Merge branch 'jh/simple-ipc'

A simple IPC interface gets introduced to build services like
fsmonitor on top.

* jh/simple-ipc:
  t0052: add simple-ipc tests and t/helper/test-simple-ipc tool
  simple-ipc: add Unix domain socket implementation
  unix-stream-server: create unix domain socket under lock
  unix-socket: disallow chdir() when creating unix domain sockets
  unix-socket: add backlog size option to unix_stream_listen()
  unix-socket: eliminate static unix_stream_socket() helper function
  simple-ipc: add win32 implementation
  simple-ipc: design documentation for new IPC mechanism
  pkt-line: add options argument to read_packetized_to_strbuf()
  pkt-line: add PACKET_READ_GENTLE_ON_READ_ERROR option
  pkt-line: do not issue flush packets in write_packetized_*()
  pkt-line: eliminate the need for static buffer in packet_write_gently()

4 files changed, 911 insertions, 0 deletions

diff --git a/t/helper/test-simple-ipc.c b/t/helper/test-simple-ipc.c
new file mode 100644
index 0000000000..42040ef81b
--- /dev/null
+++ b/t/helper/test-simple-ipc.c
@@ -0,0 +1,787 @@
+/*
+ * test-simple-ipc.c: verify that the Inter-Process Communication works.
+ */
+
+#include "test-tool.h"
+#include "cache.h"
+#include "strbuf.h"
+#include "simple-ipc.h"
+#include "parse-options.h"
+#include "thread-utils.h"
+#include "strvec.h"
+
+#ifndef SUPPORTS_SIMPLE_IPC
+int cmd__simple_ipc(int argc, const char **argv)
+{
+	die("simple IPC not available on this platform");
+}
+#else
+
+/*
+ * The test daemon defines an "application callback" that supports a
+ * series of commands (see `test_app_cb()`).
+ *
+ * Unknown commands are caught here and we send an error message back
+ * to the client process.
+ */
+static int app__unhandled_command(const char *command,
+				  ipc_server_reply_cb *reply_cb,
+				  struct ipc_server_reply_data *reply_data)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int ret;
+
+	strbuf_addf(&buf, "unhandled command: %s", command);
+	ret = reply_cb(reply_data, buf.buf, buf.len);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+/*
+ * Reply with a single very large buffer.  This is to ensure that
+ * long response are properly handled -- whether the chunking occurs
+ * in the kernel or in the (probably pkt-line) layer.
+ */
+#define BIG_ROWS (10000)
+static int app__big_command(ipc_server_reply_cb *reply_cb,
+			    struct ipc_server_reply_data *reply_data)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int row;
+	int ret;
+
+	for (row = 0; row < BIG_ROWS; row++)
+		strbuf_addf(&buf, "big: %.75d\n", row);
+
+	ret = reply_cb(reply_data, buf.buf, buf.len);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+/*
+ * Reply with a series of lines.  This is to ensure that we can incrementally
+ * compute the response and chunk it to the client.
+ */
+#define CHUNK_ROWS (10000)
+static int app__chunk_command(ipc_server_reply_cb *reply_cb,
+			      struct ipc_server_reply_data *reply_data)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int row;
+	int ret;
+
+	for (row = 0; row < CHUNK_ROWS; row++) {
+		strbuf_setlen(&buf, 0);
+		strbuf_addf(&buf, "big: %.75d\n", row);
+		ret = reply_cb(reply_data, buf.buf, buf.len);
+	}
+
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+/*
+ * Slowly reply with a series of lines.  This is to model an expensive to
+ * compute chunked response (which might happen if this callback is running
+ * in a thread and is fighting for a lock with other threads).
+ */
+#define SLOW_ROWS     (1000)
+#define SLOW_DELAY_MS (10)
+static int app__slow_command(ipc_server_reply_cb *reply_cb,
+			     struct ipc_server_reply_data *reply_data)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int row;
+	int ret;
+
+	for (row = 0; row < SLOW_ROWS; row++) {
+		strbuf_setlen(&buf, 0);
+		strbuf_addf(&buf, "big: %.75d\n", row);
+		ret = reply_cb(reply_data, buf.buf, buf.len);
+		sleep_millisec(SLOW_DELAY_MS);
+	}
+
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+/*
+ * The client sent a command followed by a (possibly very) large buffer.
+ */
+static int app__sendbytes_command(const char *received,
+				  ipc_server_reply_cb *reply_cb,
+				  struct ipc_server_reply_data *reply_data)
+{
+	struct strbuf buf_resp = STRBUF_INIT;
+	const char *p = "?";
+	int len_ballast = 0;
+	int k;
+	int errs = 0;
+	int ret;
+
+	if (skip_prefix(received, "sendbytes ", &p))
+		len_ballast = strlen(p);
+
+	/*
+	 * Verify that the ballast is n copies of a single letter.
+	 * And that the multi-threaded IO layer didn't cross the streams.
+	 */
+	for (k = 1; k < len_ballast; k++)
+		if (p[k] != p[0])
+			errs++;
+
+	if (errs)
+		strbuf_addf(&buf_resp, "errs:%d\n", errs);
+	else
+		strbuf_addf(&buf_resp, "rcvd:%c%08d\n", p[0], len_ballast);
+
+	ret = reply_cb(reply_data, buf_resp.buf, buf_resp.len);
+
+	strbuf_release(&buf_resp);
+
+	return ret;
+}
+
+/*
+ * An arbitrary fixed address to verify that the application instance
+ * data is handled properly.
+ */
+static int my_app_data = 42;
+
+static ipc_server_application_cb test_app_cb;
+
+/*
+ * This is the "application callback" that sits on top of the
+ * "ipc-server".  It completely defines the set of commands supported
+ * by this application.
+ */
+static int test_app_cb(void *application_data,
+		       const char *command,
+		       ipc_server_reply_cb *reply_cb,
+		       struct ipc_server_reply_data *reply_data)
+{
+	/*
+	 * Verify that we received the application-data that we passed
+	 * when we started the ipc-server.  (We have several layers of
+	 * callbacks calling callbacks and it's easy to get things mixed
+	 * up (especially when some are "void*").)
+	 */
+	if (application_data != (void*)&my_app_data)
+		BUG("application_cb: application_data pointer wrong");
+
+	if (!strcmp(command, "quit")) {
+		/*
+		 * The client sent a "quit" command.  This is an async
+		 * request for the server to shutdown.
+		 *
+		 * We DO NOT send the client a response message
+		 * (because we have nothing to say and the other
+		 * server threads have not yet stopped).
+		 *
+		 * Tell the ipc-server layer to start shutting down.
+		 * This includes: stop listening for new connections
+		 * on the socket/pipe and telling all worker threads
+		 * to finish/drain their outgoing responses to other
+		 * clients.
+		 *
+		 * This DOES NOT force an immediate sync shutdown.
+		 */
+		return SIMPLE_IPC_QUIT;
+	}
+
+	if (!strcmp(command, "ping")) {
+		const char *answer = "pong";
+		return reply_cb(reply_data, answer, strlen(answer));
+	}
+
+	if (!strcmp(command, "big"))
+		return app__big_command(reply_cb, reply_data);
+
+	if (!strcmp(command, "chunk"))
+		return app__chunk_command(reply_cb, reply_data);
+
+	if (!strcmp(command, "slow"))
+		return app__slow_command(reply_cb, reply_data);
+
+	if (starts_with(command, "sendbytes "))
+		return app__sendbytes_command(command, reply_cb, reply_data);
+
+	return app__unhandled_command(command, reply_cb, reply_data);
+}
+
+struct cl_args
+{
+	const char *subcommand;
+	const char *path;
+	const char *token;
+
+	int nr_threads;
+	int max_wait_sec;
+	int bytecount;
+	int batchsize;
+
+	char bytevalue;
+};
+
+static struct cl_args cl_args = {
+	.subcommand = NULL,
+	.path = "ipc-test",
+	.token = NULL,
+
+	.nr_threads = 5,
+	.max_wait_sec = 60,
+	.bytecount = 1024,
+	.batchsize = 10,
+
+	.bytevalue = 'x',
+};
+
+/*
+ * This process will run as a simple-ipc server and listen for IPC commands
+ * from client processes.
+ */
+static int daemon__run_server(void)
+{
+	int ret;
+
+	struct ipc_server_opts opts = {
+		.nr_threads = cl_args.nr_threads,
+	};
+
+	/*
+	 * Synchronously run the ipc-server.  We don't need any application
+	 * instance data, so pass an arbitrary pointer (that we'll later
+	 * verify made the round trip).
+	 */
+	ret = ipc_server_run(cl_args.path, &opts, test_app_cb, (void*)&my_app_data);
+	if (ret == -2)
+		error(_("socket/pipe already in use: '%s'"), cl_args.path);
+	else if (ret == -1)
+		error_errno(_("could not start server on: '%s'"), cl_args.path);
+
+	return ret;
+}
+
+#ifndef GIT_WINDOWS_NATIVE
+/*
+ * This is adapted from `daemonize()`.  Use `fork()` to directly create and
+ * run the daemon in a child process.
+ */
+static int spawn_server(pid_t *pid)
+{
+	struct ipc_server_opts opts = {
+		.nr_threads = cl_args.nr_threads,
+	};
+
+	*pid = fork();
+
+	switch (*pid) {
+	case 0:
+		if (setsid() == -1)
+			error_errno(_("setsid failed"));
+		close(0);
+		close(1);
+		close(2);
+		sanitize_stdfds();
+
+		return ipc_server_run(cl_args.path, &opts, test_app_cb,
+				      (void*)&my_app_data);
+
+	case -1:
+		return error_errno(_("could not spawn daemon in the background"));
+
+	default:
+		return 0;
+	}
+}
+#else
+/*
+ * Conceptually like `daemonize()` but different because Windows does not
+ * have `fork(2)`.  Spawn a normal Windows child process but without the
+ * limitations of `start_command()` and `finish_command()`.
+ */
+static int spawn_server(pid_t *pid)
+{
+	char test_tool_exe[MAX_PATH];
+	struct strvec args = STRVEC_INIT;
+	int in, out;
+
+	GetModuleFileNameA(NULL, test_tool_exe, MAX_PATH);
+
+	in = open("/dev/null", O_RDONLY);
+	out = open("/dev/null", O_WRONLY);
+
+	strvec_push(&args, test_tool_exe);
+	strvec_push(&args, "simple-ipc");
+	strvec_push(&args, "run-daemon");
+	strvec_pushf(&args, "--name=%s", cl_args.path);
+	strvec_pushf(&args, "--threads=%d", cl_args.nr_threads);
+
+	*pid = mingw_spawnvpe(args.v[0], args.v, NULL, NULL, in, out, out);
+	close(in);
+	close(out);
+
+	strvec_clear(&args);
+
+	if (*pid < 0)
+		return error(_("could not spawn daemon in the background"));
+
+	return 0;
+}
+#endif
+
+/*
+ * This is adapted from `wait_or_whine()`.  Watch the child process and
+ * let it get started and begin listening for requests on the socket
+ * before reporting our success.
+ */
+static int wait_for_server_startup(pid_t pid_child)
+{
+	int status;
+	pid_t pid_seen;
+	enum ipc_active_state s;
+	time_t time_limit, now;
+
+	time(&time_limit);
+	time_limit += cl_args.max_wait_sec;
+
+	for (;;) {
+		pid_seen = waitpid(pid_child, &status, WNOHANG);
+
+		if (pid_seen == -1)
+			return error_errno(_("waitpid failed"));
+
+		else if (pid_seen == 0) {
+			/*
+			 * The child is still running (this should be
+			 * the normal case).  Try to connect to it on
+			 * the socket and see if it is ready for
+			 * business.
+			 *
+			 * If there is another daemon already running,
+			 * our child will fail to start (possibly
+			 * after a timeout on the lock), but we don't
+			 * care (who responds) if the socket is live.
+			 */
+			s = ipc_get_active_state(cl_args.path);
+			if (s == IPC_STATE__LISTENING)
+				return 0;
+
+			time(&now);
+			if (now > time_limit)
+				return error(_("daemon not online yet"));
+
+			continue;
+		}
+
+		else if (pid_seen == pid_child) {
+			/*
+			 * The new child daemon process shutdown while
+			 * it was starting up, so it is not listening
+			 * on the socket.
+			 *
+			 * Try to ping the socket in the odd chance
+			 * that another daemon started (or was already
+			 * running) while our child was starting.
+			 *
+			 * Again, we don't care who services the socket.
+			 */
+			s = ipc_get_active_state(cl_args.path);
+			if (s == IPC_STATE__LISTENING)
+				return 0;
+
+			/*
+			 * We don't care about the WEXITSTATUS() nor
+			 * any of the WIF*(status) values because
+			 * `cmd__simple_ipc()` does the `!!result`
+			 * trick on all function return values.
+			 *
+			 * So it is sufficient to just report the
+			 * early shutdown as an error.
+			 */
+			return error(_("daemon failed to start"));
+		}
+
+		else
+			return error(_("waitpid is confused"));
+	}
+}
+
+/*
+ * This process will start a simple-ipc server in a background process and
+ * wait for it to become ready.  This is like `daemonize()` but gives us
+ * more control and better error reporting (and makes it easier to write
+ * unit tests).
+ */
+static int daemon__start_server(void)
+{
+	pid_t pid_child;
+	int ret;
+
+	/*
+	 * Run the actual daemon in a background process.
+	 */
+	ret = spawn_server(&pid_child);
+	if (pid_child <= 0)
+		return ret;
+
+	/*
+	 * Let the parent wait for the child process to get started
+	 * and begin listening for requests on the socket.
+	 */
+	ret = wait_for_server_startup(pid_child);
+
+	return ret;
+}
+
+/*
+ * This process will run a quick probe to see if a simple-ipc server
+ * is active on this path.
+ *
+ * Returns 0 if the server is alive.
+ */
+static int client__probe_server(void)
+{
+	enum ipc_active_state s;
+
+	s = ipc_get_active_state(cl_args.path);
+	switch (s) {
+	case IPC_STATE__LISTENING:
+		return 0;
+
+	case IPC_STATE__NOT_LISTENING:
+		return error("no server listening at '%s'", cl_args.path);
+
+	case IPC_STATE__PATH_NOT_FOUND:
+		return error("path not found '%s'", cl_args.path);
+
+	case IPC_STATE__INVALID_PATH:
+		return error("invalid pipe/socket name '%s'", cl_args.path);
+
+	case IPC_STATE__OTHER_ERROR:
+	default:
+		return error("other error for '%s'", cl_args.path);
+	}
+}
+
+/*
+ * Send an IPC command token to an already-running server daemon and
+ * print the response.
+ *
+ * This is a simple 1 word command/token that `test_app_cb()` (in the
+ * daemon process) will understand.
+ */
+static int client__send_ipc(void)
+{
+	const char *command = "(no-command)";
+	struct strbuf buf = STRBUF_INIT;
+	struct ipc_client_connect_options options
+		= IPC_CLIENT_CONNECT_OPTIONS_INIT;
+
+	if (cl_args.token && *cl_args.token)
+		command = cl_args.token;
+
+	options.wait_if_busy = 1;
+	options.wait_if_not_found = 0;
+
+	if (!ipc_client_send_command(cl_args.path, &options, command, &buf)) {
+		if (buf.len) {
+			printf("%s\n", buf.buf);
+			fflush(stdout);
+		}
+		strbuf_release(&buf);
+
+		return 0;
+	}
+
+	return error("failed to send '%s' to '%s'", command, cl_args.path);
+}
+
+/*
+ * Send an IPC command to an already-running server and ask it to
+ * shutdown.  "send quit" is an async request and queues a shutdown
+ * event in the server, so we spin and wait here for it to actually
+ * shutdown to make the unit tests a little easier to write.
+ */
+static int client__stop_server(void)
+{
+	int ret;
+	time_t time_limit, now;
+	enum ipc_active_state s;
+
+	time(&time_limit);
+	time_limit += cl_args.max_wait_sec;
+
+	cl_args.token = "quit";
+
+	ret = client__send_ipc();
+	if (ret)
+		return ret;
+
+	for (;;) {
+		sleep_millisec(100);
+
+		s = ipc_get_active_state(cl_args.path);
+
+		if (s != IPC_STATE__LISTENING) {
+			/*
+			 * The socket/pipe is gone and/or has stopped
+			 * responding.  Lets assume that the daemon
+			 * process has exited too.
+			 */
+			return 0;
+		}
+
+		time(&now);
+		if (now > time_limit)
+			return error(_("daemon has not shutdown yet"));
+	}
+}
+
+/*
+ * Send an IPC command followed by ballast to confirm that a large
+ * message can be sent and that the kernel or pkt-line layers will
+ * properly chunk it and that the daemon receives the entire message.
+ */
+static int do_sendbytes(int bytecount, char byte, const char *path,
+			const struct ipc_client_connect_options *options)
+{
+	struct strbuf buf_send = STRBUF_INIT;
+	struct strbuf buf_resp = STRBUF_INIT;
+
+	strbuf_addstr(&buf_send, "sendbytes ");
+	strbuf_addchars(&buf_send, byte, bytecount);
+
+	if (!ipc_client_send_command(path, options, buf_send.buf, &buf_resp)) {
+		strbuf_rtrim(&buf_resp);
+		printf("sent:%c%08d %s\n", byte, bytecount, buf_resp.buf);
+		fflush(stdout);
+		strbuf_release(&buf_send);
+		strbuf_release(&buf_resp);
+
+		return 0;
+	}
+
+	return error("client failed to sendbytes(%d, '%c') to '%s'",
+		     bytecount, byte, path);
+}
+
+/*
+ * Send an IPC command with ballast to an already-running server daemon.
+ */
+static int client__sendbytes(void)
+{
+	struct ipc_client_connect_options options
+		= IPC_CLIENT_CONNECT_OPTIONS_INIT;
+
+	options.wait_if_busy = 1;
+	options.wait_if_not_found = 0;
+	options.uds_disallow_chdir = 0;
+
+	return do_sendbytes(cl_args.bytecount, cl_args.bytevalue, cl_args.path,
+			    &options);
+}
+
+struct multiple_thread_data {
+	pthread_t pthread_id;
+	struct multiple_thread_data *next;
+	const char *path;
+	int bytecount;
+	int batchsize;
+	int sum_errors;
+	int sum_good;
+	char letter;
+};
+
+static void *multiple_thread_proc(void *_multiple_thread_data)
+{
+	struct multiple_thread_data *d = _multiple_thread_data;
+	int k;
+	struct ipc_client_connect_options options
+		= IPC_CLIENT_CONNECT_OPTIONS_INIT;
+
+	options.wait_if_busy = 1;
+	options.wait_if_not_found = 0;
+	/*
+	 * A multi-threaded client should not be randomly calling chdir().
+	 * The test will pass without this restriction because the test is
+	 * not otherwise accessing the filesystem, but it makes us honest.
+	 */
+	options.uds_disallow_chdir = 1;
+
+	trace2_thread_start("multiple");
+
+	for (k = 0; k < d->batchsize; k++) {
+		if (do_sendbytes(d->bytecount + k, d->letter, d->path, &options))
+			d->sum_errors++;
+		else
+			d->sum_good++;
+	}
+
+	trace2_thread_exit();
+	return NULL;
+}
+
+/*
+ * Start a client-side thread pool.  Each thread sends a series of
+ * IPC requests.  Each request is on a new connection to the server.
+ */
+static int client__multiple(void)
+{
+	struct multiple_thread_data *list = NULL;
+	int k;
+	int sum_join_errors = 0;
+	int sum_thread_errors = 0;
+	int sum_good = 0;
+
+	for (k = 0; k < cl_args.nr_threads; k++) {
+		struct multiple_thread_data *d = xcalloc(1, sizeof(*d));
+		d->next = list;
+		d->path = cl_args.path;
+		d->bytecount = cl_args.bytecount + cl_args.batchsize*(k/26);
+		d->batchsize = cl_args.batchsize;
+		d->sum_errors = 0;
+		d->sum_good = 0;
+		d->letter = 'A' + (k % 26);
+
+		if (pthread_create(&d->pthread_id, NULL, multiple_thread_proc, d)) {
+			warning("failed to create thread[%d] skipping remainder", k);
+			free(d);
+			break;
+		}
+
+		list = d;
+	}
+
+	while (list) {
+		struct multiple_thread_data *d = list;
+
+		if (pthread_join(d->pthread_id, NULL))
+			sum_join_errors++;
+
+		sum_thread_errors += d->sum_errors;
+		sum_good += d->sum_good;
+
+		list = d->next;
+		free(d);
+	}
+
+	printf("client (good %d) (join %d), (errors %d)\n",
+	       sum_good, sum_join_errors, sum_thread_errors);
+
+	return (sum_join_errors + sum_thread_errors) ? 1 : 0;
+}
+
+int cmd__simple_ipc(int argc, const char **argv)
+{
+	const char * const simple_ipc_usage[] = {
+		N_("test-helper simple-ipc is-active    [<name>] [<options>]"),
+		N_("test-helper simple-ipc run-daemon   [<name>] [<threads>]"),
+		N_("test-helper simple-ipc start-daemon [<name>] [<threads>] [<max-wait>]"),
+		N_("test-helper simple-ipc stop-daemon  [<name>] [<max-wait>]"),
+		N_("test-helper simple-ipc send         [<name>] [<token>]"),
+		N_("test-helper simple-ipc sendbytes    [<name>] [<bytecount>] [<byte>]"),
+		N_("test-helper simple-ipc multiple     [<name>] [<threads>] [<bytecount>] [<batchsize>]"),
+		NULL
+	};
+
+	const char *bytevalue = NULL;
+
+	struct option options[] = {
+#ifndef GIT_WINDOWS_NATIVE
+		OPT_STRING(0, "name", &cl_args.path, N_("name"), N_("name or pathname of unix domain socket")),
+#else
+		OPT_STRING(0, "name", &cl_args.path, N_("name"), N_("named-pipe name")),
+#endif
+		OPT_INTEGER(0, "threads", &cl_args.nr_threads, N_("number of threads in server thread pool")),
+		OPT_INTEGER(0, "max-wait", &cl_args.max_wait_sec, N_("seconds to wait for daemon to start or stop")),
+
+		OPT_INTEGER(0, "bytecount", &cl_args.bytecount, N_("number of bytes")),
+		OPT_INTEGER(0, "batchsize", &cl_args.batchsize, N_("number of requests per thread")),
+
+		OPT_STRING(0, "byte", &bytevalue, N_("byte"), N_("ballast character")),
+		OPT_STRING(0, "token", &cl_args.token, N_("token"), N_("command token to send to the server")),
+
+		OPT_END()
+	};
+
+	if (argc < 2)
+		usage_with_options(simple_ipc_usage, options);
+
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(simple_ipc_usage, options);
+
+	if (argc == 2 && !strcmp(argv[1], "SUPPORTS_SIMPLE_IPC"))
+		return 0;
+
+	cl_args.subcommand = argv[1];
+
+	argc--;
+	argv++;
+
+	argc = parse_options(argc, argv, NULL, options, simple_ipc_usage, 0);
+
+	if (cl_args.nr_threads < 1)
+		cl_args.nr_threads = 1;
+	if (cl_args.max_wait_sec < 0)
+		cl_args.max_wait_sec = 0;
+	if (cl_args.bytecount < 1)
+		cl_args.bytecount = 1;
+	if (cl_args.batchsize < 1)
+		cl_args.batchsize = 1;
+
+	if (bytevalue && *bytevalue)
+		cl_args.bytevalue = bytevalue[0];
+
+	/*
+	 * Use '!!' on all dispatch functions to map from `error()` style
+	 * (returns -1) style to `test_must_fail` style (expects 1).  This
+	 * makes shell error messages less confusing.
+	 */
+
+	if (!strcmp(cl_args.subcommand, "is-active"))
+		return !!client__probe_server();
+
+	if (!strcmp(cl_args.subcommand, "run-daemon"))
+		return !!daemon__run_server();
+
+	if (!strcmp(cl_args.subcommand, "start-daemon"))
+		return !!daemon__start_server();
+
+	/*
+	 * Client commands follow.  Ensure a server is running before
+	 * sending any data.  This might be overkill, but then again
+	 * this is a test harness.
+	 */
+
+	if (!strcmp(cl_args.subcommand, "stop-daemon")) {
+		if (client__probe_server())
+			return 1;
+		return !!client__stop_server();
+	}
+
+	if (!strcmp(cl_args.subcommand, "send")) {
+		if (client__probe_server())
+			return 1;
+		return !!client__send_ipc();
+	}
+
+	if (!strcmp(cl_args.subcommand, "sendbytes")) {
+		if (client__probe_server())
+			return 1;
+		return !!client__sendbytes();
+	}
+
+	if (!strcmp(cl_args.subcommand, "multiple")) {
+		if (client__probe_server())
+			return 1;
+		return !!client__multiple();
+	}
+
+	die("Unhandled subcommand: '%s'", cl_args.subcommand);
+}
+#endif
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index f97cd9f48a..287aa60023 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -65,6 +65,7 @@ static struct test_cmd cmds[] = {
 	{ "sha1", cmd__sha1 },
 	{ "sha256", cmd__sha256 },
 	{ "sigchain", cmd__sigchain },
+	{ "simple-ipc", cmd__simple_ipc },
 	{ "strcmp-offset", cmd__strcmp_offset },
 	{ "string-list", cmd__string_list },
 	{ "submodule-config", cmd__submodule_config },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index 28072c0ad5..9ea4b31011 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -55,6 +55,7 @@ int cmd__sha1(int argc, const char **argv);
 int cmd__oid_array(int argc, const char **argv);
 int cmd__sha256(int argc, const char **argv);
 int cmd__sigchain(int argc, const char **argv);
+int cmd__simple_ipc(int argc, const char **argv);
 int cmd__strcmp_offset(int argc, const char **argv);
 int cmd__string_list(int argc, const char **argv);
 int cmd__submodule_config(int argc, const char **argv);
diff --git a/t/t0052-simple-ipc.sh b/t/t0052-simple-ipc.sh
new file mode 100755
index 0000000000..ff98be31a5
--- /dev/null
+++ b/t/t0052-simple-ipc.sh
@@ -0,0 +1,122 @@
+#!/bin/sh
+
+test_description='simple command server'
+
+. ./test-lib.sh
+
+test-tool simple-ipc SUPPORTS_SIMPLE_IPC || {
+	skip_all='simple IPC not supported on this platform'
+	test_done
+}
+
+stop_simple_IPC_server () {
+	test-tool simple-ipc stop-daemon
+}
+
+test_expect_success 'start simple command server' '
+	test_atexit stop_simple_IPC_server &&
+	test-tool simple-ipc start-daemon --threads=8 &&
+	test-tool simple-ipc is-active
+'
+
+test_expect_success 'simple command server' '
+	test-tool simple-ipc send --token=ping >actual &&
+	echo pong >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'servers cannot share the same path' '
+	test_must_fail test-tool simple-ipc run-daemon &&
+	test-tool simple-ipc is-active
+'
+
+test_expect_success 'big response' '
+	test-tool simple-ipc send --token=big >actual &&
+	test_line_count -ge 10000 actual &&
+	grep -q "big: [0]*9999\$" actual
+'
+
+test_expect_success 'chunk response' '
+	test-tool simple-ipc send --token=chunk >actual &&
+	test_line_count -ge 10000 actual &&
+	grep -q "big: [0]*9999\$" actual
+'
+
+test_expect_success 'slow response' '
+	test-tool simple-ipc send --token=slow >actual &&
+	test_line_count -ge 100 actual &&
+	grep -q "big: [0]*99\$" actual
+'
+
+# Send an IPC with n=100,000 bytes of ballast.  This should be large enough
+# to force both the kernel and the pkt-line layer to chunk the message to the
+# daemon and for the daemon to receive it in chunks.
+#
+test_expect_success 'sendbytes' '
+	test-tool simple-ipc sendbytes --bytecount=100000 --byte=A >actual &&
+	grep "sent:A00100000 rcvd:A00100000" actual
+'
+
+# Start a series of <threads> client threads that each make <batchsize>
+# IPC requests to the server.  Each (<threads> * <batchsize>) request
+# will open a new connection to the server and randomly bind to a server
+# thread.  Each client thread exits after completing its batch.  So the
+# total number of live client threads will be smaller than the total.
+# Each request will send a message containing at least <bytecount> bytes
+# of ballast.  (Responses are small.)
+#
+# The purpose here is to test threading in the server and responding to
+# many concurrent client requests (regardless of whether they come from
+# 1 client process or many).  And to test that the server side of the
+# named pipe/socket is stable.  (On Windows this means that the server
+# pipe is properly recycled.)
+#
+# On Windows it also lets us adjust the connection timeout in the
+# `ipc_client_send_command()`.
+#
+# Note it is easy to drive the system into failure by requesting an
+# insane number of threads on client or server and/or increasing the
+# per-thread batchsize or the per-request bytecount (ballast).
+# On Windows these failures look like "pipe is busy" errors.
+# So I've chosen fairly conservative values for now.
+#
+# We expect output of the form "sent:<letter><length> ..."
+# With terms (7, 19, 13) we expect:
+#   <letter> in [A-G]
+#   <length> in [19+0 .. 19+(13-1)]
+# and (7 * 13) successful responses.
+#
+test_expect_success 'stress test threads' '
+	test-tool simple-ipc multiple \
+		--threads=7 \
+		--bytecount=19 \
+		--batchsize=13 \
+		>actual &&
+	test_line_count = 92 actual &&
+	grep "good 91" actual &&
+	grep "sent:A" <actual >actual_a &&
+	cat >expect_a <<-EOF &&
+		sent:A00000019 rcvd:A00000019
+		sent:A00000020 rcvd:A00000020
+		sent:A00000021 rcvd:A00000021
+		sent:A00000022 rcvd:A00000022
+		sent:A00000023 rcvd:A00000023
+		sent:A00000024 rcvd:A00000024
+		sent:A00000025 rcvd:A00000025
+		sent:A00000026 rcvd:A00000026
+		sent:A00000027 rcvd:A00000027
+		sent:A00000028 rcvd:A00000028
+		sent:A00000029 rcvd:A00000029
+		sent:A00000030 rcvd:A00000030
+		sent:A00000031 rcvd:A00000031
+	EOF
+	test_cmp expect_a actual_a
+'
+
+test_expect_success 'stop-daemon works' '
+	test-tool simple-ipc stop-daemon &&
+	test_must_fail test-tool simple-ipc is-active &&
+	test_must_fail test-tool simple-ipc send --token=ping
+'
+
+test_done