merge: skip syntactic path checks in [_checkunknownfile]

We don't need to check the paths syntactically, since they are coming from
diffing the revisions, so hopefully already checked on the way in.

We still need to check what's on the filesystem, to avoid traversing the
symlinks or subdirs, which we can't know about statically.

Also, we use the directory audit to elide [isfileorlink],
this removing ~all lstat calls from hg updates from-empty.

mercurial/merge.py

@@ -46,7 +46,7 b' def _getcheckunknownconfig(repo, section'
     return config
 
 
-def _checkunknownfile(repo, wctx, mctx, f, f2=None):
+def _checkunknownfile(repo, dircache, wctx, mctx, f, f2=None):
     if wctx.isinmemory():
         # Nothing to do in IMM because nothing in the "working copy" can be an
         # unknown file.
@@ -58,8 +58,7 b' def _checkunknownfile(repo, wctx, mctx, '
     if f2 is None:
         f2 = f
     return (
-        repo.wvfs.audit.check(f)
-        and repo.wvfs.isfileorlink(f)
+        repo.wvfs.isfileorlink_checkdir(dircache, f)
         and repo.dirstate.normalize(f) not in repo.dirstate
         and mctx[f2].cmp(wctx[f])
     )
@@ -136,6 +135,7 b' def _checkunknownfiles(repo, wctx, mctx,'
     pathconfig = repo.ui.configbool(
         b'experimental', b'merge.checkpathconflicts'
     )
+    dircache = dict()
     if not force:
 
         def collectconflicts(conflicts, config):
@@ -145,19 +145,18 b' def _checkunknownfiles(repo, wctx, mctx,'
                 warnconflicts.update(conflicts)
 
         checkunknowndirs = _unknowndirschecker()
-        with repo.wvfs.audit.cached():
-            for f in mresult.files(
-                (
-                    mergestatemod.ACTION_CREATED,
-                    mergestatemod.ACTION_DELETED_CHANGED,
-                )
-            ):
-                if _checkunknownfile(repo, wctx, mctx, f):
-                    fileconflicts.add(f)
-                elif pathconfig and f not in wctx:
-                    path = checkunknowndirs(repo, wctx, f)
-                    if path is not None:
-                        pathconflicts.add(path)
+        for f in mresult.files(
+            (
+                mergestatemod.ACTION_CREATED,
+                mergestatemod.ACTION_DELETED_CHANGED,
+            )
+        ):
+            if _checkunknownfile(repo, dircache, wctx, mctx, f):
+                fileconflicts.add(f)
+            elif pathconfig and f not in wctx:
+                path = checkunknowndirs(repo, wctx, f)
+                if path is not None:
+                    pathconflicts.add(path)
         for f, args, msg in mresult.getactions(
             [mergestatemod.ACTION_LOCAL_DIR_RENAME_GET]
         ):

mercurial/vfs.py

@@ -422,6 +422,25 b' class vfs(abstractvfs):'
                 raise error.Abort(b"%s: %r" % (r, path))
             self.audit(path, mode=mode)
 
+    def isfileorlink_checkdir(
+        self, dircache, path: Optional[bytes] = None
+    ) -> bool:
+        """return True if the path is a regular file or a symlink and
+        the directories along the path are "normal", that is
+        not symlinks or nested hg repositories."""
+        try:
+            for prefix in pathutil.finddirs_rev_noroot(util.localpath(path)):
+                if prefix in dircache:
+                    res = dircache[prefix]
+                else:
+                    res = self.audit._checkfs_exists(prefix, path)
+                    dircache[prefix] = res
+                if not res:
+                    return False
+        except (OSError, error.Abort):
+            return False
+        return self.isfileorlink(path)
+
     def __call__(
         self,
         path: bytes,