fsmonitor: implement filesystem change listener for Linux

Implement the built-in fsmonitor daemon for Linux using the inotify
API, bringing it to feature parity with the existing Windows and macOS
implementations.

The implementation uses inotify rather than fanotify because fanotify
requires either CAP_SYS_ADMIN or CAP_PERFMON capabilities, making it
unsuitable for an unprivileged user-space daemon.  While inotify has
the limitation of requiring a separate watch on every directory (unlike
macOS's FSEvents, which can monitor an entire directory tree with a
single watch), it operates without elevated privileges and provides
the per-file event granularity needed for fsmonitor.

The listener uses inotify_init1(O_NONBLOCK) with a poll loop that
checks for events with a 50-millisecond timeout, keeping the inotify
queue well-drained to minimize the risk of overflows.  Bidirectional
hashmaps map between watch descriptors and directory paths for efficient
event resolution.  Directory renames are tracked using inotify's cookie
mechanism to correlate IN_MOVED_FROM and IN_MOVED_TO event pairs; a
periodic check detects stale renames where the matching IN_MOVED_TO
never arrived, forcing a resync.

New directory creation triggers recursive watch registration to ensure
all subdirectories are monitored.  The IN_MASK_CREATE flag is used
where available to prevent modifying existing watches, with a fallback
for older kernels.  When IN_MASK_CREATE is available and
inotify_add_watch returns EEXIST, it means another thread or recursive
scan has already registered the watch, so it is safe to ignore.

Remote filesystem detection uses statfs() to identify network-mounted
filesystems (NFS, CIFS, SMB, FUSE, etc.) via their magic numbers.
Mount point information is read from /proc/mounts and matched against
the statfs f_fsid to get accurate, human-readable filesystem type names
for logging.  When the .git directory is on a remote filesystem, the
IPC socket falls back to $HOME or a user-configured directory via the
fsmonitor.socketDir setting.

Based-on-patch-by: Eric DeCosta <edecosta@mathworks.com>
Based-on-patch-by: Marziyeh Esipreh <marziyeh.esipreh@gmail.com>
Signed-off-by: Paul Tarjan <github@paulisageek.com>

Author: ptarjan

Documentation/config/fsmonitor--daemon.adoc

@@ -4,8 +4,8 @@ fsmonitor.allowRemote::
     behavior.  Only respected when `core.fsmonitor` is set to `true`.
 
 fsmonitor.socketDir::
-    This Mac OS-specific option, if set, specifies the directory in
+    This Mac OS and Linux-specific option, if set, specifies the directory in
     which to create the Unix domain socket used for communication
     between the fsmonitor daemon and various Git commands. The directory must
-    reside on a native Mac OS filesystem.  Only respected when `core.fsmonitor`
+    reside on a native filesystem.  Only respected when `core.fsmonitor`
     is set to `true`.

Documentation/git-fsmonitor--daemon.adoc

@@ -76,9 +76,9 @@ repositories; this may be overridden by setting `fsmonitor.allowRemote` to
 correctly with all network-mounted repositories, so such use is considered
 experimental.
 
-On Mac OS, the inter-process communication (IPC) between various Git
+On Mac OS and Linux, the inter-process communication (IPC) between various Git
 commands and the fsmonitor daemon is done via a Unix domain socket (UDS) -- a
-special type of file -- which is supported by native Mac OS filesystems,
+special type of file -- which is supported by native Mac OS and Linux filesystems,
 but not on network-mounted filesystems, NTFS, or FAT32.  Other filesystems
 may or may not have the needed support; the fsmonitor daemon is not guaranteed
 to work with these filesystems and such use is considered experimental.
@@ -87,13 +87,33 @@ By default, the socket is created in the `.git` directory.  However, if the
 `.git` directory is on a network-mounted filesystem, it will instead be
 created at `$HOME/.git-fsmonitor-*` unless `$HOME` itself is on a
 network-mounted filesystem, in which case you must set the configuration
-variable `fsmonitor.socketDir` to the path of a directory on a Mac OS native
+variable `fsmonitor.socketDir` to the path of a directory on a native
 filesystem in which to create the socket file.
 
 If none of the above directories (`.git`, `$HOME`, or `fsmonitor.socketDir`)
-is on a native Mac OS file filesystem the fsmonitor daemon will report an
+is on a native filesystem the fsmonitor daemon will report an
 error that will cause the daemon and the currently running command to exit.
 
+LINUX CAVEATS
+~~~~~~~~~~~~~
+
+On Linux, the fsmonitor daemon uses inotify to monitor filesystem events.
+The inotify system has per-user limits on the number of watches that can
+be created.  The default limit is typically 8192 watches per user.
+
+For large repositories with many directories, you may need to increase
+this limit.  Check the current limit with:
+
+    cat /proc/sys/fs/inotify/max_user_watches
+
+To temporarily increase the limit:
+
+    sudo sysctl fs.inotify.max_user_watches=65536
+
+To make the change permanent, add to `/etc/sysctl.conf`:
+
+    fs.inotify.max_user_watches=65536
+
 CONFIGURATION
 -------------
 

compat/fsmonitor/fsm-health-linux.c

@@ -0,0 +1,33 @@
+#include "git-compat-util.h"
+#include "config.h"
+#include "fsmonitor-ll.h"
+#include "fsm-health.h"
+#include "fsmonitor--daemon.h"
+
+/*
+ * The Linux fsmonitor implementation uses inotify which has its own
+ * mechanisms for detecting filesystem unmount and other events that
+ * would require the daemon to shutdown.  Therefore, we don't need
+ * a separate health thread like Windows does.
+ *
+ * These stub functions satisfy the interface requirements.
+ */
+
+int fsm_health__ctor(struct fsmonitor_daemon_state *state UNUSED)
+{
+	return 0;
+}
+
+void fsm_health__dtor(struct fsmonitor_daemon_state *state UNUSED)
+{
+	return;
+}
+
+void fsm_health__loop(struct fsmonitor_daemon_state *state UNUSED)
+{
+	return;
+}
+
+void fsm_health__stop_async(struct fsmonitor_daemon_state *state UNUSED)
+{
+}