Significantly speed up file handling error paths (python#17920)

This can have a huge overall impact on mypy performance when search paths are long

mypy/build.py

@@ -736,8 +736,8 @@ def maybe_swap_for_shadow_path(self, path: str) -> str:
         shadow_file = self.shadow_equivalence_map.get(path)
         return shadow_file if shadow_file else path
 
-    def get_stat(self, path: str) -> os.stat_result:
-        return self.fscache.stat(self.maybe_swap_for_shadow_path(path))
+    def get_stat(self, path: str) -> os.stat_result | None:
+        return self.fscache.stat_or_none(self.maybe_swap_for_shadow_path(path))
 
     def getmtime(self, path: str) -> int:
         """Return a file's mtime; but 0 in bazel mode.
@@ -1394,9 +1394,9 @@ def validate_meta(
     if bazel:
         # Normalize path under bazel to make sure it isn't absolute
         path = normpath(path, manager.options)
-    try:
-        st = manager.get_stat(path)
-    except OSError:
+
+    st = manager.get_stat(path)
+    if st is None:
         return None
     if not stat.S_ISDIR(st.st_mode) and not stat.S_ISREG(st.st_mode):
         manager.log(f"Metadata abandoned for {id}: file or directory {path} does not exist")
@@ -1572,10 +1572,9 @@ def write_cache(
     plugin_data = manager.plugin.report_config_data(ReportConfigContext(id, path, is_check=False))
 
     # Obtain and set up metadata
-    try:
-        st = manager.get_stat(path)
-    except OSError as err:
-        manager.log(f"Cannot get stat for {path}: {err}")
+    st = manager.get_stat(path)
+    if st is None:
+        manager.log(f"Cannot get stat for {path}")
         # Remove apparently-invalid cache files.
         # (This is purely an optimization.)
         for filename in [data_json, meta_json]:

mypy/fscache.py

@@ -51,8 +51,8 @@ def set_package_root(self, package_root: list[str]) -> None:
 
     def flush(self) -> None:
         """Start another transaction and empty all caches."""
-        self.stat_cache: dict[str, os.stat_result] = {}
-        self.stat_error_cache: dict[str, OSError] = {}
+        self.stat_or_none_cache: dict[str, os.stat_result | None] = {}
+
         self.listdir_cache: dict[str, list[str]] = {}
         self.listdir_error_cache: dict[str, OSError] = {}
         self.isfile_case_cache: dict[str, bool] = {}
@@ -62,24 +62,21 @@ def flush(self) -> None:
         self.hash_cache: dict[str, str] = {}
         self.fake_package_cache: set[str] = set()
 
-    def stat(self, path: str) -> os.stat_result:
-        if path in self.stat_cache:
-            return self.stat_cache[path]
-        if path in self.stat_error_cache:
-            raise copy_os_error(self.stat_error_cache[path])
+    def stat_or_none(self, path: str) -> os.stat_result | None:
+        if path in self.stat_or_none_cache:
+            return self.stat_or_none_cache[path]
+
+        st = None
         try:
             st = os.stat(path)
-        except OSError as err:
+        except OSError:
             if self.init_under_package_root(path):
                 try:
-                    return self._fake_init(path)
+                    st = self._fake_init(path)
                 except OSError:
                     pass
-            # Take a copy to get rid of associated traceback and frame objects.
-            # Just assigning to __traceback__ doesn't free them.
-            self.stat_error_cache[path] = copy_os_error(err)
-            raise err
-        self.stat_cache[path] = st
+
+        self.stat_or_none_cache[path] = st
         return st
 
     def init_under_package_root(self, path: str) -> bool:
@@ -112,9 +109,9 @@ def init_under_package_root(self, path: str) -> bool:
         if not os.path.basename(dirname).isidentifier():
             # Can't put an __init__.py in a place that's not an identifier
             return False
-        try:
-            st = self.stat(dirname)
-        except OSError:
+
+        st = self.stat_or_none(dirname)
+        if st is None:
             return False
         else:
             if not stat.S_ISDIR(st.st_mode):
@@ -145,15 +142,14 @@ def _fake_init(self, path: str) -> os.stat_result:
         assert basename == "__init__.py", path
         assert not os.path.exists(path), path  # Not cached!
         dirname = os.path.normpath(dirname)
-        st = self.stat(dirname)  # May raise OSError
+        st = os.stat(dirname)  # May raise OSError
         # Get stat result as a list so we can modify it.
         seq: list[float] = list(st)
         seq[stat.ST_MODE] = stat.S_IFREG | 0o444
         seq[stat.ST_INO] = 1
         seq[stat.ST_NLINK] = 1
         seq[stat.ST_SIZE] = 0
         st = os.stat_result(seq)
-        self.stat_cache[path] = st
         # Make listdir() and read() also pretend this file exists.
         self.fake_package_cache.add(dirname)
         return st
@@ -181,9 +177,8 @@ def listdir(self, path: str) -> list[str]:
         return results
 
     def isfile(self, path: str) -> bool:
-        try:
-            st = self.stat(path)
-        except OSError:
+        st = self.stat_or_none(path)
+        if st is None:
             return False
         return stat.S_ISREG(st.st_mode)
 
@@ -248,18 +243,14 @@ def exists_case(self, path: str, prefix: str) -> bool:
         return res
 
     def isdir(self, path: str) -> bool:
-        try:
-            st = self.stat(path)
-        except OSError:
+        st = self.stat_or_none(path)
+        if st is None:
             return False
         return stat.S_ISDIR(st.st_mode)
 
     def exists(self, path: str) -> bool:
-        try:
-            self.stat(path)
-        except FileNotFoundError:
-            return False
-        return True
+        st = self.stat_or_none(path)
+        return st is not None
 
     def read(self, path: str) -> bytes:
         if path in self.read_cache:
@@ -269,7 +260,7 @@ def read(self, path: str) -> bytes:
 
         # Need to stat first so that the contents of file are from no
         # earlier instant than the mtime reported by self.stat().
-        self.stat(path)
+        self.stat_or_none(path)
 
         dirname, basename = os.path.split(path)
         dirname = os.path.normpath(dirname)
@@ -294,8 +285,10 @@ def hash_digest(self, path: str) -> str:
         return self.hash_cache[path]
 
     def samefile(self, f1: str, f2: str) -> bool:
-        s1 = self.stat(f1)
-        s2 = self.stat(f2)
+        s1 = self.stat_or_none(f1)
+        s2 = self.stat_or_none(f2)
+        if s1 is None or s2 is None:
+            return False
         return os.path.samestat(s1, s2)
 
 

mypy/fswatcher.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 from typing import AbstractSet, Iterable, NamedTuple
 
 from mypy.fscache import FileSystemCache
@@ -56,18 +57,16 @@ def remove_watched_paths(self, paths: Iterable[str]) -> None:
                 del self._file_data[path]
         self._paths -= set(paths)
 
-    def _update(self, path: str) -> None:
-        st = self.fs.stat(path)
+    def _update(self, path: str, st: os.stat_result) -> None:
         hash_digest = self.fs.hash_digest(path)
         self._file_data[path] = FileData(st.st_mtime, st.st_size, hash_digest)
 
     def _find_changed(self, paths: Iterable[str]) -> AbstractSet[str]:
         changed = set()
         for path in paths:
             old = self._file_data[path]
-            try:
-                st = self.fs.stat(path)
-            except FileNotFoundError:
+            st = self.fs.stat_or_none(path)
+            if st is None:
                 if old is not None:
                     # File was deleted.
                     changed.add(path)
@@ -76,13 +75,13 @@ def _find_changed(self, paths: Iterable[str]) -> AbstractSet[str]:
                 if old is None:
                     # File is new.
                     changed.add(path)
-                    self._update(path)
+                    self._update(path, st)
                 # Round mtimes down, to match the mtimes we write to meta files
                 elif st.st_size != old.st_size or int(st.st_mtime) != int(old.st_mtime):
                     # Only look for changes if size or mtime has changed as an
                     # optimization, since calculating hash is expensive.
                     new_hash = self.fs.hash_digest(path)
-                    self._update(path)
+                    self._update(path, st)
                     if st.st_size != old.st_size or new_hash != old.hash:
                         # Changed file.
                         changed.add(path)