fsmonitor-settings: VFS for Git virtual repos are incompatible

VFS for Git virtual repositories are incompatible with FSMonitor.

VFS for Git is a downstream fork of Git.  It contains its own custom
file system watcher that is aware of the virtualization.  If a working
directory is being managed by VFS for Git, we should not try to watch
it because we may get incomplete results.

We do not know anything about how VFS for Git works, but we do
know that VFS for Git working directories contain a well-defined
config setting.  If it is set, mark the working directory as
incompatible.

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

compat/fsmonitor/fsm-settings-win32.c

@@ -3,7 +3,33 @@
 #include "repository.h"
 #include "fsmonitor-settings.h"
 
-enum fsmonitor_reason fsm_os__incompatible(struct repository *r)
+/*
+ * VFS for Git is incompatible with FSMonitor.
+ *
+ * Granted, core Git does not know anything about VFS for Git and we
+ * shouldn't make assumptions about a downstream feature, but users
+ * can install both versions.  And this can lead to incorrect results
+ * from core Git commands.  So, without bringing in any of the VFS for
+ * Git code, do a simple config test for a published config setting.
+ * (We do not look at the various *_TEST_* environment variables.)
+ */
+static enum fsmonitor_reason check_vfs4git(struct repository *r)
 {
+	const char *const_str;
+
+	if (!repo_config_get_value(r, "core.virtualfilesystem", &const_str))
+		return FSMONITOR_REASON_VFS4GIT;
+
+	return FSMONITOR_REASON_OK;
+}
+
+enum fsmonitor_reason fsm_os__incompatible(struct repository *r)
+{
+	enum fsmonitor_reason reason;
+
+	reason = check_vfs4git(r);
+	if (reason != FSMONITOR_REASON_OK)
+		return reason;
+
 	return FSMONITOR_REASON_OK;
 }

fsmonitor-settings.c

@@ -207,6 +207,12 @@ char *fsm_settings__get_incompatible_msg(const struct repository *r,
 			    _("bare repository '%s' is incompatible with fsmonitor"),
 			    xgetcwd());
 		goto done;
+
+	case FSMONITOR_REASON_VFS4GIT:
+		strbuf_addf(&msg,
+			    _("virtual repository '%s' is incompatible with fsmonitor"),
+			    r->worktree);
+		goto done;
 	}
 
 	BUG("Unhandled case in fsm_settings__get_incompatible_msg: '%d'",

fsmonitor-settings.h

@@ -17,6 +17,7 @@ enum fsmonitor_reason {
 	FSMONITOR_REASON_UNTESTED = 0,
 	FSMONITOR_REASON_OK, /* no incompatibility or when disabled */
 	FSMONITOR_REASON_BARE,
+	FSMONITOR_REASON_VFS4GIT, /* VFS for Git virtualization */
 };
 
 void fsm_settings__set_ipc(struct repository *r);

t/t7519-status-fsmonitor.sh

@@ -78,6 +78,15 @@ test_expect_success FSMONITOR_DAEMON 'run fsmonitor-daemon in bare repo' '
 	grep "bare repository .* is incompatible with fsmonitor" actual
 '
 
+test_expect_success MINGW,FSMONITOR_DAEMON 'run fsmonitor-daemon in virtual repo' '
+	test_when_finished "rm -rf ./fake-virtual-clone actual" &&
+	git init fake-virtual-clone &&
+	test_must_fail git -C ./fake-virtual-clone \
+			   -c core.virtualfilesystem=true \
+			   fsmonitor--daemon run 2>actual &&
+	grep "virtual repository .* is incompatible with fsmonitor" actual
+'
+
 test_expect_success 'setup' '
 	mkdir -p .git/hooks &&
 	: >tracked &&