[PATCH v3 05/26] Add git-file-watcher and basic connection handling logic

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

git-file-watcher is a daemon (*) that watches file changes and tells
git about them. The intent is to avoid lstat() calls at index refresh
time, which could be a lot on big working directory.

The actual monitoring needs support from OS (inotify, FSEvents,
FindFirstChangeNotification or kqueue) and is not part of this patch
or the next few yet. This patch only provides a UNIX socket server.

(*) it will be a a daemon in this end, but in this patch it runs in
the foreground.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 .gitignore                               |   1 +
 Documentation/git-file-watcher.txt (new) |  29 +++++++
 Makefile                                 |   1 +
 file-watcher.c (new)                     | 140 +++++++++++++++++++++++++++++++
 4 files changed, 171 insertions(+)
 create mode 100644 Documentation/git-file-watcher.txt
 create mode 100644 file-watcher.c

diff --git a/.gitignore b/.gitignore
index b5f9def..12c78f0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -56,6 +56,7 @@
 /git-fast-import
 /git-fetch
 /git-fetch-pack
+/git-file-watcher
 /git-filter-branch
 /git-fmt-merge-msg
 /git-for-each-ref
diff --git a/Documentation/git-file-watcher.txt b/Documentation/git-file-watcher.txt
new file mode 100644
index 0000000..625a389
--- /dev/null
+++ b/Documentation/git-file-watcher.txt
@@ -0,0 +1,29 @@
+git-file-watcher(1)
+===================
+
+NAME
+----
+git-file-watcher - File update notification daemon
+
+SYNOPSIS
+--------
+[verse]
+'git file-watcher' [options] <socket directory>
+
+DESCRIPTION
+-----------
+This program watches file changes in a git working directory and let
+Git now what files have been changed so that Git does not have to call
+lstat(2) to detect that itself.
+
+OPTIONS
+-------
+
+SEE ALSO
+--------
+linkgit:git-update-index[1],
+linkgit:git-config[1]
+
+GIT
+---
+Part of the linkgit:git[1] suite
diff --git a/Makefile b/Makefile
index dddaf4f..8eef0d6 100644
--- a/Makefile
+++ b/Makefile
@@ -536,6 +536,7 @@ PROGRAMS += $(EXTRA_PROGRAMS)
 PROGRAM_OBJS += credential-store.o
 PROGRAM_OBJS += daemon.o
 PROGRAM_OBJS += fast-import.o
+PROGRAM_OBJS += file-watcher.o
 PROGRAM_OBJS += http-backend.o
 PROGRAM_OBJS += imap-send.o
 PROGRAM_OBJS += sh-i18n--envsubst.o
diff --git a/file-watcher.c b/file-watcher.c
new file mode 100644
index 0000000..a6d7f17
--- /dev/null
+++ b/file-watcher.c
@@ -0,0 +1,140 @@
+#include "cache.h"
+#include "sigchain.h"
+#include "parse-options.h"
+#include "exec_cmd.h"
+#include "unix-socket.h"
+
+static const char *const file_watcher_usage[] = {
+	N_("git file-watcher [options] <socket directory>"),
+	NULL
+};
+
+struct connection {
+	int sock;
+};
+
+static struct connection **conns;
+static struct pollfd *pfd;
+static int conns_alloc, pfd_nr, pfd_alloc;
+
+static int shutdown_connection(int id)
+{
+	struct connection *conn = conns[id];
+	conns[id] = NULL;
+	pfd[id].fd = -1;
+	if (!conn)
+		return 0;
+	close(conn->sock);
+	free(conn);
+	return 0;
+}
+
+static int handle_command(int conn_id)
+{
+	return 0;
+}
+
+static void accept_connection(int fd)
+{
+	struct connection *conn;
+	int client = accept(fd, NULL, NULL);
+	if (client < 0) {
+		warning(_("accept failed: %s"), strerror(errno));
+		return;
+	}
+
+	ALLOC_GROW(pfd, pfd_nr + 1, pfd_alloc);
+	pfd[pfd_nr].fd = client;
+	pfd[pfd_nr].events = POLLIN;
+	pfd[pfd_nr].revents = 0;
+
+	ALLOC_GROW(conns, pfd_nr + 1, conns_alloc);
+	conn = xmalloc(sizeof(*conn));
+	memset(conn, 0, sizeof(*conn));
+	conn->sock = client;
+	conns[pfd_nr] = conn;
+	pfd_nr++;
+}
+
+int main(int argc, const char **argv)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int i, new_nr, fd, quit = 0, nr_common;
+	const char *socket_path = NULL;
+	struct option options[] = {
+		OPT_END()
+	};
+
+	git_extract_argv0_path(argv[0]);
+	git_setup_gettext();
+	argc = parse_options(argc, argv, NULL, options,
+			     file_watcher_usage, 0);
+	if (argc < 1)
+		die(_("socket path missing"));
+	else if (argc > 1)
+		die(_("too many arguments"));
+
+	socket_path = argv[0];
+	strbuf_addf(&sb, "%s/socket", socket_path);
+	fd = unix_stream_listen(sb.buf, 0);
+	if (fd == -1)
+		die_errno(_("unable to listen at %s"), sb.buf);
+	strbuf_reset(&sb);
+
+	nr_common = 1;
+	pfd_alloc = pfd_nr = nr_common;
+	pfd = xmalloc(sizeof(*pfd) * pfd_alloc);
+	pfd[0].fd = fd;
+	pfd[0].events = POLLIN;
+
+	while (!quit) {
+		if (poll(pfd, pfd_nr, -1) < 0) {
+			if (errno != EINTR) {
+				error("Poll failed, resuming: %s",
+				      strerror(errno));
+				sleep(1);
+			}
+			continue;
+		}
+
+		for (new_nr = i = nr_common; i < pfd_nr; i++) {
+			if (pfd[i].revents & (POLLERR | POLLNVAL))
+				shutdown_connection(i);
+			else if ((pfd[i].revents & POLLIN) && conns[i]) {
+				unsigned int avail = 1;
+				/*
+				 * pkt-line is not gentle with eof, at
+				 * least not with
+				 * packet_read_line(). Avoid feeding
+				 * eof to it.
+				 */
+				if ((pfd[i].revents & POLLHUP) &&
+				    ioctl(pfd[i].fd, FIONREAD, &avail))
+					die_errno("unable to FIONREAD inotify handle");
+				/*
+				 * We better have a way to handle all
+				 * packets in one go...
+				 */
+				if (avail)
+					handle_command(i);
+				else
+					shutdown_connection(i);
+			} else if (pfd[i].revents & POLLHUP)
+				shutdown_connection(i);
+			if (!conns[i])
+				continue;
+			if (i != new_nr) { /* pfd[] is shrunk, move pfd[i] up */
+				conns[new_nr] = conns[i];
+				pfd[new_nr] = pfd[i];
+			}
+			new_nr++; /* keep the good socket */
+		}
+		pfd_nr = new_nr;
+
+		if (pfd[0].revents & POLLIN)
+			accept_connection(pfd[0].fd);
+		if (pfd[0].revents & (POLLHUP | POLLERR | POLLNVAL))
+			die(_("error on listening socket"));
+	}
+	return 0;
+}
-- 
1.8.5.2.240.g8478abd