From d8f646d1b27f533c7b47e154235e22b047b6682c Mon Sep 17 00:00:00 2001
From: Victoria Dye <vdye@github.com>
Date: Mon, 4 Apr 2022 15:38:58 -0700
Subject: [PATCH] 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 |  3 +++
 advice.c                        |  1 +
 advice.h                        |  1 +
 fsmonitor-settings.c            | 33 +++++++++++++++++++++++++++++++--
 4 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/Documentation/config/advice.txt b/Documentation/config/advice.txt
index a00d0100a82ba7..0d1e04f1ed631c 100644
--- a/Documentation/config/advice.txt
+++ b/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.
 --
diff --git a/advice.c b/advice.c
index fd189689437c75..1d75981d05db60 100644
--- a/advice.c
+++ b/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 },
 };
 
diff --git a/advice.h b/advice.h
index 07e0f76833e780..edfb86ecaa8857 100644
--- a/advice.h
+++ b/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,
 };
diff --git a/fsmonitor-settings.c b/fsmonitor-settings.c
index 464424a1e924c6..5377222651d09e 100644
--- a/fsmonitor-settings.c
+++ b/fsmonitor-settings.c
@@ -47,6 +47,31 @@ 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;
@@ -72,12 +97,16 @@ 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 */