fsmonitor--daemon: man page and documentation

Create a manual page describing the `git fsmonitor--daemon` feature.

Update references to `core.fsmonitor`, `core.fsmonitorHookVersion` and
pointers to `watchman` to mention the built-in FSMonitor.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>

Documentation/config/core.txt

@@ -66,18 +66,43 @@ core.fsmonitor::
 	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].
++
+See the "fsmonitor-watchman" section of linkgit:githooks[5].
++
+Note: FSMonitor hooks (and this config setting) are ignored if the
+built-in FSMonitor 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 version of hook that is to be used when calling the
+	FSMonitor hook (as configured via `core.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.
++
+Note: FSMonitor hooks (and this config setting) are ignored if the
+built-in FSMonitor is enabled (see `core.useBuiltinFSMonitor`).
+
+core.useBuiltinFSMonitor::
+	If set to true, enable the built-in filesystem event watcher (for
+	technical details, see linkgit:git-fsmonitor--daemon[1]).
++
+Like external (hook-based) FSMonitors, the built-in FSMonitor can speed up
+Git commands that need to refresh the Git index (e.g. `git status`) in a
+worktree with many files. The built-in FSMonitor facility eliminates the
+need to install and maintain an external third-party monitoring tool.
++
+The built-in FSMonitor is currently available only on a limited set of
+supported platforms.
++
+Note: if this config setting is set to `true`, any FSMonitor hook
+configured via `core.fsmonitor` (and possibly `core.fsmonitorHookVersion`)
+is ignored.
 
 core.trustctime::
 	If false, the ctime differences between the index and the

Documentation/git-fsmonitor--daemon.txt

@@ -0,0 +1,104 @@
+git-fsmonitor--daemon(1)
+========================
+
+NAME
+----
+git-fsmonitor--daemon - Builtin file system monitor daemon
+
+SYNOPSIS
+--------
+[verse]
+'git fsmonitor--daemon' --start
+'git fsmonitor--daemon' --run
+'git fsmonitor--daemon' --stop
+'git fsmonitor--daemon' --is-running
+'git fsmonitor--daemon' --is-supported
+'git fsmonitor--daemon' --query <token>
+'git fsmonitor--daemon' --query-index
+'git fsmonitor--daemon' --flush
+
+DESCRIPTION
+-----------
+
+Monitors files and directories in the working directory for changes using
+platform-specific file system notification facilities.
+
+It communicates directly with commands like `git status` using the
+link:technical/api-simple-ipc.html[simple IPC] interface instead of
+the slower linkgit:githooks[5] interface.
+
+OPTIONS
+-------
+
+--start::
+	Starts the fsmonitor daemon in the background.
+
+--run::
+	Runs the fsmonitor daemon in the foreground.
+
+--stop::
+	Stops the fsmonitor daemon running for the current working
+	directory, if present.
+
+--is-running::
+	Exits with zero status if the fsmonitor daemon is watching the
+	current working directory.
+
+--is-supported::
+	Exits with zero status if the fsmonitor daemon feature is supported
+	on this platform.
+
+--query <token>::
+	Connects to the fsmonitor daemon (starting it if necessary) and
+	requests the list of changed files and directories since the
+	given token.
+	This is intended for testing purposes.
+
+--query-index::
+	Read the current `<token>` from the File System Monitor index
+	extension (if present) and use it to query the fsmonitor daemon.
+	This is intended for testing purposes.
+
+--flush::
+	Force the fsmonitor daemon to flush its in-memory cache and
+	re-sync with the file system.
+	This is intended for testing purposes.
+
+REMARKS
+-------
+The fsmonitor daemon is a long running process that will watch a single
+working directory.  Commands, such as `git status`, should automatically
+start it (if necessary) when `core.useBuiltinFSMonitor` is set to `true`
+(see linkgit:git-config[1]).
+
+Configure the built-in FSMonitor via `core.useBuiltinFSMonitor` in each
+working directory separately, or globally via `git config --global
+core.useBuiltinFSMonitor true`.
+
+Tokens are opaque strings.  They are used by the fsmonitor daemon to
+mark a point in time and the associated internal state.  Callers should
+make no assumptions about the content of the token.  In particular,
+the should not assume that it is a timestamp.
+
+Query commands send a request-token to the daemon and it responds with
+a summary of the changes that have occurred since that token was
+created.  The daemon also returns a response-token that the client can
+use in a future query.
+
+For more information see the "File System Monitor" section in
+linkgit:git-update-index[1].
+
+CAVEATS
+-------
+
+The fsmonitor daemon does not currently know about submodules and does
+not know to filter out file system events that happen within a
+submodule.  If fsmonitor daemon is watching a super repo and a file is
+modified within the working directory of a submodule, it will report
+the change (as happening against the super repo).  However, the client
+should properly ignore these extra events, so performance may be affected
+but it should not cause an incorrect result.
+
+GIT
+---
+Part of the linkgit:git[1] suite

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.

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.