bpo-36352: Avoid hardcoded MAXPATHLEN size in getpath.c (GH-12423)

* Use Py_ARRAY_LENGTH() rather than hardcoded MAXPATHLEN in getpath.c.
* Pass string length to functions modifying strings.

Misc/NEWS.d/next/Core and Builtins/2019-03-19-02-36-40.bpo-36352.qj2trz.rst

+Python initialization now fails with an error, rather than silently
+truncating paths, if a path is too long.

Python/fileutils.c

@@ -1716,7 +1716,7 @@ _Py_wrealpath(const wchar_t *path,
 }
 #endif
 
-/* Get the current directory. size is the buffer size in wide characters
+/* Get the current directory. buflen is the buffer size in wide characters
    including the null character. Decode the path from the locale encoding.
 
    Return NULL on getcwd() error, on decoding error, or if 'buf' is

Python/pathconfig.c

@@ -578,8 +578,8 @@ _PyPathConfig_ComputeArgv0(const _PyWstrList *argv)
     int have_script_arg = 0;
     int have_module_arg = 0;
 #ifdef HAVE_READLINK
-    wchar_t link[MAXPATHLEN+1];
-    wchar_t argv0copy[2*MAXPATHLEN+1];
+    wchar_t link[MAXPATHLEN + 1];
+    wchar_t argv0copy[2 * MAXPATHLEN + 1];
     int nr = 0;
 #endif
 #if defined(HAVE_REALPATH)
@@ -607,7 +607,7 @@ _PyPathConfig_ComputeArgv0(const _PyWstrList *argv)
 
 #ifdef HAVE_READLINK
     if (have_script_arg)
-        nr = _Py_wreadlink(argv0, link, MAXPATHLEN);
+        nr = _Py_wreadlink(argv0, link, Py_ARRAY_LENGTH(link));
     if (nr > 0) {
         /* It's a symlink */
         link[nr] = '\0';
@@ -692,11 +692,12 @@ _Py_FindEnvConfigValue(FILE *env_file, const wchar_t *key,
                        wchar_t *value, size_t value_size)
 {
     int result = 0; /* meaning not found */
-    char buffer[MAXPATHLEN*2+1];  /* allow extra for key, '=', etc. */
+    char buffer[MAXPATHLEN * 2 + 1];  /* allow extra for key, '=', etc. */
+    buffer[Py_ARRAY_LENGTH(buffer)-1] = '\0';
 
     fseek(env_file, 0, SEEK_SET);
     while (!feof(env_file)) {
-        char * p = fgets(buffer, MAXPATHLEN*2, env_file);
+        char * p = fgets(buffer, Py_ARRAY_LENGTH(buffer) - 1, env_file);
 
         if (p == NULL) {
             break;
@@ -721,7 +722,8 @@ _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, MAXPATHLEN);
+                        wcsncpy(value, tok, value_size - 1);
+                        value[value_size - 1] = L'\0';
                         result = 1;
                         PyMem_RawFree(tmpbuffer);
                         break;