hooks: improve multiprocessing hook to work with pytorch

cx_Freeze/hooks/multiprocessing.py

@@ -25,8 +25,9 @@ def load_multiprocessing(_, module: Module) -> None:
     """
     # Support for:
     # - fork in Unix (including macOS) is native;
-    # - spawn in Windows is native (since 4.3.4) but was improved in v6.2;
-    # - spawn and forkserver in Unix is implemented here.
+    # - spawn in Windows is native since 4.3.4, but was improved in 6.2;
+    # - spawn and forkserver in Unix is implemented here in 6.15.4 #1956;
+    # - monkeypath get_context to do automatic freeze_support in 7.1 #2382;
     if IS_MINGW or IS_WINDOWS:
         return
     if module.file.suffix == ".pyc":  # source unavailable
@@ -40,12 +41,18 @@ def load_multiprocessing(_, module: Module) -> None:
         if re.search(r"^from multiprocessing.* import main.*", cmd):
             exec(cmd)
             sys.exit()
-    # workaround for python docs: run the freeze_support to avoid infinite loop
-    from multiprocessing.spawn import freeze_support as spawn_freeze_support
-    spawn_freeze_support()
-    del spawn_freeze_support
-    # disable it, cannot run twice
-    freeze_support = lambda: None
+    # workaround: inject freeze_support call to avoid an infinite loop
+    from multiprocessing.spawn import freeze_support as _spawn_freeze_support
+    from multiprocessing.context import BaseContext
+    BaseContext._get_context = BaseContext.get_context
+    def _get_freeze_context(self, method=None):
+        ctx = self._get_context(method)
+        _spawn_freeze_support()
+        return ctx
+    BaseContext.get_context = \
+        lambda self, method=None: _get_freeze_context(self, method)
+    # disable freeze_support, because it cannot be run twice
+    BaseContext.freeze_support = lambda self: None
     # cx_Freeze patch end
     """
     code_string = module.file.read_text(encoding="utf_8") + dedent(source)

doc/src/faq.rst

@@ -179,3 +179,37 @@ sources.
 
  Or install patchelf from `sources
  <https://github.com/NixOS/patchelf#compiling-and-testing>`_.
+
+
+Multiprocessing support
+-----------------------
+
+On Linux and macOS, multiprocessing support is automatically managed by
+cx_Freeze, including supporting it in pyTorch.
+
+However, to produce a Windows executable, you must use
+`multiprocessing.freeze_support()`.
+
+One needs to call this function straight after the if __name__ == '__main__'
+line of the main module. For example:
+
+  .. code-block:: python
+
+    from multiprocessing import Process, freeze_support
+
+
+    def f():
+        print("Hello from cx_Freeze")
+
+
+    if __name__ == "__main__":
+        freeze_support()
+        Process(target=f).start()
+
+If the freeze_support() line is omitted then trying to run the frozen
+executable will raise RuntimeError.
+
+Calling freeze_support() has no effect when invoked on any operating system
+other than Windows. In addition, if the module is being run normally by the
+Python interpreter on Windows (the program has not been frozen), then
+freeze_support() has no effect.

source/bases/common.c

@@ -78,13 +78,10 @@ static wchar_t *get_sys_path(wchar_t *lib_dir)
     wcscpy(filename, lib_dir);
     wcscat(filename, L"\\");
     wcscat(filename, L"library.dat");
-    fprintf(stderr, "--filename: %ls\n", filename);
-    fprintf(stderr, "--MAXPATHLEN: %zd/%zd\n", wcslen(filename), (size_t)MAXPATHLEN);
     if ((fp = _wfopen(filename, L"rt")) != NULL) {
         int i = fread(buffer, sizeof(*buffer), sizeof(buffer), fp);
         buffer[i] = 0;
         fclose(fp);
-        fprintf(stderr, "--buffer: %s=%zd\n", buffer, strlen(buffer));
         wbuffer = Py_DecodeLocale(buffer, NULL);
         if (!wbuffer) {
             FatalError("Unable to convert path to string!");
@@ -93,23 +90,18 @@ static wchar_t *get_sys_path(wchar_t *lib_dir)
             wcscat(filename, L"\\");
             wcscat(filename, wbuffer);
             PyMem_RawFree(wbuffer);
-            fprintf(stderr, "--filename: %ls=%zd\n", filename, wcslen(filename));
             wcscat(buf_path, filename);
-            fprintf(stderr, "--buf_path: %ls\n", buf_path);
         }
     }
     if (wcslen(buf_path) != 0)
         wcscat(buf_path, L";");
-    fprintf(stderr, "--buf_path: %ls\n", buf_path);
     wcscat(buf_path, lib_dir);
-    fprintf(stderr, "--buf_path: %ls\n", buf_path);
 
     sys_path = PyMem_RawMalloc(sizeof(wchar_t) * wcslen(buf_path) + 1);
     if (!sys_path) {
         FatalError("Out of memory creating sys_path!");
         return NULL;
     }
-    fprintf(stderr, "--sys_path: %ls\n", wcscpy(sys_path, buf_path));
     return wcscpy(sys_path, buf_path);
 }
 #else
@@ -207,16 +199,13 @@ static char *get_sys_path(char *lib_dir)
     strcpy(filename, lib_dir);
     strcat(filename, "/");
     strcat(filename, "library.dat");
-    fprintf(stderr, "--filename: %s\n", filename);
-    fprintf(stderr, "--MAXPATHLEN: %d\n", MAXPATHLEN);
     if ((fp = fopen(filename, "r")) != NULL) {
         int i = fread(buffer, sizeof(*buffer), sizeof(buffer), fp);
         buffer[i] = 0;
         fclose(fp);
         strcpy(filename, lib_dir);
         strcat(filename, "/");
         strcat(filename, buffer);
-        fprintf(stderr, "--filename: %s\n", filename);
         strcat(buf_path, filename);
     }
     if (strlen(buf_path) != 0)