PR 138189: Speed up "git checkout -b foo" by avoiding the call to merge_working_tree

Speed up "git checkout -b foo" by avoiding the call to merge_working_tree when certain conditions are met.  Basically you can only checkout a new branch with the current commit as this path does not need the merge to occur. Any other options force it through the old code path.  With this optimization "git checkout -b test" takes 16 vs 165 seconds (cold HDD) and 3 seconds vs 44 seconds (warm HDD) on the ost2 repo.  Note, all git tests still pass with this change but I'm not sure if they would take this upstream as it still changes the behavior.

Author: Ben Peart

builtin/checkout.c

@@ -20,6 +20,7 @@
 #include "resolve-undo.h"
 #include "submodule-config.h"
 #include "submodule.h"
+#include "gvfs.h"
 
 static const char * const checkout_usage[] = {
 	N_("git checkout [<options>] <branch>"),
@@ -824,10 +825,26 @@ static int switch_branches(const struct checkout_opts *opts,
 		parse_commit_or_die(new->commit);
 	}
 
-	ret = merge_working_tree(opts, &old, new, &writeout_error);
-	if (ret) {
-		free(path_to_free);
-		return ret;
+	/*
+	 * Optimize the performance of "git checkout foo" by skipping the call
+	 * to merge_working_tree. Make this as restrictive as possible, only
+	 * checkout a new branch with the current commit. Any other options force
+	 * it through the old path.
+	 */
+	if (!gvfs_config_is_set(GVFS_SKIP_MERGE_IN_CHECKOUT)
+		|| !old.commit || !new->commit
+		|| oidcmp(&old.commit->object.oid, &new->commit->object.oid)
+		|| !opts->new_branch || opts->new_branch_force || opts->new_orphan_branch
+		|| opts->patch_mode || opts->merge || opts->force || opts->force_detach
+		|| opts->writeout_stage || !opts->overwrite_ignore
+		|| opts->ignore_skipworktree || opts->ignore_other_worktrees
+		|| opts->new_branch_log || opts->branch_exists || opts->prefix
+		|| opts->source_tree) {
+		ret = merge_working_tree(opts, &old, new, &writeout_error);
+		if (ret) {
+			free(path_to_free);
+			return ret;
+		}
 	}
 
 	if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)

gvfs.h

@@ -20,6 +20,7 @@
 #define GVFS_FETCH_SKIP_REACHABILITY_AND_UPLOADPACK 16
 #define GVFS_LOWER_DEFAULT_SLOP 32
 #define GVFS_BLOCK_FILTERS_AND_EOL_CONVERSIONS 64
+#define GVFS_SKIP_MERGE_IN_CHECKOUT 128
 
 static inline BOOL gvfs_config_is_set(int mask) {
 	return (core_gvfs & mask) == mask;