fsmonitor-settings: remote repos on macOS are incompatible

Teach Git to detect remote working directories on macOS and mark them as
incompatible with FSMonitor.

With this, `git fsmonitor--daemon run` will error out with a message
like it does for bare repos.

Client commands, like `git status`, will not attempt to start the daemon.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/fsmonitor/fsm-settings-darwin.c

@@ -2,8 +2,74 @@
 #include "config.h"
 #include "repository.h"
 #include "fsmonitor-settings.h"
+#include "fsmonitor.h"
+#include <sys/param.h>
+#include <sys/mount.h>
+
+/*
+ * Remote working directories are problematic for FSMonitor.
+ *
+ * The underlying file system on the server machine and/or the remote
+ * mount type (NFS, SAMBA, etc.) dictates whether notification events
+ * are available at all to remote client machines.
+ *
+ * Kernel differences between the server and client machines also
+ * dictate the how (buffering, frequency, de-dup) the events are
+ * delivered to client machine processes.
+ *
+ * A client machine (such as a laptop) may choose to suspend/resume
+ * and it is unclear (without lots of testing) whether the watcher can
+ * resync after a resume.  We might be able to treat this as a normal
+ * "events were dropped by the kernel" event and do our normal "flush
+ * and resync" --or-- we might need to close the existing (zombie?)
+ * notification fd and create a new one.
+ *
+ * In theory, the above issues need to be addressed whether we are
+ * using the Hook or IPC API.
+ *
+ * For the builtin FSMonitor, we create the Unix domain socket for the
+ * IPC in the .git directory.  If the working directory is remote,
+ * then the socket will be created on the remote file system.  This
+ * can fail if the remote file system does not support UDS file types
+ * (e.g. smbfs to a Windows server) or if the remote kernel does not
+ * allow a non-local process to bind() the socket.  (These problems
+ * could be fixed by moving the UDS out of the .git directory and to a
+ * well-known local directory on the client machine, but care should
+ * be taken to ensure that $HOME is actually local and not a managed
+ * file share.)
+ *
+ * So (for now at least), mark remote working directories as
+ * incompatible.
+ */
+static enum fsmonitor_reason check_remote(struct repository *r)
+{
+	struct statfs fs;
+
+	if (statfs(r->worktree, &fs) == -1) {
+		int saved_errno = errno;
+		trace_printf_key(&trace_fsmonitor, "statfs('%s') failed: %s",
+				 r->worktree, strerror(saved_errno));
+		errno = saved_errno;
+		return FSMONITOR_REASON_ERROR;
+	}
+
+	trace_printf_key(&trace_fsmonitor,
+			 "statfs('%s') [type 0x%08x][flags 0x%08x] '%s'",
+			 r->worktree, fs.f_type, fs.f_flags, fs.f_fstypename);
+
+	if (!(fs.f_flags & MNT_LOCAL))
+		return FSMONITOR_REASON_REMOTE;
+
+	return FSMONITOR_REASON_OK;
+}
 
 enum fsmonitor_reason fsm_os__incompatible(struct repository *r)
 {
+	enum fsmonitor_reason reason;
+
+	reason = check_remote(r);
+	if (reason != FSMONITOR_REASON_OK)
+		return reason;
+
 	return FSMONITOR_REASON_OK;
 }

fsmonitor-settings.c

@@ -208,6 +208,18 @@ char *fsm_settings__get_incompatible_msg(const struct repository *r,
 			    xgetcwd());
 		goto done;
 
+	case FSMONITOR_REASON_ERROR:
+		strbuf_addf(&msg,
+			    _("repository '%s' is incompatible with fsmonitor due to errors"),
+			    r->worktree);
+		goto done;
+
+	case FSMONITOR_REASON_REMOTE:
+		strbuf_addf(&msg,
+			    _("remote repository '%s' is incompatible with fsmonitor"),
+			    r->worktree);
+		goto done;
+
 	case FSMONITOR_REASON_VFS4GIT:
 		strbuf_addf(&msg,
 			    _("virtual repository '%s' is incompatible with fsmonitor"),

fsmonitor-settings.h

@@ -17,6 +17,8 @@ enum fsmonitor_reason {
 	FSMONITOR_REASON_UNTESTED = 0,
 	FSMONITOR_REASON_OK, /* no incompatibility or when disabled */
 	FSMONITOR_REASON_BARE,
+	FSMONITOR_REASON_ERROR, /* FS error probing for compatibility */
+	FSMONITOR_REASON_REMOTE,
 	FSMONITOR_REASON_VFS4GIT, /* VFS for Git virtualization */
 };