Optimize symlink/directory detection

This is the base for making symlink detection in the middle fo a pathname
saner and (much) more efficient.

Under various loads, we want to verify that the full path leading up to a
filename is a real directory tree, and that when we successfully do an
'lstat()' on a filename, we don't get a false positive due to a symlink in
the middle of the path that git should have seen as a symlink, not as a
normal path component.

The 'has_symlink_leading_path()' function already did this, and cached
a single level of symlink information, but didn't cache the _lack_ of a
symlink, so the normal behaviour was actually the wrong way around, and we
ended up doing an 'lstat()' on each path component to check that it was a
real directory.

This caches the last detected full directory and symlink entries, and
speeds up especially deep directory structures a lot by avoiding to
lstat() all the directories leading up to each entry in the index.

[ This can - and should - probably be extended upon so that we eventually
  never do a bare 'lstat()' on any path entries at *all* when checking the
  index, but always check the full path carefully. Right now we do not
  generally check the whole path for all our normal quick index
  revalidation.

  We should also make sure that we're careful about all the invalidation,
  ie when we remove a link and replace it by a directory we should
  invalidate the symlink cache if it matches (and vice versa for the
  directory cache).

  But regardless, the basic function needs to be sane to do that. The old
  'has_symlink_leading_path()' was not capable enough - or indeed the code
  readable enough - to really do that sanely. So I'm pushing this as not
  just an optimization, but as a base for further work. ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-apply.c

@@ -2247,7 +2247,7 @@ static int check_to_create_blob(const char *new_name, int ok_if_exists)
 		 * In such a case, path "new_name" does not exist as
 		 * far as git is concerned.
 		 */
-		if (has_symlink_leading_path(new_name, NULL))
+		if (has_symlink_leading_path(strlen(new_name), new_name))
 			return 0;
 
 		return error("%s: already exists in working directory", new_name);

cache.h

@@ -598,7 +598,7 @@ struct checkout {
 };
 
 extern int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *topath);
-extern int has_symlink_leading_path(const char *name, char *last_symlink);
+extern int has_symlink_leading_path(int len, const char *name);
 
 extern struct alternate_object_database {
 	struct alternate_object_database *next;

diff-lib.c

@@ -347,14 +347,14 @@ int run_diff_files_cmd(struct rev_info *revs, int argc, const char **argv)
  * exists for ce that is a submodule -- it is a submodule that is not
  * checked out).  Return negative for an error.
  */
-static int check_removed(const struct cache_entry *ce, struct stat *st, char *symcache)
+static int check_removed(const struct cache_entry *ce, struct stat *st)
 {
 	if (lstat(ce->name, st) < 0) {
 		if (errno != ENOENT && errno != ENOTDIR)
 			return -1;
 		return 1;
 	}
-	if (has_symlink_leading_path(ce->name, symcache))
+	if (has_symlink_leading_path(ce_namelen(ce), ce->name))
 		return 1;
 	if (S_ISDIR(st->st_mode)) {
 		unsigned char sub[20];
@@ -421,7 +421,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 			memset(&(dpath->parent[0]), 0,
 			       sizeof(struct combine_diff_parent)*5);
 
-			changed = check_removed(ce, &st, symcache);
+			changed = check_removed(ce, &st);
 			if (!changed)
 				dpath->mode = ce_mode_from_stat(ce, st.st_mode);
 			else {
@@ -485,7 +485,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		if (ce_uptodate(ce))
 			continue;
 
-		changed = check_removed(ce, &st, symcache);
+		changed = check_removed(ce, &st);
 		if (changed) {
 			if (changed < 0) {
 				perror(ce->name);
@@ -546,7 +546,7 @@ static int get_stat_data(struct cache_entry *ce,
 	if (!cached) {
 		int changed;
 		struct stat st;
-		changed = check_removed(ce, &st, cbdata->symcache);
+		changed = check_removed(ce, &st);
 		if (changed < 0)
 			return -1;
 		else if (changed) {

symlinks.c

@@ -1,48 +1,64 @@
 #include "cache.h"
 
-int has_symlink_leading_path(const char *name, char *last_symlink)
-{
+struct pathname {
+	int len;
 	char path[PATH_MAX];
-	const char *sp, *ep;
-	char *dp;
+};
 
-	sp = name;
-	dp = path;
+/* Return matching pathname prefix length, or zero if not matching */
+static inline int match_pathname(int len, const char *name, struct pathname *match)
+{
+	int match_len = match->len;
+	return (len > match_len &&
+		name[match_len] == '/' &&
+		!memcmp(name, match->path, match_len)) ? match_len : 0;
+}
 
-	if (last_symlink && *last_symlink) {
-		size_t last_len = strlen(last_symlink);
-		size_t len = strlen(name);
-		if (last_len < len &&
-		    !strncmp(name, last_symlink, last_len) &&
-		    name[last_len] == '/')
-			return 1;
-		*last_symlink = '\0';
+static inline void set_pathname(int len, const char *name, struct pathname *match)
+{
+	if (len < PATH_MAX) {
+		match->len = len;
+		memcpy(match->path, name, len);
+		match->path[len] = 0;
 	}
+}
 
-	while (1) {
-		size_t len;
-		struct stat st;
+int has_symlink_leading_path(int len, const char *name)
+{
+	static struct pathname link, nonlink;
+	char path[PATH_MAX];
+	struct stat st;
+	char *sp;
+	int known_dir;
 
-		ep = strchr(sp, '/');
-		if (!ep)
-			break;
-		len = ep - sp;
-		if (PATH_MAX <= dp + len - path + 2)
-			return 0; /* new name is longer than that??? */
-		memcpy(dp, sp, len);
-		dp[len] = 0;
+	/*
+	 * See if the last known symlink cache matches.
+	 */
+	if (match_pathname(len, name, &link))
+		return 1;
+
+	/*
+	 * Get rid of the last known directory part
+	 */
+	known_dir = match_pathname(len, name, &nonlink);
+
+	while ((sp = strchr(name + known_dir + 1, '/')) != NULL) {
+		int thislen = sp - name ;
+		memcpy(path, name, thislen);
+		path[thislen] = 0;
 
 		if (lstat(path, &st))
 			return 0;
+		if (S_ISDIR(st.st_mode)) {
+			set_pathname(thislen, path, &nonlink);
+			known_dir = thislen;
+			continue;
+		}
 		if (S_ISLNK(st.st_mode)) {
-			if (last_symlink)
-				strcpy(last_symlink, path);
+			set_pathname(thislen, path, &link);
 			return 1;
 		}
-
-		dp[len++] = '/';
-		dp = dp + len;
-		sp = ep + 1;
+		break;
 	}
 	return 0;
 }

unpack-trees.c

@@ -26,11 +26,12 @@ static void add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
  * directories, in case this unlink is the removal of the
  * last entry in the directory -- empty directories are removed.
  */
-static void unlink_entry(char *name, char *last_symlink)
+static void unlink_entry(struct cache_entry *ce)
 {
 	char *cp, *prev;
+	char *name = ce->name;
 
-	if (has_symlink_leading_path(name, last_symlink))
+	if (has_symlink_leading_path(ce_namelen(ce), ce->name))
 		return;
 	if (unlink(name))
 		return;
@@ -58,7 +59,6 @@ static int check_updates(struct unpack_trees_options *o)
 {
 	unsigned cnt = 0, total = 0;
 	struct progress *progress = NULL;
-	char last_symlink[PATH_MAX];
 	struct index_state *index = &o->result;
 	int i;
 	int errs = 0;
@@ -75,14 +75,13 @@ static int check_updates(struct unpack_trees_options *o)
 		cnt = 0;
 	}
 
-	*last_symlink = '\0';
 	for (i = 0; i < index->cache_nr; i++) {
 		struct cache_entry *ce = index->cache[i];
 
 		if (ce->ce_flags & CE_REMOVE) {
 			display_progress(progress, ++cnt);
 			if (o->update)
-				unlink_entry(ce->name, last_symlink);
+				unlink_entry(ce);
 			remove_index_entry_at(&o->result, i);
 			i--;
 			continue;
@@ -97,7 +96,6 @@ static int check_updates(struct unpack_trees_options *o)
 			ce->ce_flags &= ~CE_UPDATE;
 			if (o->update) {
 				errs |= checkout_entry(ce, &state, NULL);
-				*last_symlink = '\0';
 			}
 		}
 	}
@@ -553,7 +551,7 @@ static int verify_absent(struct cache_entry *ce, const char *action,
 	if (o->index_only || o->reset || !o->update)
 		return 0;
 
-	if (has_symlink_leading_path(ce->name, NULL))
+	if (has_symlink_leading_path(ce_namelen(ce), ce->name))
 		return 0;
 
 	if (!lstat(ce->name, &st)) {