Make orphaned commit checks configurable

Documentation/config/advice.adoc

@@ -38,6 +38,12 @@ all advice messages.
 		configuration variable for how to set a given remote
 		to be used by default in some situations where this
 		advice would be printed.
+	checkoutOrphanedCommitWarning::
+		Shown when linkgit:git-checkout[1] or linkgit:git-switch[1]
+		takes more than 2 seconds to check for commits left behind
+		when leaving a detached `HEAD`. Suggests setting
+		`checkout.orphanedCommitWarning` to `false` to skip the check.
+		Disabling this advice does not disable the check itself.
 	commitBeforeMerge::
 		Shown when linkgit:git-merge[1] refuses to
 		merge to avoid overwriting local changes.

Documentation/config/checkout.adoc

@@ -22,6 +22,17 @@ commands or functionality in the future.
 	option in `git checkout` and `git switch`. See
 	linkgit:git-switch[1] and linkgit:git-checkout[1].
 
+`checkout.orphanedCommitWarning`::
+	When leaving a detached `HEAD`, linkgit:git-checkout[1] and
+	linkgit:git-switch[1] check whether any commits are being left
+	behind that are not reachable from any ref or the new `HEAD`,
+	and warn if so. This check can be expensive in large repositories.
+	Set this variable to `false` to skip the check and its warning
+	without suppressing other checkout output. Defaults to `true`.
++
+When disabled, commits left behind can still be found in the `HEAD`
+reflog until its entries expire. See linkgit:git-reflog[1].
+
 `checkout.workers`::
 	The number of parallel workers to use when updating the working tree.
 	The default is one, i.e. sequential execution. If set to a value less

Documentation/scalar.adoc

@@ -214,6 +214,13 @@ am.keepCR=true::
 	and non-Windows platforms and keeping carriage return (`\r`) characters
 	in certain workflows.
 
+checkout.orphanedCommitWarning=false::
+	This disables the reachability check that warns about commits left
+	behind when `git checkout` or `git switch` leaves a detached `HEAD`.
+	The check can be expensive in large repositories. Other checkout
+	output is unchanged, and commits left behind can still be found
+	in the `HEAD` reflog until its entries expire.
+
 commitGraph.changedPaths=true::
 	This setting helps the background maintenance steps that compute the
 	serialized commit-graph to also store changed-path Bloom filters. This

advice.c

@@ -50,6 +50,7 @@ static struct {
 	[ADVICE_AMBIGUOUS_FETCH_REFSPEC]		= { "ambiguousFetchRefspec" },
 	[ADVICE_AM_WORK_DIR] 				= { "amWorkDir" },
 	[ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME] 	= { "checkoutAmbiguousRemoteBranchName" },
+	[ADVICE_CHECKOUT_ORPHANED_COMMIT_WARNING]		= { "checkoutOrphanedCommitWarning" },
 	[ADVICE_COMMIT_BEFORE_MERGE]			= { "commitBeforeMerge" },
 	[ADVICE_DEFAULT_BRANCH_NAME]			= { "defaultBranchName" },
 	[ADVICE_DETACHED_HEAD]				= { "detachedHead" },

advice.h

@@ -17,6 +17,7 @@ enum advice_type {
 	ADVICE_AMBIGUOUS_FETCH_REFSPEC,
 	ADVICE_AM_WORK_DIR,
 	ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME,
+	ADVICE_CHECKOUT_ORPHANED_COMMIT_WARNING,
 	ADVICE_COMMIT_BEFORE_MERGE,
 	ADVICE_DEFAULT_BRANCH_NAME, /* To be retired sometime after Git 3.0 */
 	ADVICE_DETACHED_HEAD,

builtin/checkout.c

@@ -35,6 +35,7 @@
 #include "strvec.h"
 #include "submodule.h"
 #include "symlinks.h"
+#include "trace.h"
 #include "trace2.h"
 #include "tree.h"
 #include "tree-walk.h"
@@ -51,6 +52,7 @@ struct checkout_opts {
 	int patch_interhunk_context;
 	int auto_advance;
 	int quiet;
+	int orphaned_commit_warning;
 	int merge;
 	int force;
 	int force_detach;
@@ -99,6 +101,7 @@ struct checkout_opts {
 	.patch_context = -1, \
 	.patch_interhunk_context = -1, \
 	.auto_advance = 1, \
+	.orphaned_commit_warning = 1, \
 }
 
 #define MERGE_WORKING_TREE_UNPACK_FAILED (-2)
@@ -1131,7 +1134,10 @@ static void orphaned_commit_warning(struct commit *old_commit, struct commit *ne
 {
 	struct rev_info revs;
 	struct object *object = &old_commit->object;
+	uint64_t t_begin = getnanotime();
+	uint64_t t_delta_in_ms;
 
+	trace2_region_enter("checkout", "orphaned_commit_warning", the_repository);
 	repo_init_revisions(the_repository, &revs, NULL);
 	setup_revisions(0, NULL, &revs, NULL);
 
@@ -1155,6 +1161,17 @@ static void orphaned_commit_warning(struct commit *old_commit, struct commit *ne
 	/* Clean up objects used, as they will be reused. */
 	repo_clear_commit_marks(the_repository, ALL_REV_FLAGS);
 	release_revisions(&revs);
+	trace2_region_leave("checkout", "orphaned_commit_warning", the_repository);
+
+	t_delta_in_ms = (getnanotime() - t_begin) / 1000000;
+	if (t_delta_in_ms > 2000 ||
+	    git_env_bool("GIT_TEST_CHECKOUT_DELAY_WARNING", 0))
+		advise_if_enabled(ADVICE_CHECKOUT_ORPHANED_COMMIT_WARNING,
+				  _("Checking for orphaned commits took %.2f seconds.\n"
+				    "You can use 'git config set checkout.orphanedCommitWarning false'\n"
+				    "to skip this check, but you will no longer be warned when leaving\n"
+				    "commits behind that are not connected to any of your branches."),
+				  t_delta_in_ms / 1000.0);
 }
 
 static int switch_branches(const struct checkout_opts *opts,
@@ -1253,8 +1270,14 @@ static int switch_branches(const struct checkout_opts *opts,
 		}
 	}
 
-	if (!opts->quiet && !old_branch_info.path && old_branch_info.commit && new_branch_info->commit != old_branch_info.commit)
-		orphaned_commit_warning(old_branch_info.commit, new_branch_info->commit);
+	if (!opts->quiet && !old_branch_info.path && old_branch_info.commit &&
+	    new_branch_info->commit != old_branch_info.commit) {
+		if (opts->orphaned_commit_warning)
+			orphaned_commit_warning(old_branch_info.commit, new_branch_info->commit);
+		else
+			describe_detached_head(_("Previous HEAD position was"),
+					       old_branch_info.commit);
+	}
 
 	if (autostash_res == STASH_APPLY_CONFLICT && !opts->quiet)
 		fputc('\n', stderr);
@@ -1295,6 +1318,10 @@ static int git_checkout_config(const char *var, const char *value,
 		opts->dwim_new_local_branch = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "checkout.orphanedcommitwarning")) {
+		opts->orphaned_commit_warning = git_config_bool(var, value);
+		return 0;
+	}
 
 	if (starts_with(var, "submodule."))
 		return git_default_submodule_config(var, value, NULL);

scalar.c

@@ -138,6 +138,7 @@ static int set_recommended_config(int reconfigure)
 	 */
 	struct scalar_config config[] = {
 		{ "am.keepCR", "true" },
+		{ "checkout.orphanedCommitWarning", "false" },
 		{ "commitGraph.changedPaths", "true" },
 		{ "commitGraph.generationVersion", "1" },
 		{ "core.autoCRLF", "false" },

t/README

@@ -371,6 +371,11 @@ test suite. Accept any boolean values that are accepted by git-config.
 GIT_TEST_PROTOCOL_VERSION=<n>, when set, makes 'protocol.version'
 default to n.
 
+GIT_TEST_CHECKOUT_DELAY_WARNING=<boolean>, when true, forces the
+slow orphaned-commit check advice to be considered regardless of the
+elapsed time. It does not change the measured duration or override
+advice settings.
+
 GIT_TEST_FULL_IN_PACK_ARRAY=<boolean> exercises the uncommon
 pack-objects code path where there are more than 1024 packs even if
 the actual number of packs in repository is below this limit. Accept

t/t2020-checkout-detach.sh

@@ -164,6 +164,75 @@ test_expect_success 'checkout does not warn leaving reachable commit' '
 	check_no_orphan_warning stderr
 '
 
+for command in checkout switch
+do
+	test_expect_success "$command honors checkout.orphanedCommitWarning" '
+		git checkout --detach "$orphan2" &&
+		GIT_TRACE2_EVENT="$(pwd)/trace-enabled-$command" \
+			git -c checkout.orphanedCommitWarning=true \
+			$command main 2>stderr &&
+		check_orphan_warning stderr "2 commits" &&
+		test_grep "\"label\":\"orphaned_commit_warning\"" \
+			trace-enabled-$command &&
+
+		git checkout --detach "$orphan2" &&
+		GIT_TRACE2_EVENT="$(pwd)/trace-disabled-$command" \
+			git -c checkout.orphanedCommitWarning=false \
+			$command main 2>stderr &&
+		check_no_orphan_warning stderr &&
+		test_grep "Switched to branch" stderr &&
+		test_grep ! "\"label\":\"orphaned_commit_warning\"" \
+			trace-disabled-$command &&
+		check_not_detached
+	'
+
+	test_expect_success "$command advises when the orphan check is slow" '
+		git checkout --detach "$orphan2" &&
+		GIT_TEST_CHECKOUT_DELAY_WARNING=true \
+			git $command main 2>stderr &&
+		check_orphan_warning stderr "2 commits" &&
+		test_grep -E "^hint: Checking for orphaned commits took [0-9]+[.][0-9]{2} seconds[.]$" stderr &&
+		test_grep "git config set checkout.orphanedCommitWarning false" stderr &&
+		test_grep "git config set advice.checkoutOrphanedCommitWarning false" stderr
+	'
+
+	test_expect_success "$command can suppress slow orphan check advice" '
+		git checkout --detach "$orphan2" &&
+		GIT_TEST_CHECKOUT_DELAY_WARNING=true \
+			git -c advice.checkoutOrphanedCommitWarning=false \
+			$command main 2>stderr &&
+		check_orphan_warning stderr "2 commits" &&
+		test_grep ! "^hint:" stderr &&
+
+		git checkout --detach "$orphan2" &&
+		GIT_TEST_CHECKOUT_DELAY_WARNING=true GIT_ADVICE=0 \
+			git $command main 2>stderr &&
+		check_orphan_warning stderr "2 commits" &&
+		test_grep ! "^hint:" stderr
+	'
+
+	test_expect_success "$command does not advise when the orphan check is skipped" '
+		git checkout --detach "$orphan2" &&
+		GIT_TEST_CHECKOUT_DELAY_WARNING=true \
+			git -c checkout.orphanedCommitWarning=false \
+			$command main 2>stderr &&
+		check_no_orphan_warning stderr &&
+		test_grep ! "^hint:" stderr &&
+
+		git checkout --detach "$orphan2" &&
+		GIT_TEST_CHECKOUT_DELAY_WARNING=true \
+			git $command --quiet main 2>stderr &&
+		test_must_be_empty stderr &&
+		check_not_detached
+	'
+
+	test_expect_success "$command rejects invalid checkout.orphanedCommitWarning" '
+		test_must_fail git -c checkout.orphanedCommitWarning=invalid \
+			$command main 2>stderr &&
+		test_grep "bad boolean config value.*checkout.orphanedcommitwarning" stderr
+	'
+done
+
 cat >expect <<'EOF'
 Your branch is behind 'main' by 1 commit, and can be fast-forwarded.
   (use "git pull" to update your local branch)