bpo-45020: Default to using frozen modules unless running from source tree. (gh-28940)

The default was "off".  Switching it to "on" means users get the benefit of frozen stdlib modules without having to do anything.  There's a special-case for running-in-source-tree, so contributors don't get surprised when their stdlib changes don't get used.

https://bugs.python.org/issue45020

2 files changed, 73 insertions, 19 deletions

diff --git a/Python/fileutils.c b/Python/fileutils.c
index 173d34dd23..3d8f3a4f16 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -2169,6 +2169,18 @@ _Py_add_relfile(wchar_t *dirname, const wchar_t *relfile, size_t bufsize)
 }
 
 
+size_t
+_Py_find_basename(const wchar_t *filename)
+{
+    for (size_t i = wcslen(filename); i > 0; --i) {
+        if (filename[i] == SEP) {
+            return i + 1;
+        }
+    }
+    return 0;
+}
+
+
 /* Get the current directory. buflen is the buffer size in wide characters
    including the null character. Decode the path from the locale encoding.
 
diff --git a/Python/initconfig.c b/Python/initconfig.c
index b0d54b0472..c916e2f7c0 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -739,6 +739,7 @@ _PyConfig_InitCompatConfig(PyConfig *config)
 #ifdef MS_WINDOWS
     config->legacy_windows_stdio = -1;
 #endif
+    config->use_frozen_modules = -1;
 }
 
 
@@ -2090,6 +2091,44 @@ config_init_fs_encoding(PyConfig *config, const PyPreConfig *preconfig)
 }
 
 
+/* Determine if the current build is a "development" build (e.g. running
+   out of the source tree) or not.
+
+   A return value of -1 indicates that we do not know.
+  */
+static int
+is_dev_env(PyConfig *config)
+{
+    // This should only ever get called early in runtime initialization,
+    // before the global path config is written.  Otherwise we would
+    // use Py_GetProgramFullPath() and _Py_GetStdlibDir().
+    assert(config != NULL);
+
+    const wchar_t *executable = config->executable;
+    const wchar_t *stdlib = config->stdlib_dir;
+    if (executable == NULL || *executable == L'\0' ||
+            stdlib == NULL || *stdlib == L'\0') {
+        // _PyPathConfig_Calculate() hasn't run yet.
+        return -1;
+    }
+    size_t len = _Py_find_basename(executable);
+    if (wcscmp(executable + len, L"python") != 0 &&
+            wcscmp(executable + len, L"python.exe") != 0) {
+        return 0;
+    }
+    /* If dirname() is the same for both then it is a dev build. */
+    if (len != _Py_find_basename(stdlib)) {
+        return 0;
+    }
+    // We do not bother normalizing the two filenames first since
+    // for config_init_import() is does the right thing as-is.
+    if (wcsncmp(stdlib, executable, len) != 0) {
+        return 0;
+    }
+    return 1;
+}
+
+
 static PyStatus
 config_init_import(PyConfig *config, int compute_path_config)
 {
@@ -2101,25 +2140,28 @@ config_init_import(PyConfig *config, int compute_path_config)
     }
 
     /* -X frozen_modules=[on|off] */
-    const wchar_t *value = config_get_xoption_value(config, L"frozen_modules");
-    if (value == NULL) {
-        // For now we always default to "off".
-        // In the near future we will be factoring in PGO and in-development.
-        config->use_frozen_modules = 0;
-    }
-    else if (wcscmp(value, L"on") == 0) {
-        config->use_frozen_modules = 1;
-    }
-    else if (wcscmp(value, L"off") == 0) {
-        config->use_frozen_modules = 0;
-    }
-    else if (wcslen(value) == 0) {
-        // "-X frozen_modules" and "-X frozen_modules=" both imply "on".
-        config->use_frozen_modules = 1;
-    }
-    else {
-        return PyStatus_Error("bad value for option -X frozen_modules "
-                              "(expected \"on\" or \"off\")");
+    if (config->use_frozen_modules < 0) {
+        const wchar_t *value = config_get_xoption_value(config, L"frozen_modules");
+        if (value == NULL) {
+            int isdev = is_dev_env(config);
+            if (isdev >= 0) {
+                config->use_frozen_modules = !isdev;
+            }
+        }
+        else if (wcscmp(value, L"on") == 0) {
+            config->use_frozen_modules = 1;
+        }
+        else if (wcscmp(value, L"off") == 0) {
+            config->use_frozen_modules = 0;
+        }
+        else if (wcslen(value) == 0) {
+            // "-X frozen_modules" and "-X frozen_modules=" both imply "on".
+            config->use_frozen_modules = 1;
+        }
+        else {
+            return PyStatus_Error("bad value for option -X frozen_modules "
+                                  "(expected \"on\" or \"off\")");
+        }
     }
 
     return _PyStatus_OK();