bpo-36352: Clarify fileutils.h documentation (GH-12406)

The last parameter of _Py_wreadlink(), _Py_wrealpath() and
_Py_wgetcwd() is a length, not a size: number of characters including
the trailing NUL character.

Enhance also documentation of error conditions.

Include/fileutils.h

@@ -140,19 +140,25 @@ PyAPI_FUNC(Py_ssize_t) _Py_write_noraise(
 PyAPI_FUNC(int) _Py_wreadlink(
     const wchar_t *path,
     wchar_t *buf,
-    size_t bufsiz);
+    /* Number of characters of 'buf' buffer
+       including the trailing NUL character */
+    size_t buflen);
 #endif
 
 #ifdef HAVE_REALPATH
 PyAPI_FUNC(wchar_t*) _Py_wrealpath(
     const wchar_t *path,
     wchar_t *resolved_path,
-    size_t resolved_path_size);
+    /* Number of characters of 'resolved_path' buffer
+       including the trailing NUL character */
+    size_t resolved_path_len);
 #endif
 
 PyAPI_FUNC(wchar_t*) _Py_wgetcwd(
     wchar_t *buf,
-    size_t size);
+    /* Number of characters of 'buf' buffer
+       including the trailing NUL character */
+    size_t buflen);
 
 PyAPI_FUNC(int) _Py_get_inheritable(int fd);
 

Python/fileutils.c

@@ -1629,10 +1629,12 @@ _Py_write_noraise(int fd, const void *buf, size_t count)
 #ifdef HAVE_READLINK
 
 /* Read value of symbolic link. Encode the path to the locale encoding, decode
-   the result from the locale encoding. Return -1 on error. */
+   the result from the locale encoding.
 
+   Return -1 on encoding error, on readlink() error, if the internal buffer is
+   too short, on decoding error, or if 'buf' is too short. */
 int
-_Py_wreadlink(const wchar_t *path, wchar_t *buf, size_t bufsiz)
+_Py_wreadlink(const wchar_t *path, wchar_t *buf, size_t buflen)
 {
     char *cpath;
     char cbuf[MAXPATHLEN];
@@ -1659,12 +1661,13 @@ _Py_wreadlink(const wchar_t *path, wchar_t *buf, size_t bufsiz)
         errno = EINVAL;
         return -1;
     }
-    if (bufsiz <= r1) {
+    /* wbuf must have space to store the trailing NUL character */
+    if (buflen <= r1) {
         PyMem_RawFree(wbuf);
         errno = EINVAL;
         return -1;
     }
-    wcsncpy(buf, wbuf, bufsiz);
+    wcsncpy(buf, wbuf, buflen);
     PyMem_RawFree(wbuf);
     return (int)r1;
 }
@@ -1674,11 +1677,12 @@ _Py_wreadlink(const wchar_t *path, wchar_t *buf, size_t bufsiz)
 
 /* Return the canonicalized absolute pathname. Encode path to the locale
    encoding, decode the result from the locale encoding.
-   Return NULL on error. */
 
+   Return NULL on encoding error, realpath() error, decoding error
+   or if 'resolved_path' is too short. */
 wchar_t*
 _Py_wrealpath(const wchar_t *path,
-              wchar_t *resolved_path, size_t resolved_path_size)
+              wchar_t *resolved_path, size_t resolved_path_len)
 {
     char *cpath;
     char cresolved_path[MAXPATHLEN];
@@ -1700,12 +1704,13 @@ _Py_wrealpath(const wchar_t *path,
         errno = EINVAL;
         return NULL;
     }
-    if (resolved_path_size <= r) {
+    /* wresolved_path must have space to store the trailing NUL character */
+    if (resolved_path_len <= r) {
         PyMem_RawFree(wresolved_path);
         errno = EINVAL;
         return NULL;
     }
-    wcsncpy(resolved_path, wresolved_path, resolved_path_size);
+    wcsncpy(resolved_path, wresolved_path, resolved_path_len);
     PyMem_RawFree(wresolved_path);
     return resolved_path;
 }
@@ -1713,14 +1718,15 @@ _Py_wrealpath(const wchar_t *path,
 
 /* Get the current directory. size is the buffer size in wide characters
    including the null character. Decode the path from the locale encoding.
-   Return NULL on error. */
 
+   Return NULL on getcwd() error, on decoding error, or if 'buf' is
+   too short. */
 wchar_t*
-_Py_wgetcwd(wchar_t *buf, size_t size)
+_Py_wgetcwd(wchar_t *buf, size_t buflen)
 {
 #ifdef MS_WINDOWS
-    int isize = (int)Py_MIN(size, INT_MAX);
-    return _wgetcwd(buf, isize);
+    int ibuflen = (int)Py_MIN(buflen, INT_MAX);
+    return _wgetcwd(buf, ibuflen);
 #else
     char fname[MAXPATHLEN];
     wchar_t *wname;
@@ -1731,11 +1737,12 @@ _Py_wgetcwd(wchar_t *buf, size_t size)
     wname = Py_DecodeLocale(fname, &len);
     if (wname == NULL)
         return NULL;
-    if (size <= len) {
+    /* wname must have space to store the trailing NUL character */
+    if (buflen <= len) {
         PyMem_RawFree(wname);
         return NULL;
     }
-    wcsncpy(buf, wname, size);
+    wcsncpy(buf, wname, buflen);
     PyMem_RawFree(wname);
     return buf;
 #endif