fsmonitor: config settings are repository-specific

Move fsmonitor config settings to a new and opaque
`struct fsmonitor_settings` structure.  Add a lazily-loaded pointer
to this into `struct repo_settings`

Create an `enum fsmonitor_mode` type in `struct fsmonitor_settings` to
represent the state of fsmonitor.  This lets us represent which, if
any, fsmonitor provider (hook or IPC) is enabled.

Create `fsm_settings__get_*()` getters to lazily look up fsmonitor-
related config settings.

Get rid of the `core_fsmonitor` global variable.  Move the code to
lookup the existing `core.fsmonitor` config value into the fsmonitor
settings.

Create a hook pathname variable in `struct fsmonitor-settings` and
only set it when in hook mode.

Extend the definition of `core.fsmonitor` to be either a boolean
or a hook pathname.  When true, the builtin FSMonitor is used.
When false or unset, no FSMonitor (neither builtin nor hook) is
used.

The existing `core_fsmonitor` global variable was used to store the
pathname to the fsmonitor hook *and* it was used as a boolean to see
if fsmonitor was enabled.  This dual usage and global visibility leads
to confusion when we add the IPC-based provider.  So lets hide the
details in fsmonitor-settings.c and let it decide which provider to
use in the case of multiple settings.  This avoids cluttering up
repo-settings.c with these private details.

A future commit in builtin-fsmonitor series will add the ability to
disqualify worktrees for various reasons, such as being mounted from a
remote volume, where fsmonitor should not be started.  Having the
config settings hidden in fsmonitor-settings.c allows such worktree
restrictions to override the config values used.

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

fsmonitor-settings.c

@@ -0,0 +1,114 @@
+#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;
+};
+
+static void lookup_fsmonitor_settings(struct repository *r)
+{
+	struct fsmonitor_settings *s;
+	const char *const_str;
+	int bool_value;
+
+	if (r->settings.fsmonitor)
+		return;
+
+	CALLOC_ARRAY(s, 1);
+	s->mode = FSMONITOR_MODE_DISABLED;
+
+	r->settings.fsmonitor = s;
+
+	/*
+	 * Overload the existing "core.fsmonitor" config setting (which
+	 * has historically been either unset or a hook pathname) to
+	 * now allow a boolean value to enable the builtin FSMonitor
+	 * or to turn everything off.  (This does imply that you can't
+	 * use a hook script named "true" or "false", but that's OK.)
+	 */
+	switch (repo_config_get_maybe_bool(r, "core.fsmonitor", &bool_value)) {
+
+	case 0: /* config value was set to <bool> */
+		if (bool_value)
+			fsm_settings__set_ipc(r);
+		return;
+
+	case 1: /* config value was unset */
+		const_str = getenv("GIT_TEST_FSMONITOR");
+		break;
+
+	case -1: /* config value set to an arbitrary string */
+		if (repo_config_get_pathname(r, "core.fsmonitor", &const_str))
+			return; /* should not happen */
+		break;
+
+	default: /* should not happen */
+		return;
+	}
+
+	if (!const_str || !*const_str)
+		return;
+
+	fsm_settings__set_hook(r, const_str);
+}
+
+enum fsmonitor_mode fsm_settings__get_mode(struct repository *r)
+{
+	if (!r)
+		r = the_repository;
+
+	lookup_fsmonitor_settings(r);
+
+	return r->settings.fsmonitor->mode;
+}
+
+const char *fsm_settings__get_hook_path(struct repository *r)
+{
+	if (!r)
+		r = the_repository;
+
+	lookup_fsmonitor_settings(r);
+
+	return r->settings.fsmonitor->hook_path;
+}
+
+void fsm_settings__set_ipc(struct repository *r)
+{
+	if (!r)
+		r = the_repository;
+
+	lookup_fsmonitor_settings(r);
+
+	r->settings.fsmonitor->mode = FSMONITOR_MODE_IPC;
+	FREE_AND_NULL(r->settings.fsmonitor->hook_path);
+}
+
+void fsm_settings__set_hook(struct repository *r, const char *path)
+{
+	if (!r)
+		r = the_repository;
+
+	lookup_fsmonitor_settings(r);
+
+	r->settings.fsmonitor->mode = FSMONITOR_MODE_HOOK;
+	FREE_AND_NULL(r->settings.fsmonitor->hook_path);
+	r->settings.fsmonitor->hook_path = strdup(path);
+}
+
+void fsm_settings__set_disabled(struct repository *r)
+{
+	if (!r)
+		r = the_repository;
+
+	lookup_fsmonitor_settings(r);
+
+	r->settings.fsmonitor->mode = FSMONITOR_MODE_DISABLED;
+	FREE_AND_NULL(r->settings.fsmonitor->hook_path);
+}