Merge branch 'sb/submodule-parallel-fetch'

Add a framework to spawn a group of processes in parallel, and use
it to run "git fetch --recurse-submodules" in parallel.

Rerolled and this seems to be a lot cleaner.  The merge of the
earlier one to 'next' has been reverted.

* sb/submodule-parallel-fetch:
  submodules: allow parallel fetching, add tests and documentation
  fetch_populated_submodules: use new parallel job processing
  run-command: add an asynchronous parallel child processor
  sigchain: add command to pop all common signals
  strbuf: add strbuf_read_once to read without blocking
  xread: poll on non blocking fds
  submodule.c: write "Fetching submodule <foo>" to stderr

1 files changed, 335 insertions, 0 deletions

diff --git a/run-command.c b/run-command.c
index 13fa452e8c..51fd72c427 100644
--- a/run-command.c
+++ b/run-command.c
@@ -3,6 +3,8 @@
 #include "exec_cmd.h"
 #include "sigchain.h"
 #include "argv-array.h"
+#include "thread-utils.h"
+#include "strbuf.h"
 
 void child_process_init(struct child_process *child)
 {
@@ -865,3 +867,336 @@ int capture_command(struct child_process *cmd, struct strbuf *buf, size_t hint)
 	close(cmd->out);
 	return finish_command(cmd);
 }
+
+enum child_state {
+	GIT_CP_FREE,
+	GIT_CP_WORKING,
+	GIT_CP_WAIT_CLEANUP,
+};
+
+struct parallel_processes {
+	void *data;
+
+	int max_processes;
+	int nr_processes;
+
+	get_next_task_fn get_next_task;
+	start_failure_fn start_failure;
+	task_finished_fn task_finished;
+
+	struct {
+		enum child_state state;
+		struct child_process process;
+		struct strbuf err;
+		void *data;
+	} *children;
+	/*
+	 * The struct pollfd is logically part of *children,
+	 * but the system call expects it as its own array.
+	 */
+	struct pollfd *pfd;
+
+	unsigned shutdown : 1;
+
+	int output_owner;
+	struct strbuf buffered_output; /* of finished children */
+};
+
+static int default_start_failure(struct child_process *cp,
+				 struct strbuf *err,
+				 void *pp_cb,
+				 void *pp_task_cb)
+{
+	int i;
+
+	strbuf_addstr(err, "Starting a child failed:");
+	for (i = 0; cp->argv[i]; i++)
+		strbuf_addf(err, " %s", cp->argv[i]);
+
+	return 0;
+}
+
+static int default_task_finished(int result,
+				 struct child_process *cp,
+				 struct strbuf *err,
+				 void *pp_cb,
+				 void *pp_task_cb)
+{
+	int i;
+
+	if (!result)
+		return 0;
+
+	strbuf_addf(err, "A child failed with return code %d:", result);
+	for (i = 0; cp->argv[i]; i++)
+		strbuf_addf(err, " %s", cp->argv[i]);
+
+	return 0;
+}
+
+static void kill_children(struct parallel_processes *pp, int signo)
+{
+	int i, n = pp->max_processes;
+
+	for (i = 0; i < n; i++)
+		if (pp->children[i].state == GIT_CP_WORKING)
+			kill(pp->children[i].process.pid, signo);
+}
+
+static struct parallel_processes *pp_for_signal;
+
+static void handle_children_on_signal(int signo)
+{
+	kill_children(pp_for_signal, signo);
+	sigchain_pop(signo);
+	raise(signo);
+}
+
+static void pp_init(struct parallel_processes *pp,
+		    int n,
+		    get_next_task_fn get_next_task,
+		    start_failure_fn start_failure,
+		    task_finished_fn task_finished,
+		    void *data)
+{
+	int i;
+
+	if (n < 1)
+		n = online_cpus();
+
+	pp->max_processes = n;
+
+	trace_printf("run_processes_parallel: preparing to run up to %d tasks", n);
+
+	pp->data = data;
+	if (!get_next_task)
+		die("BUG: you need to specify a get_next_task function");
+	pp->get_next_task = get_next_task;
+
+	pp->start_failure = start_failure ? start_failure : default_start_failure;
+	pp->task_finished = task_finished ? task_finished : default_task_finished;
+
+	pp->nr_processes = 0;
+	pp->output_owner = 0;
+	pp->shutdown = 0;
+	pp->children = xcalloc(n, sizeof(*pp->children));
+	pp->pfd = xcalloc(n, sizeof(*pp->pfd));
+	strbuf_init(&pp->buffered_output, 0);
+
+	for (i = 0; i < n; i++) {
+		strbuf_init(&pp->children[i].err, 0);
+		child_process_init(&pp->children[i].process);
+		pp->pfd[i].events = POLLIN | POLLHUP;
+		pp->pfd[i].fd = -1;
+	}
+
+	pp_for_signal = pp;
+	sigchain_push_common(handle_children_on_signal);
+}
+
+static void pp_cleanup(struct parallel_processes *pp)
+{
+	int i;
+
+	trace_printf("run_processes_parallel: done");
+	for (i = 0; i < pp->max_processes; i++) {
+		strbuf_release(&pp->children[i].err);
+		child_process_clear(&pp->children[i].process);
+	}
+
+	free(pp->children);
+	free(pp->pfd);
+
+	/*
+	 * When get_next_task added messages to the buffer in its last
+	 * iteration, the buffered output is non empty.
+	 */
+	fputs(pp->buffered_output.buf, stderr);
+	strbuf_release(&pp->buffered_output);
+
+	sigchain_pop_common();
+}
+
+/* returns
+ *  0 if a new task was started.
+ *  1 if no new jobs was started (get_next_task ran out of work, non critical
+ *    problem with starting a new command)
+ * <0 no new job was started, user wishes to shutdown early. Use negative code
+ *    to signal the children.
+ */
+static int pp_start_one(struct parallel_processes *pp)
+{
+	int i, code;
+
+	for (i = 0; i < pp->max_processes; i++)
+		if (pp->children[i].state == GIT_CP_FREE)
+			break;
+	if (i == pp->max_processes)
+		die("BUG: bookkeeping is hard");
+
+	code = pp->get_next_task(&pp->children[i].process,
+				 &pp->children[i].err,
+				 pp->data,
+				 &pp->children[i].data);
+	if (!code) {
+		strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
+		strbuf_reset(&pp->children[i].err);
+		return 1;
+	}
+	pp->children[i].process.err = -1;
+	pp->children[i].process.stdout_to_stderr = 1;
+	pp->children[i].process.no_stdin = 1;
+
+	if (start_command(&pp->children[i].process)) {
+		code = pp->start_failure(&pp->children[i].process,
+					 &pp->children[i].err,
+					 pp->data,
+					 &pp->children[i].data);
+		strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
+		strbuf_reset(&pp->children[i].err);
+		if (code)
+			pp->shutdown = 1;
+		return code;
+	}
+
+	pp->nr_processes++;
+	pp->children[i].state = GIT_CP_WORKING;
+	pp->pfd[i].fd = pp->children[i].process.err;
+	return 0;
+}
+
+static void pp_buffer_stderr(struct parallel_processes *pp, int output_timeout)
+{
+	int i;
+
+	while ((i = poll(pp->pfd, pp->max_processes, output_timeout)) < 0) {
+		if (errno == EINTR)
+			continue;
+		pp_cleanup(pp);
+		die_errno("poll");
+	}
+
+	/* Buffer output from all pipes. */
+	for (i = 0; i < pp->max_processes; i++) {
+		if (pp->children[i].state == GIT_CP_WORKING &&
+		    pp->pfd[i].revents & (POLLIN | POLLHUP)) {
+			int n = strbuf_read_once(&pp->children[i].err,
+						 pp->children[i].process.err, 0);
+			if (n == 0) {
+				close(pp->children[i].process.err);
+				pp->children[i].state = GIT_CP_WAIT_CLEANUP;
+			} else if (n < 0)
+				if (errno != EAGAIN)
+					die_errno("read");
+		}
+	}
+}
+
+static void pp_output(struct parallel_processes *pp)
+{
+	int i = pp->output_owner;
+	if (pp->children[i].state == GIT_CP_WORKING &&
+	    pp->children[i].err.len) {
+		fputs(pp->children[i].err.buf, stderr);
+		strbuf_reset(&pp->children[i].err);
+	}
+}
+
+static int pp_collect_finished(struct parallel_processes *pp)
+{
+	int i, code;
+	int n = pp->max_processes;
+	int result = 0;
+
+	while (pp->nr_processes > 0) {
+		for (i = 0; i < pp->max_processes; i++)
+			if (pp->children[i].state == GIT_CP_WAIT_CLEANUP)
+				break;
+		if (i == pp->max_processes)
+			break;
+
+		code = finish_command(&pp->children[i].process);
+
+		code = pp->task_finished(code, &pp->children[i].process,
+					 &pp->children[i].err, pp->data,
+					 &pp->children[i].data);
+
+		if (code)
+			result = code;
+		if (code < 0)
+			break;
+
+		pp->nr_processes--;
+		pp->children[i].state = GIT_CP_FREE;
+		pp->pfd[i].fd = -1;
+		child_process_init(&pp->children[i].process);
+
+		if (i != pp->output_owner) {
+			strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
+			strbuf_reset(&pp->children[i].err);
+		} else {
+			fputs(pp->children[i].err.buf, stderr);
+			strbuf_reset(&pp->children[i].err);
+
+			/* Output all other finished child processes */
+			fputs(pp->buffered_output.buf, stderr);
+			strbuf_reset(&pp->buffered_output);
+
+			/*
+			 * Pick next process to output live.
+			 * NEEDSWORK:
+			 * For now we pick it randomly by doing a round
+			 * robin. Later we may want to pick the one with
+			 * the most output or the longest or shortest
+			 * running process time.
+			 */
+			for (i = 0; i < n; i++)
+				if (pp->children[(pp->output_owner + i) % n].state == GIT_CP_WORKING)
+					break;
+			pp->output_owner = (pp->output_owner + i) % n;
+		}
+	}
+	return result;
+}
+
+int run_processes_parallel(int n,
+			   get_next_task_fn get_next_task,
+			   start_failure_fn start_failure,
+			   task_finished_fn task_finished,
+			   void *pp_cb)
+{
+	int i, code;
+	int output_timeout = 100;
+	int spawn_cap = 4;
+	struct parallel_processes pp;
+
+	pp_init(&pp, n, get_next_task, start_failure, task_finished, pp_cb);
+	while (1) {
+		for (i = 0;
+		    i < spawn_cap && !pp.shutdown &&
+		    pp.nr_processes < pp.max_processes;
+		    i++) {
+			code = pp_start_one(&pp);
+			if (!code)
+				continue;
+			if (code < 0) {
+				pp.shutdown = 1;
+				kill_children(&pp, -code);
+			}
+			break;
+		}
+		if (!pp.nr_processes)
+			break;
+		pp_buffer_stderr(&pp, output_timeout);
+		pp_output(&pp);
+		code = pp_collect_finished(&pp);
+		if (code) {
+			pp.shutdown = 1;
+			if (code < 0)
+				kill_children(&pp, -code);
+		}
+	}
+
+	pp_cleanup(&pp);
+	return 0;
+}