Merge branch 'try-v4-fsmonitor-part2' into try-v4-fsmonitor

Documentation/config/core.txt

@@ -62,22 +62,50 @@ core.protectNTFS::
 	Defaults to `true` on Windows, and `false` elsewhere.
 
 core.fsmonitor::
-	If set, the value of this variable is used as a command which
-	will identify all files that may have changed since the
-	requested date/time. This information is used to speed up git by
-	avoiding unnecessary processing of files that have not changed.
-	See the "fsmonitor-watchman" section of linkgit:githooks[5].
+	If set, this variable contains the pathname of the "fsmonitor"
+	hook command.
++
+This hook command is used to identify all files that may have changed
+since the requested date/time. This information is used to speed up
+git by avoiding unnecessary scanning of files that have not changed.
++
+See the "fsmonitor-watchman" section of linkgit:githooks[5].
++
+Note: The value of this config setting is ignored if the
+built-in file system monitor is enabled (see `core.useBuiltinFSMonitor`).
 
 core.fsmonitorHookVersion::
-	Sets the version of hook that is to be used when calling fsmonitor.
-	There are currently versions 1 and 2. When this is not set,
-	version 2 will be tried first and if it fails then version 1
-	will be tried. Version 1 uses a timestamp as input to determine
-	which files have changes since that time but some monitors
-	like watchman have race conditions when used with a timestamp.
-	Version 2 uses an opaque string so that the monitor can return
-	something that can be used to determine what files have changed
-	without race conditions.
+	Sets the protocol version to be used when invoking the
+	"fsmonitor" hook.
++
+There are currently versions 1 and 2. When this is not set,
+version 2 will be tried first and if it fails then version 1
+will be tried. Version 1 uses a timestamp as input to determine
+which files have changes since that time but some monitors
+like Watchman have race conditions when used with a timestamp.
+Version 2 uses an opaque string so that the monitor can return
+something that can be used to determine what files have changed
+without race conditions.
++
+Note: The value of this config setting is ignored if the
+built-in file system monitor is enabled (see `core.useBuiltinFSMonitor`).
+
+core.useBuiltinFSMonitor::
+	If set to true, enable the built-in file system monitor
+	daemon for this working directory (linkgit:git-fsmonitor--daemon[1]).
++
+Like hook-based file system monitors, the built-in file system monitor
+can speed up Git commands that need to refresh the Git index
+(e.g. `git status`) in a working directory with many files.  The
+built-in monitor eliminates the need to install and maintain an
+external third-party tool.
++
+The built-in file system monitor is currently available only on a
+limited set of supported platforms.  Currently, this includes Windows
+and MacOS.
++
+Note: if this config setting is set to `true`, the values of
+`core.fsmonitor` and `core.fsmonitorHookVersion` are ignored.
 
 core.trustctime::
 	If false, the ctime differences between the index and the

Documentation/git-update-index.txt

@@ -498,7 +498,9 @@ FILE SYSTEM MONITOR
 This feature is intended to speed up git operations for repos that have
 large working directories.
 
-It enables git to work together with a file system monitor (see the
+It enables git to work together with a file system monitor (see
+linkgit:git-fsmonitor--daemon[1]
+and the
 "fsmonitor-watchman" section of linkgit:githooks[5]) that can
 inform it as to what files have been modified. This enables git to avoid
 having to lstat() every file to find modified files.
@@ -508,17 +510,18 @@ performance by avoiding the cost of scanning the entire working directory
 looking for new files.
 
 If you want to enable (or disable) this feature, it is easier to use
-the `core.fsmonitor` configuration variable (see
-linkgit:git-config[1]) than using the `--fsmonitor` option to
-`git update-index` in each repository, especially if you want to do so
-across all repositories you use, because you can set the configuration
-variable in your `$HOME/.gitconfig` just once and have it affect all
-repositories you touch.
+the `core.fsmonitor` or `core.useBuiltinFSMonitor` configuration
+variable (see linkgit:git-config[1]) than using the `--fsmonitor`
+option to `git update-index` in each repository, especially if you
+want to do so across all repositories you use, because you can set the
+configuration variable in your `$HOME/.gitconfig` just once and have
+it affect all repositories you touch.
 
-When the `core.fsmonitor` configuration variable is changed, the
-file system monitor is added to or removed from the index the next time
-a command reads the index. When `--[no-]fsmonitor` are used, the file
-system monitor is immediately added to or removed from the index.
+When the `core.fsmonitor` or `core.useBuiltinFSMonitor` configuration
+variable is changed, the file system monitor is added to or removed
+from the index the next time a command reads the index. When
+`--[no-]fsmonitor` are used, the file system monitor is immediately
+added to or removed from the index.
 
 CONFIGURATION
 -------------

Documentation/githooks.txt

@@ -593,7 +593,8 @@ fsmonitor-watchman
 
 This hook is invoked when the configuration option `core.fsmonitor` is
 set to `.git/hooks/fsmonitor-watchman` or `.git/hooks/fsmonitor-watchmanv2`
-depending on the version of the hook to use.
+depending on the version of the hook to use, unless overridden via
+`core.useBuiltinFSMonitor` (see linkgit:git-config[1]).
 
 Version 1 takes two arguments, a version (1) and the time in elapsed
 nanoseconds since midnight, January 1, 1970.

Makefile

@@ -899,6 +899,8 @@ LIB_OBJS += fetch-pack.o
 LIB_OBJS += fmt-merge-msg.o
 LIB_OBJS += fsck.o
 LIB_OBJS += fsmonitor.o
+LIB_OBJS += fsmonitor-ipc.o
+LIB_OBJS += fsmonitor-settings.o
 LIB_OBJS += gettext.o
 LIB_OBJS += gpg-interface.o
 LIB_OBJS += graph.o

builtin/update-index.c

@@ -1214,14 +1214,25 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	}
 
 	if (fsmonitor > 0) {
-		if (git_config_get_fsmonitor() == 0)
+		enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
+
+		if (fsm_mode == FSMONITOR_MODE_DISABLED) {
+			warning(_("core.useBuiltinFSMonitor is unset; "
+				"set it if you really want to enable the "
+				"builtin fsmonitor"));
 			warning(_("core.fsmonitor is unset; "
-				"set it if you really want to "
-				"enable fsmonitor"));
+				"set it if you really want to enable the "
+				"hook-based fsmonitor"));
+		}
 		add_fsmonitor(&the_index);
 		report(_("fsmonitor enabled"));
 	} else if (!fsmonitor) {
-		if (git_config_get_fsmonitor() == 1)
+		enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
+		if (fsm_mode == FSMONITOR_MODE_IPC)
+			warning(_("core.useBuiltinFSMonitor is set; "
+				"remove it if you really want to "
+				"disable fsmonitor"));
+		if (fsm_mode == FSMONITOR_MODE_HOOK)
 			warning(_("core.fsmonitor is set; "
 				"remove it if you really want to "
 				"disable fsmonitor"));

cache.h

@@ -999,7 +999,6 @@ extern int core_preload_index;
 extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
-extern const char *core_fsmonitor;
 
 extern int core_apply_sparse_checkout;
 extern int core_sparse_checkout_cone;

config.c

@@ -2508,20 +2508,6 @@ int git_config_get_max_percent_split_change(void)
 	return -1; /* default value */
 }
 
-int git_config_get_fsmonitor(void)
-{
-	if (git_config_get_pathname("core.fsmonitor", &core_fsmonitor))
-		core_fsmonitor = getenv("GIT_TEST_FSMONITOR");
-
-	if (core_fsmonitor && !*core_fsmonitor)
-		core_fsmonitor = NULL;
-
-	if (core_fsmonitor)
-		return 1;
-
-	return 0;
-}
-
 int git_config_get_index_threads(int *dest)
 {
 	int is_bool, val;

config.h

@@ -611,7 +611,6 @@ int git_config_get_pathname(const char *key, const char **dest);
 int git_config_get_index_threads(int *dest);
 int git_config_get_split_index(void);
 int git_config_get_max_percent_split_change(void);
-int git_config_get_fsmonitor(void);
 
 /* This dies if the configured or default date is in the future */
 int git_config_get_expiry(const char *key, const char **output);

environment.c

@@ -84,7 +84,6 @@ int protect_hfs = PROTECT_HFS_DEFAULT;
 #define PROTECT_NTFS_DEFAULT 1
 #endif
 int protect_ntfs = PROTECT_NTFS_DEFAULT;
-const char *core_fsmonitor;
 
 /*
  * The character that begins a commented line in user-editable file

fsmonitor-ipc.c

@@ -0,0 +1,176 @@
+#include "cache.h"
+#include "fsmonitor.h"
+#include "simple-ipc.h"
+#include "fsmonitor-ipc.h"
+#include "run-command.h"
+#include "strbuf.h"
+#include "trace2.h"
+
+#ifdef HAVE_FSMONITOR_DAEMON_BACKEND
+
+int fsmonitor_ipc__is_supported(void)
+{
+	return 1;
+}
+
+GIT_PATH_FUNC(fsmonitor_ipc__get_path, "fsmonitor--daemon.ipc")
+
+enum ipc_active_state fsmonitor_ipc__get_state(void)
+{
+	return ipc_get_active_state(fsmonitor_ipc__get_path());
+}
+
+static int spawn_daemon(void)
+{
+	const char *args[] = { "fsmonitor--daemon", "start", NULL };
+
+	return run_command_v_opt_tr2(args, RUN_COMMAND_NO_STDIN | RUN_GIT_CMD,
+				    "fsmonitor");
+}
+
+int fsmonitor_ipc__send_query(const char *since_token,
+			      struct strbuf *answer)
+{
+	int ret = -1;
+	int tried_to_spawn = 0;
+	enum ipc_active_state state = IPC_STATE__OTHER_ERROR;
+	struct ipc_client_connection *connection = NULL;
+	struct ipc_client_connect_options options
+		= IPC_CLIENT_CONNECT_OPTIONS_INIT;
+	const char *tok = since_token ? since_token : "";
+	size_t tok_len = since_token ? strlen(since_token) : 0;
+
+	options.wait_if_busy = 1;
+	options.wait_if_not_found = 0;
+
+	trace2_region_enter("fsm_client", "query", NULL);
+	trace2_data_string("fsm_client", NULL, "query/command", tok);
+
+try_again:
+	state = ipc_client_try_connect(fsmonitor_ipc__get_path(), &options,
+				       &connection);
+
+	switch (state) {
+	case IPC_STATE__LISTENING:
+		ret = ipc_client_send_command_to_connection(
+			connection, tok, tok_len, answer);
+		ipc_client_close_connection(connection);
+
+		trace2_data_intmax("fsm_client", NULL,
+				   "query/response-length", answer->len);
+
+		if (fsmonitor_is_trivial_response(answer))
+			trace2_data_intmax("fsm_client", NULL,
+					   "query/trivial-response", 1);
+
+		goto done;
+
+	case IPC_STATE__NOT_LISTENING:
+	case IPC_STATE__PATH_NOT_FOUND:
+		if (tried_to_spawn)
+			goto done;
+
+		tried_to_spawn++;
+		if (spawn_daemon())
+			goto done;
+
+		/*
+		 * Try again, but this time give the daemon a chance to
+		 * actually create the pipe/socket.
+		 *
+		 * Granted, the daemon just started so it can't possibly have
+		 * any FS cached yet, so we'll always get a trivial answer.
+		 * BUT the answer should include a new token that can serve
+		 * as the basis for subsequent requests.
+		 */
+		options.wait_if_not_found = 1;
+		goto try_again;
+
+	case IPC_STATE__INVALID_PATH:
+		ret = error(_("fsmonitor_ipc__send_query: invalid path '%s'"),
+			    fsmonitor_ipc__get_path());
+		goto done;
+
+	case IPC_STATE__OTHER_ERROR:
+	default:
+		ret = error(_("fsmonitor_ipc__send_query: unspecified error on '%s'"),
+			    fsmonitor_ipc__get_path());
+		goto done;
+	}
+
+done:
+	trace2_region_leave("fsm_client", "query", NULL);
+
+	return ret;
+}
+
+int fsmonitor_ipc__send_command(const char *command,
+				struct strbuf *answer)
+{
+	struct ipc_client_connection *connection = NULL;
+	struct ipc_client_connect_options options
+		= IPC_CLIENT_CONNECT_OPTIONS_INIT;
+	int ret;
+	enum ipc_active_state state;
+	const char *c = command ? command : "";
+	size_t c_len = command ? strlen(command) : 0;
+
+	strbuf_reset(answer);
+
+	options.wait_if_busy = 1;
+	options.wait_if_not_found = 0;
+
+	state = ipc_client_try_connect(fsmonitor_ipc__get_path(), &options,
+				       &connection);
+	if (state != IPC_STATE__LISTENING) {
+		die("fsmonitor--daemon is not running");
+		return -1;
+	}
+
+	ret = ipc_client_send_command_to_connection(connection, c, c_len,
+						    answer);
+	ipc_client_close_connection(connection);
+
+	if (ret == -1) {
+		die("could not send '%s' command to fsmonitor--daemon", c);
+		return -1;
+	}
+
+	return 0;
+}
+
+#else
+
+/*
+ * A trivial implementation of the fsmonitor_ipc__ API for unsupported
+ * platforms.
+ */
+
+int fsmonitor_ipc__is_supported(void)
+{
+	return 0;
+}
+
+const char *fsmonitor_ipc__get_path(void)
+{
+	return NULL;
+}
+
+enum ipc_active_state fsmonitor_ipc__get_state(void)
+{
+	return IPC_STATE__OTHER_ERROR;
+}
+
+int fsmonitor_ipc__send_query(const char *since_token,
+			      struct strbuf *answer)
+{
+	return -1;
+}
+
+int fsmonitor_ipc__send_command(const char *command,
+				struct strbuf *answer)
+{
+	return -1;
+}
+
+#endif

fsmonitor-ipc.h

@@ -0,0 +1,48 @@
+#ifndef FSMONITOR_IPC_H
+#define FSMONITOR_IPC_H
+
+#include "simple-ipc.h"
+
+/*
+ * Returns true if built-in file system monitor daemon is defined
+ * for this platform.
+ */
+int fsmonitor_ipc__is_supported(void);
+
+/*
+ * Returns the pathname to the IPC named pipe or Unix domain socket
+ * where a `git-fsmonitor--daemon` process will listen.  This is a
+ * per-worktree value.
+ *
+ * Returns NULL if the daemon is not supported on this platform.
+ */
+const char *fsmonitor_ipc__get_path(void);
+
+/*
+ * Try to determine whether there is a `git-fsmonitor--daemon` process
+ * listening on the IPC pipe/socket.
+ */
+enum ipc_active_state fsmonitor_ipc__get_state(void);
+
+/*
+ * Connect to a `git-fsmonitor--daemon` process via simple-ipc
+ * and ask for the set of changed files since the given token.
+ *
+ * Spawn a daemon process in the background if necessary.
+ *
+ * Returns -1 on error; 0 on success.
+ */
+int fsmonitor_ipc__send_query(const char *since_token,
+			      struct strbuf *answer);
+
+/*
+ * Connect to a `git-fsmonitor--daemon` process via simple-ipc and
+ * send a command verb.  If no daemon is available, we DO NOT try to
+ * start one.
+ *
+ * Returns -1 on error; 0 on success.
+ */
+int fsmonitor_ipc__send_command(const char *command,
+				struct strbuf *answer);
+
+#endif /* FSMONITOR_IPC_H */

fsmonitor-settings.c

@@ -0,0 +1,97 @@
+#include "cache.h"
+#include "config.h"
+#include "repository.h"
+#include "fsmonitor-settings.h"
+
+/*
+ * We keep this structure defintion private and have getters
+ * for all fields so that we can lazy load it as needed.
+ */
+struct fsmonitor_settings {
+	enum fsmonitor_mode mode;
+	char *hook_path;
+};
+
+void fsm_settings__set_ipc(struct repository *r)
+{
+	struct fsmonitor_settings *s = r->settings.fsmonitor;
+
+	s->mode = FSMONITOR_MODE_IPC;
+}
+
+void fsm_settings__set_hook(struct repository *r, const char *path)
+{
+	struct fsmonitor_settings *s = r->settings.fsmonitor;
+
+	s->mode = FSMONITOR_MODE_HOOK;
+	s->hook_path = strdup(path);
+}
+
+void fsm_settings__set_disabled(struct repository *r)
+{
+	struct fsmonitor_settings *s = r->settings.fsmonitor;
+
+	s->mode = FSMONITOR_MODE_DISABLED;
+	FREE_AND_NULL(s->hook_path);
+}
+
+static int check_for_ipc(struct repository *r)
+{
+	int value;
+
+	if (!repo_config_get_bool(r, "core.usebuiltinfsmonitor", &value) &&
+	    value) {
+		fsm_settings__set_ipc(r);
+		return 1;
+	}
+
+	return 0;
+}
+
+static int check_for_hook(struct repository *r)
+{
+	const char *const_str;
+
+	if (repo_config_get_pathname(r, "core.fsmonitor", &const_str))
+		const_str = getenv("GIT_TEST_FSMONITOR");
+
+	if (const_str && *const_str) {
+		fsm_settings__set_hook(r, const_str);
+		return 1;
+	}
+
+	return 0;
+}
+
+static void lookup_fsmonitor_settings(struct repository *r)
+{
+	struct fsmonitor_settings *s;
+
+	CALLOC_ARRAY(s, 1);
+
+	r->settings.fsmonitor = s;
+
+	if (check_for_ipc(r))
+		return;
+
+	if (check_for_hook(r))
+		return;
+
+	fsm_settings__set_disabled(r);
+}
+
+enum fsmonitor_mode fsm_settings__get_mode(struct repository *r)
+{
+	if (!r->settings.fsmonitor)
+		lookup_fsmonitor_settings(r);
+
+	return r->settings.fsmonitor->mode;
+}
+
+const char *fsm_settings__get_hook_path(struct repository *r)
+{
+	if (!r->settings.fsmonitor)
+		lookup_fsmonitor_settings(r);
+
+	return r->settings.fsmonitor->hook_path;
+}

fsmonitor-settings.h

@@ -0,0 +1,21 @@
+#ifndef FSMONITOR_SETTINGS_H
+#define FSMONITOR_SETTINGS_H
+
+struct repository;
+
+enum fsmonitor_mode {
+	FSMONITOR_MODE_DISABLED = 0,
+	FSMONITOR_MODE_HOOK = 1, /* core.fsmonitor */
+	FSMONITOR_MODE_IPC = 2,  /* core.useBuiltinFSMonitor */
+};
+
+void fsm_settings__set_ipc(struct repository *r);
+void fsm_settings__set_hook(struct repository *r, const char *path);
+void fsm_settings__set_disabled(struct repository *r);
+
+enum fsmonitor_mode fsm_settings__get_mode(struct repository *r);
+const char *fsm_settings__get_hook_path(struct repository *r);
+
+struct fsmonitor_settings;
+
+#endif /* FSMONITOR_SETTINGS_H */

fsmonitor.c

@@ -3,6 +3,7 @@
 #include "dir.h"
 #include "ewah/ewok.h"
 #include "fsmonitor.h"
+#include "fsmonitor-ipc.h"
 #include "run-command.h"
 #include "strbuf.h"
 
@@ -148,15 +149,18 @@ void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
 /*
  * Call the query-fsmonitor hook passing the last update token of the saved results.
  */
-static int query_fsmonitor(int version, const char *last_update, struct strbuf *query_result)
+static int query_fsmonitor_hook(struct repository *r,
+				int version,
+				const char *last_update,
+				struct strbuf *query_result)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
 	int result;
 
-	if (!core_fsmonitor)
+	if (fsm_settings__get_mode(r) != FSMONITOR_MODE_HOOK)
 		return -1;
 
-	strvec_push(&cp.args, core_fsmonitor);
+	strvec_push(&cp.args, fsm_settings__get_hook_path(r));
 	strvec_pushf(&cp.args, "%d", version);
 	strvec_pushf(&cp.args, "%s", last_update);
 	cp.use_shell = 1;
@@ -238,17 +242,57 @@ void refresh_fsmonitor(struct index_state *istate)
 	struct strbuf last_update_token = STRBUF_INIT;
 	char *buf;
 	unsigned int i;
+	struct repository *r = istate->repo ? istate->repo : the_repository;
+	enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
 
-	if (!core_fsmonitor || istate->fsmonitor_has_run_once)
+	if (fsm_mode <= FSMONITOR_MODE_DISABLED ||
+	    istate->fsmonitor_has_run_once)
 		return;
 
-	hook_version = fsmonitor_hook_version();
-
 	istate->fsmonitor_has_run_once = 1;
 
 	trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");
+
+	if (fsm_mode == FSMONITOR_MODE_IPC) {
+		query_success = !fsmonitor_ipc__send_query(
+			istate->fsmonitor_last_update ?
+			istate->fsmonitor_last_update : "builtin:fake",
+			&query_result);
+		if (query_success) {
+			/*
+			 * The response contains a series of nul terminated
+			 * strings.  The first is the new token.
+			 *
+			 * Use `char *buf` as an interlude to trick the CI
+			 * static analysis to let us use `strbuf_addstr()`
+			 * here (and only copy the token) rather than
+			 * `strbuf_addbuf()`.
+			 */
+			buf = query_result.buf;
+			strbuf_addstr(&last_update_token, buf);
+			bol = last_update_token.len + 1;
+		} else {
+			/*
+			 * The builtin daemon is not available on this
+			 * platform -OR- we failed to get a response.
+			 *
+			 * Generate a fake token (rather than a V1
+			 * timestamp) for the index extension.  (If
+			 * they switch back to the hook API, we don't
+			 * want ambiguous state.)
+			 */
+			strbuf_addstr(&last_update_token, "builtin:fake");
+		}
+
+		goto apply_results;
+	}
+
+	assert(fsm_mode == FSMONITOR_MODE_HOOK);
+
+	hook_version = fsmonitor_hook_version();
+
 	/*
-	 * This could be racy so save the date/time now and query_fsmonitor
+	 * This could be racy so save the date/time now and query_fsmonitor_hook
 	 * should be inclusive to ensure we don't miss potential changes.
 	 */
 	last_update = getnanotime();
@@ -256,13 +300,14 @@ void refresh_fsmonitor(struct index_state *istate)
 		strbuf_addf(&last_update_token, "%"PRIu64"", last_update);
 
 	/*
-	 * If we have a last update token, call query_fsmonitor for the set of
+	 * If we have a last update token, call query_fsmonitor_hook for the set of
 	 * changes since that token, else assume everything is possibly dirty
 	 * and check it all.
 	 */
 	if (istate->fsmonitor_last_update) {
 		if (hook_version == -1 || hook_version == HOOK_INTERFACE_VERSION2) {
-			query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION2,
+			query_success = !query_fsmonitor_hook(
+				r, HOOK_INTERFACE_VERSION2,
 				istate->fsmonitor_last_update, &query_result);
 
 			if (query_success) {
@@ -292,18 +337,39 @@ void refresh_fsmonitor(struct index_state *istate)
 		}
 
 		if (hook_version == HOOK_INTERFACE_VERSION1) {
-			query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION1,
+			query_success = !query_fsmonitor_hook(
+				r, HOOK_INTERFACE_VERSION1,
 				istate->fsmonitor_last_update, &query_result);
 		}
 
-		trace_performance_since(last_update, "fsmonitor process '%s'", core_fsmonitor);
-		trace_printf_key(&trace_fsmonitor, "fsmonitor process '%s' returned %s",
-			core_fsmonitor, query_success ? "success" : "failure");
+		trace_performance_since(last_update, "fsmonitor process '%s'",
+					fsm_settings__get_hook_path(r));
+		trace_printf_key(&trace_fsmonitor,
+				 "fsmonitor process '%s' returned %s",
+				 fsm_settings__get_hook_path(r),
+				 query_success ? "success" : "failure");
 	}
 
-	/* a fsmonitor process can return '/' to indicate all entries are invalid */
+apply_results:
+	/*
+	 * The response from FSMonitor (excluding the header token) is
+	 * either:
+	 *
+	 * [a] a (possibly empty) list of NUL delimited relative
+	 *     pathnames of changed paths.  This list can contain
+	 *     files and directories.  Directories have a trailing
+	 *     slash.
+	 *
+	 * [b] a single '/' to indicate the provider had no
+	 *     information and that we should consider everything
+	 *     invalid.  We call this a trivial response.
+	 */
 	if (query_success && query_result.buf[bol] != '/') {
-		/* Mark all entries returned by the monitor as dirty */
+		/*
+		 * Mark all pathnames returned by the monitor as dirty.
+		 *
+		 * This updates both the cache-entries and the untracked-cache.
+		 */
 		buf = query_result.buf;
 		for (i = bol; i < query_result.len; i++) {
 			if (buf[i] != '\0')
@@ -318,11 +384,15 @@ void refresh_fsmonitor(struct index_state *istate)
 		if (istate->untracked)
 			istate->untracked->use_fsmonitor = 1;
 	} else {
-
-		/* We only want to run the post index changed hook if we've actually changed entries, so keep track
-		 * if we actually changed entries or not */
+		/*
+		 * We received a trivial response, so invalidate everything.
+		 *
+		 * We only want to run the post index changed hook if
+		 * we've actually changed entries, so keep track if we
+		 * actually changed entries or not.
+		 */
 		int is_cache_changed = 0;
-		/* Mark all entries invalid */
+
 		for (i = 0; i < istate->cache_nr; i++) {
 			if (istate->cache[i]->ce_flags & CE_FSMONITOR_VALID) {
 				is_cache_changed = 1;
@@ -330,7 +400,10 @@ void refresh_fsmonitor(struct index_state *istate)
 			}
 		}
 
-		/* If we're going to check every file, ensure we save the results */
+		/*
+		 * If we're going to check every file, ensure we save
+		 * the results.
+		 */
 		if (is_cache_changed)
 			istate->cache_changed |= FSMONITOR_CHANGED;
 
@@ -411,7 +484,8 @@ void remove_fsmonitor(struct index_state *istate)
 void tweak_fsmonitor(struct index_state *istate)
 {
 	unsigned int i;
-	int fsmonitor_enabled = git_config_get_fsmonitor();
+	struct repository *r = istate->repo ? istate->repo : the_repository;
+	int fsmonitor_enabled = (fsm_settings__get_mode(r) > FSMONITOR_MODE_DISABLED);
 
 	if (istate->fsmonitor_dirty) {
 		if (fsmonitor_enabled) {
@@ -431,16 +505,8 @@ void tweak_fsmonitor(struct index_state *istate)
 		istate->fsmonitor_dirty = NULL;
 	}
 
-	switch (fsmonitor_enabled) {
-	case -1: /* keep: do nothing */
-		break;
-	case 0: /* false */
-		remove_fsmonitor(istate);
-		break;
-	case 1: /* true */
+	if (fsmonitor_enabled)
 		add_fsmonitor(istate);
-		break;
-	default: /* unknown value: do nothing */
-		break;
-	}
+	else
+		remove_fsmonitor(istate);
 }

fsmonitor.h

@@ -3,6 +3,7 @@
 
 #include "cache.h"
 #include "dir.h"
+#include "fsmonitor-settings.h"
 
 extern struct trace_key trace_fsmonitor;
 
@@ -57,7 +58,11 @@ int fsmonitor_is_trivial_response(const struct strbuf *query_result);
  */
 static inline int is_fsmonitor_refreshed(const struct index_state *istate)
 {
-	return !core_fsmonitor || istate->fsmonitor_has_run_once;
+	struct repository *r = istate->repo ? istate->repo : the_repository;
+	enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
+
+	return fsm_mode <= FSMONITOR_MODE_DISABLED ||
+		istate->fsmonitor_has_run_once;
 }
 
 /*
@@ -67,7 +72,11 @@ static inline int is_fsmonitor_refreshed(const struct index_state *istate)
  */
 static inline void mark_fsmonitor_valid(struct index_state *istate, struct cache_entry *ce)
 {
-	if (core_fsmonitor && !(ce->ce_flags & CE_FSMONITOR_VALID)) {
+	struct repository *r = istate->repo ? istate->repo : the_repository;
+	enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
+
+	if (fsm_mode > FSMONITOR_MODE_DISABLED &&
+	    !(ce->ce_flags & CE_FSMONITOR_VALID)) {
 		istate->cache_changed = 1;
 		ce->ce_flags |= CE_FSMONITOR_VALID;
 		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_clean '%s'", ce->name);
@@ -83,7 +92,10 @@ static inline void mark_fsmonitor_valid(struct index_state *istate, struct cache
  */
 static inline void mark_fsmonitor_invalid(struct index_state *istate, struct cache_entry *ce)
 {
-	if (core_fsmonitor) {
+	struct repository *r = istate->repo ? istate->repo : the_repository;
+	enum fsmonitor_mode fsm_mode = fsm_settings__get_mode(r);
+
+	if (fsm_mode > FSMONITOR_MODE_DISABLED) {
 		ce->ce_flags &= ~CE_FSMONITOR_VALID;
 		untracked_cache_invalidate_path(istate, ce->name, 1);
 		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_invalid '%s'", ce->name);

repo-settings.c

@@ -63,6 +63,8 @@ void prepare_repo_settings(struct repository *r)
 	/*
 	 * Non-boolean config
 	 */
+	r->settings.fsmonitor = NULL; /* lazy loaded */
+
 	if (!repo_config_get_int(r, "index.version", &value))
 		r->settings.index_version = value;
 

repository.h

@@ -4,6 +4,7 @@
 #include "path.h"
 
 struct config_set;
+struct fsmonitor_settings;
 struct git_hash_algo;
 struct index_state;
 struct lock_file;
@@ -35,6 +36,8 @@ struct repo_settings {
 	int command_requires_full_index;
 	int sparse_index;
 
+	struct fsmonitor_settings *fsmonitor; /* lazy loaded */
+
 	int index_version;
 	enum untracked_cache_setting core_untracked_cache;
 

t/README

@@ -405,8 +405,8 @@ every 'git commit-graph write', as if the `--changed-paths` option was
 passed in.
 
 GIT_TEST_FSMONITOR=$PWD/t7519/fsmonitor-all exercises the fsmonitor
-code path for utilizing a file system monitor to speed up detecting
-new or changed files.
+code path for utilizing a (hook based) file system monitor to speed up
+detecting new or changed files.
 
 GIT_TEST_INDEX_VERSION=<n> exercises the index read/write code path
 for the index version specified.  Can be set to any valid version