FindModuleCache: optionally leverage BuildSourceSet (#12616)

Given a large codebase with folder hierarchy of the form

```
foo/
    company/
        __init__.py
        foo/
bar/
    company/
        __init__.py
        bar/
baz/
    company/
        __init__.py
        baz/
    ...
```

with >100 toplevel folders, the time spent in load_graph
is dominated by find_module because this operation is
itself O(n) where n is the number of input files, which
ends up being O(n**2) because it is called for every import
statement in the codebase and the way find_module work,
it will always scan through each and every one of those
toplevel directories for each and every import statement
of company.*

Introduce a fast path that leverages the fact that for
imports within the code being typechecked, we already
have a mapping of module import path to file path in
BuildSourceSet

Gated behind a command line flag (--fast-module-lookup) to 
assuage concerns about subtle issues in module lookup being 
introduced by this fast path.

docs/source/command_line.rst

@@ -212,6 +212,29 @@ imports.
     By default, mypy will suppress any error messages generated within :pep:`561`
     compliant packages. Adding this flag will disable this behavior.
 
+.. option:: --fast-module-lookup
+
+    The default logic used to scan through search paths to resolve imports has a
+    quadratic worse-case behavior in some cases, which is for instance triggered
+    by a large number of folders sharing a top-level namespace as in:
+
+        foo/
+            company/
+                foo/
+                    a.py
+        bar/
+            company/
+                bar/
+                    b.py
+        baz/
+            company/
+                baz/
+                    c.py
+       ...
+
+    If you are in this situation, you can enable an experimental fast path by
+    setting the :option:`--fast-module-lookup` option.
+
 
 .. _platform-configuration:
 

docs/source/running_mypy.rst

@@ -516,6 +516,7 @@ same directory on the search path, only the stub file is used.
 (However, if the files are in different directories, the one found
 in the earlier directory is used.)
 
+
 Other advice and best practices
 *******************************
 

mypy/build.py

@@ -43,8 +43,8 @@
     from mypy.report import Reports  # Avoid unconditional slow import
 from mypy.fixup import fixup_module
 from mypy.modulefinder import (
-    BuildSource, compute_search_paths, FindModuleCache, SearchPaths, ModuleSearchResult,
-    ModuleNotFoundReason
+    BuildSource, BuildSourceSet, compute_search_paths, FindModuleCache, SearchPaths,
+    ModuleSearchResult, ModuleNotFoundReason
 )
 from mypy.nodes import Expression
 from mypy.options import Options
@@ -107,33 +107,6 @@ def __init__(self, manager: 'BuildManager', graph: Graph) -> None:
         self.errors: List[str] = []  # Filled in by build if desired
 
 
-class BuildSourceSet:
-    """Efficiently test a file's membership in the set of build sources."""
-
-    def __init__(self, sources: List[BuildSource]) -> None:
-        self.source_text_present = False
-        self.source_modules: Set[str] = set()
-        self.source_paths: Set[str] = set()
-
-        for source in sources:
-            if source.text is not None:
-                self.source_text_present = True
-            elif source.path:
-                self.source_paths.add(source.path)
-            else:
-                self.source_modules.add(source.module)
-
-    def is_source(self, file: MypyFile) -> bool:
-        if file.path and file.path in self.source_paths:
-            return True
-        elif file._fullname in self.source_modules:
-            return True
-        elif self.source_text_present:
-            return True
-        else:
-            return False
-
-
 def build(sources: List[BuildSource],
           options: Options,
           alt_lib_path: Optional[str] = None,
@@ -630,7 +603,8 @@ def __init__(self, data_dir: str,
                                    or options.use_fine_grained_cache)
                               and not has_reporters)
         self.fscache = fscache
-        self.find_module_cache = FindModuleCache(self.search_paths, self.fscache, self.options)
+        self.find_module_cache = FindModuleCache(self.search_paths, self.fscache, self.options,
+                                                 source_set=self.source_set)
         self.metastore = create_metastore(options)
 
         # a mapping from source files to their corresponding shadow files

mypy/main.py

@@ -881,6 +881,10 @@ def add_invertible_flag(flag: str,
         '--explicit-package-bases', default=False,
         help="Use current directory and MYPYPATH to determine module names of files passed",
         group=code_group)
+    add_invertible_flag(
+        '--fast-module-lookup', default=False,
+        help=argparse.SUPPRESS,
+        group=code_group)
     code_group.add_argument(
         "--exclude",
         action="append",

mypy/modulefinder.py

@@ -23,6 +23,7 @@
 from typing_extensions import Final, TypeAlias as _TypeAlias
 
 from mypy.fscache import FileSystemCache
+from mypy.nodes import MypyFile
 from mypy.options import Options
 from mypy.stubinfo import is_legacy_bundled_package
 from mypy import pyinfo
@@ -126,6 +127,33 @@ def __repr__(self) -> str:
             self.base_dir)
 
 
+class BuildSourceSet:
+    """Helper to efficiently test a file's membership in a set of build sources."""
+
+    def __init__(self, sources: List[BuildSource]) -> None:
+        self.source_text_present = False
+        self.source_modules = {}  # type: Dict[str, str]
+        self.source_paths = set()  # type: Set[str]
+
+        for source in sources:
+            if source.text is not None:
+                self.source_text_present = True
+            if source.path:
+                self.source_paths.add(source.path)
+            if source.module:
+                self.source_modules[source.module] = source.path or ''
+
+    def is_source(self, file: MypyFile) -> bool:
+        if file.path and file.path in self.source_paths:
+            return True
+        elif file._fullname in self.source_modules:
+            return True
+        elif self.source_text_present:
+            return True
+        else:
+            return False
+
+
 class FindModuleCache:
     """Module finder with integrated cache.
 
@@ -141,8 +169,10 @@ def __init__(self,
                  search_paths: SearchPaths,
                  fscache: Optional[FileSystemCache],
                  options: Optional[Options],
-                 stdlib_py_versions: Optional[StdlibVersions] = None) -> None:
+                 stdlib_py_versions: Optional[StdlibVersions] = None,
+                 source_set: Optional[BuildSourceSet] = None) -> None:
         self.search_paths = search_paths
+        self.source_set = source_set
         self.fscache = fscache or FileSystemCache()
         # Cache for get_toplevel_possibilities:
         # search_paths -> (toplevel_id -> list(package_dirs))
@@ -164,6 +194,53 @@ def clear(self) -> None:
         self.initial_components.clear()
         self.ns_ancestors.clear()
 
+    def find_module_via_source_set(self, id: str) -> Optional[ModuleSearchResult]:
+        """Fast path to find modules by looking through the input sources
+
+        This is only used when --fast-module-lookup is passed on the command line."""
+        if not self.source_set:
+            return None
+
+        p = self.source_set.source_modules.get(id, None)
+        if p and self.fscache.isfile(p):
+            # We need to make sure we still have __init__.py all the way up
+            # otherwise we might have false positives compared to slow path
+            # in case of deletion of init files, which is covered by some tests.
+            # TODO: are there some combination of flags in which this check should be skipped?
+            d = os.path.dirname(p)
+            for _ in range(id.count('.')):
+                if not any(self.fscache.isfile(os.path.join(d, '__init__' + x))
+                           for x in PYTHON_EXTENSIONS):
+                    return None
+                d = os.path.dirname(d)
+            return p
+
+        idx = id.rfind('.')
+        if idx != -1:
+            # When we're looking for foo.bar.baz and can't find a matching module
+            # in the source set, look up for a foo.bar module.
+            parent = self.find_module_via_source_set(id[:idx])
+            if parent is None or not isinstance(parent, str):
+                return None
+
+            basename, ext = os.path.splitext(parent)
+            if (not any(parent.endswith('__init__' + x) for x in PYTHON_EXTENSIONS)
+                    and (ext in PYTHON_EXTENSIONS and not self.fscache.isdir(basename))):
+                # If we do find such a *module* (and crucially, we don't want a package,
+                # hence the filtering out of __init__ files, and checking for the presence
+                # of a folder with a matching name), then we can be pretty confident that
+                # 'baz' will either be a top-level variable in foo.bar, or will not exist.
+                #
+                # Either way, spelunking in other search paths for another 'foo.bar.baz'
+                # module should be avoided because:
+                #  1. in the unlikely event that one were found, it's highly likely that
+                #     it would be unrelated to the source being typechecked and therefore
+                #     more likely to lead to erroneous results
+                #  2. as described in _find_module, in some cases the search itself could
+                #  potentially waste significant amounts of time
+                return ModuleNotFoundReason.NOT_FOUND
+        return None
+
     def find_lib_path_dirs(self, id: str, lib_path: Tuple[str, ...]) -> PackageDirs:
         """Find which elements of a lib_path have the directory a module needs to exist.
 
@@ -229,7 +306,7 @@ def find_module(self, id: str, *, fast_path: bool = False) -> ModuleSearchResult
             elif top_level in self.stdlib_py_versions:
                 use_typeshed = self._typeshed_has_version(top_level)
             self.results[id] = self._find_module(id, use_typeshed)
-            if (not fast_path
+            if (not (fast_path or (self.options is not None and self.options.fast_module_lookup))
                     and self.results[id] is ModuleNotFoundReason.NOT_FOUND
                     and self._can_find_module_in_parent_dir(id)):
                 self.results[id] = ModuleNotFoundReason.WRONG_WORKING_DIRECTORY
@@ -295,6 +372,39 @@ def _can_find_module_in_parent_dir(self, id: str) -> bool:
     def _find_module(self, id: str, use_typeshed: bool) -> ModuleSearchResult:
         fscache = self.fscache
 
+        # Fast path for any modules in the current source set.
+        # This is particularly important when there are a large number of search
+        # paths which share the first (few) component(s) due to the use of namespace
+        # packages, for instance:
+        # foo/
+        #    company/
+        #        __init__.py
+        #        foo/
+        # bar/
+        #    company/
+        #        __init__.py
+        #        bar/
+        # baz/
+        #    company/
+        #        __init__.py
+        #        baz/
+        #
+        # mypy gets [foo/company/foo, bar/company/bar, baz/company/baz, ...] as input
+        # and computes [foo, bar, baz, ...] as the module search path.
+        #
+        # This would result in O(n) search for every import of company.*, leading to
+        # O(n**2) behavior in load_graph as such imports are unsurprisingly present
+        # at least once, and usually many more times than that, in each and every file
+        # being parsed.
+        #
+        # Thankfully, such cases are efficiently handled by looking up the module path
+        # via BuildSourceSet.
+        p = (self.find_module_via_source_set(id)
+             if (self.options is not None and self.options.fast_module_lookup)
+             else None)
+        if p:
+            return p
+
         # If we're looking for a module like 'foo.bar.baz', it's likely that most of the
         # many elements of lib_path don't even have a subdirectory 'foo/bar'.  Discover
         # that only once and cache it for when we look for modules like 'foo.bar.blah'

mypy/options.py

@@ -293,6 +293,8 @@ def __init__(self) -> None:
         self.cache_map: Dict[str, Tuple[str, str]] = {}
         # Don't properly free objects on exit, just kill the current process.
         self.fast_exit = True
+        # fast path for finding modules from source set
+        self.fast_module_lookup = False
         # Used to transform source code before parsing if not None
         # TODO: Make the type precise (AnyStr -> AnyStr)
         self.transform_source: Optional[Callable[[Any], Any]] = None

mypy/test/testcheck.py

@@ -46,6 +46,7 @@
     'check-multiple-inheritance.test',
     'check-super.test',
     'check-modules.test',
+    'check-modules-fast.test',
     'check-typevar-values.test',
     'check-unsupported.test',
     'check-unreachable-code.test',