bpo-38353: getpath.c: allocates strings on the heap (GH-16585)

* _Py_FindEnvConfigValue() now returns a string allocated
  by PyMem_RawMalloc().
* calculate_init() now decodes VPATH macro.
* Add calculate_open_pyenv() function.
* Add substring() and joinpath2() functions.

* Fix add_exe_suffix()

And a few cleanup changes.

Include/internal/pycore_pathconfig.h

@@ -56,11 +56,10 @@ extern PyStatus _PyPathConfig_Calculate(
 extern int _PyPathConfig_ComputeSysPath0(
     const PyWideStringList *argv,
     PyObject **path0);
-extern int _Py_FindEnvConfigValue(
+extern PyStatus _Py_FindEnvConfigValue(
     FILE *env_file,
     const wchar_t *key,
-    wchar_t *value,
-    size_t value_size);
+    wchar_t **value_p);
 
 #ifdef MS_WINDOWS
 extern wchar_t* _Py_GetDLLPath(void);

PC/getpathp.c

@@ -752,7 +752,7 @@ calculate_pth_file(PyCalculatePath *calculate, _PyPathConfig *pathconfig,
    executable's directory and then in the parent directory.
    If found, open it for use when searching for prefixes.
 */
-static void
+static PyStatus
 calculate_pyvenv_file(PyCalculatePath *calculate,
                       wchar_t *argv0_path, size_t argv0_path_len)
 {
@@ -775,17 +775,23 @@ calculate_pyvenv_file(PyCalculatePath *calculate,
         env_file = _Py_wfopen(filename, L"r");
         if (env_file == NULL) {
             errno = 0;
-            return;
+            return _PyStatus_OK();
         }
     }
 
     /* Look for a 'home' variable and set argv0_path to it, if found */
-    wchar_t home[MAXPATHLEN+1];
-    if (_Py_FindEnvConfigValue(env_file, L"home",
-                               home, Py_ARRAY_LENGTH(home))) {
+    wchar_t *home = NULL;
+    PyStatus status = _Py_FindEnvConfigValue(env_file, L"home", &home);
+    if (_PyStatus_EXCEPTION(status)) {
+        fclose(env_file);
+        return status;
+    }
+    if (home) {
         wcscpy_s(argv0_path, argv0_path_len, home);
+        PyMem_RawFree(home);
     }
     fclose(env_file);
+    return _PyStatus_OK();
 }
 
 
@@ -1022,7 +1028,11 @@ calculate_path(PyCalculatePath *calculate, _PyPathConfig *pathconfig)
         goto done;
     }
 
-    calculate_pyvenv_file(calculate, argv0_path, Py_ARRAY_LENGTH(argv0_path));
+    status = calculate_pyvenv_file(calculate,
+                                   argv0_path, Py_ARRAY_LENGTH(argv0_path));
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     /* Calculate zip archive path from DLL or exe path */
     wchar_t zip_path[MAXPATHLEN+1];

Python/pathconfig.c

@@ -777,12 +777,17 @@ _PyPathConfig_ComputeSysPath0(const PyWideStringList *argv, PyObject **path0_p)
 #endif
 
 /* Search for a prefix value in an environment file (pyvenv.cfg).
-   If found, copy it into the provided buffer. */
-int
+
+   - If found, copy it into *value_p: string which must be freed by
+     PyMem_RawFree().
+   - If not found, *value_p is set to NULL.
+*/
+PyStatus
 _Py_FindEnvConfigValue(FILE *env_file, const wchar_t *key,
-                       wchar_t *value, size_t value_size)
+                       wchar_t **value_p)
 {
-    int result = 0; /* meaning not found */
+    *value_p = NULL;
+
     char buffer[MAXPATHLEN * 2 + 1];  /* allow extra for key, '=', etc. */
     buffer[Py_ARRAY_LENGTH(buffer)-1] = '\0';
 
@@ -812,18 +817,24 @@ _Py_FindEnvConfigValue(FILE *env_file, const wchar_t *key,
                 if ((tok != NULL) && !wcscmp(tok, L"=")) {
                     tok = WCSTOK(NULL, L"\r\n", &state);
                     if (tok != NULL) {
-                        wcsncpy(value, tok, value_size - 1);
-                        value[value_size - 1] = L'\0';
-                        result = 1;
+                        *value_p = _PyMem_RawWcsdup(tok);
                         PyMem_RawFree(tmpbuffer);
-                        break;
+
+                        if (*value_p == NULL) {
+                            return _PyStatus_NO_MEMORY();
+                        }
+
+                        /* found */
+                        return _PyStatus_OK();
                     }
                 }
             }
             PyMem_RawFree(tmpbuffer);
         }
     }
-    return result;
+
+    /* not found */
+    return _PyStatus_OK();
 }
 
 #ifdef __cplusplus