fsmonitor: reintroduce core.useBuiltinFSMonitor

Reintroduce the 'core.useBuiltinFSMonitor' config setting (originally added
in 0a756b2a25 (fsmonitor: config settings are repository-specific,
2021-03-05)) after its removal from the upstream version of FSMonitor.

Upstream, the 'core.useBuiltinFSMonitor' setting was rendered obsolete by
"overloading" the 'core.fsmonitor' setting to take a boolean value. However,
several applications (e.g., 'scalar') utilize the original config setting,
so it should be preserved for a deprecation period before complete removal:

* if 'core.fsmonitor' is a boolean, the user is correctly using the new
  config syntax; do not use 'core.useBuiltinFSMonitor'.
* if 'core.fsmonitor' is unspecified, use 'core.useBuiltinFSMonitor'.
* if 'core.fsmonitor' is a path, override and use the builtin FSMonitor if
  'core.useBuiltinFSMonitor' is 'true'; otherwise, use the FSMonitor hook
  indicated by the path.

Additionally, for this deprecation period, advise users to switch to using
'core.fsmonitor' to specify their use of the builtin FSMonitor.

Signed-off-by: Victoria Dye <vdye@github.com>

Documentation/config/advice.txt

@@ -136,4 +136,7 @@ advice.*::
 		Advice shown when either linkgit:git-add[1] or linkgit:git-rm[1]
 		is asked to update index entries outside the current sparse
 		checkout.
+	useCoreFSMonitorConfig::
+		Advice shown if the deprecated 'core.useBuiltinFSMonitor' config
+		setting is in use.
 --

advice.c

@@ -74,6 +74,7 @@ static struct {
 	[ADVICE_SUBMODULE_ALTERNATE_ERROR_STRATEGY_DIE] = { "submoduleAlternateErrorStrategyDie", 1 },
 	[ADVICE_SUBMODULES_NOT_UPDATED] 		= { "submodulesNotUpdated", 1 },
 	[ADVICE_UPDATE_SPARSE_PATH]			= { "updateSparsePath", 1 },
+	[ADVICE_USE_CORE_FSMONITOR_CONFIG]		= { "useCoreFSMonitorConfig", 1 },
 	[ADVICE_WAITING_FOR_EDITOR]			= { "waitingForEditor", 1 },
 };
 

advice.h

@@ -48,6 +48,7 @@ struct string_list;
 	ADVICE_SUBMODULE_ALTERNATE_ERROR_STRATEGY_DIE,
 	ADVICE_SUBMODULES_NOT_UPDATED,
 	ADVICE_UPDATE_SPARSE_PATH,
+	ADVICE_USE_CORE_FSMONITOR_CONFIG,
 	ADVICE_WAITING_FOR_EDITOR,
 	ADVICE_SKIPPED_CHERRY_PICKS,
 };

fsmonitor-settings.c

@@ -98,6 +98,31 @@ static struct fsmonitor_settings *alloc_settings(void)
 	return s;
 }
 
+static int check_deprecated_builtin_config(struct repository *r)
+{
+	int core_use_builtin_fsmonitor = 0;
+
+	/*
+	 * If 'core.useBuiltinFSMonitor' is set, print a deprecation warning
+	 * suggesting the use of 'core.fsmonitor' instead. If the config is
+	 * set to true, set the appropriate mode and return 1 indicating that
+	 * the check resulted the config being set by this (deprecated) setting.
+	 */
+	if(!repo_config_get_bool(r, "core.useBuiltinFSMonitor", &core_use_builtin_fsmonitor) &&
+	   core_use_builtin_fsmonitor) {
+		if (!git_env_bool("GIT_SUPPRESS_USEBUILTINFSMONITOR_ADVICE", 0)) {
+			advise_if_enabled(ADVICE_USE_CORE_FSMONITOR_CONFIG,
+					  _("core.useBuiltinFSMonitor=true is deprecated;"
+					    "please set core.fsmonitor=true instead"));
+			setenv("GIT_SUPPRESS_USEBUILTINFSMONITOR_ADVICE", "1", 1);
+		}
+		fsm_settings__set_ipc(r);
+		return 1;
+	}
+
+	return 0;
+}
+
 static void lookup_fsmonitor_settings(struct repository *r)
 {
 	const char *const_str;
@@ -123,12 +148,16 @@ static void lookup_fsmonitor_settings(struct repository *r)
 		return;
 
 	case 1: /* config value was unset */
+		if (check_deprecated_builtin_config(r))
+			return;
+
 		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 */
+		if (check_deprecated_builtin_config(r) ||
+		    repo_config_get_pathname(r, "core.fsmonitor", &const_str))
+			return;
 		break;
 
 	default: /* should not happen */