;
+	for (;;) {
+		line = packet_read_line(fd, NULL);
+		if (!line)
+			break;
+		pair = strbuf_split_str(line, '=', 2);
+		if (pair[0] && pair[0]->len && pair[1]) {
+			/* the last "status=<foo>" line wins */
+			if (!strcmp(pair[0]->buf, "status=")) {
+				strbuf_reset(status);
+				strbuf_addbuf(status, pair[1]);
+			}
+		}
+		strbuf_list_free(pair);
+	}
+}
+
+void subprocess_stop(struct hashmap *hashmap, struct subprocess_entry *entry)
+{
+	if (!entry)
+		return;
+
+	entry->process.clean_on_exit = 0;
+	kill(entry->process.pid, SIGTERM);
+	finish_command(&entry->process);
+
+	hashmap_remove(hashmap, entry, NULL);
+}
+
+static void subprocess_exit_handler(struct child_process *process)
+{
+	sigchain_push(SIGPIPE, SIG_IGN);
+	/* Closing the pipe signals the subprocess to initiate a shutdown. */
+	close(process->in);
+	close(process->out);
+	sigchain_pop(SIGPIPE);
+	/* Finish command will wait until the shutdown is complete. */
+	finish_command(process);
+}
+
+int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, const char *cmd,
+	subprocess_start_fn startfn)
+{
+	int err;
+	struct child_process *process;
+	const char *argv[] = { cmd, NULL };
+
+	entry->cmd = cmd;
+	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 = subprocess_exit_handler;
+
+	err = start_command(process);
+	if (err) {
+		error("cannot fork to run subprocess '%s'", cmd);
+		return err;
+	}
+
+	hashmap_entry_init(entry, strhash(cmd));
+
+	err = startfn(entry);
+	if (err) {
+		error("initialization for subprocess '%s' failed", cmd);
+		subprocess_stop(hashmap, entry);
+		return err;
+	}
+
+	hashmap_add(hashmap, entry);
+	return 0;
+}