Don't abort on FT2Font weakref.

anntzer · anntzer · commit 77bff5ffec7a · 2017-08-22T17:37:40.000-07:00
Currently,

    import weakref, matplotlib.ft2font
    weakref.ref(matplotlib.ft2font.FT2Font("/usr/share/fonts/TTF/DejaVuSans.ttf"))

aborts Py3 with

    terminate called after throwing an instance of 'char const*'
    Fatal Python error: Aborted

whereas on Py2 a normal `TypeError: cannot create weak reference to
'matplotlib.ft2font.FT2Font' object` is raised.

This is because the following happens:
- Py3 sets the TypeError for failure to create a weakref.
- The FT2Font object gets GC'd as nothing is referring to it (yes,
  creating a weakref to a temporary is a bit silly).
- The FT2Font destructor calls close_file_callback, which calls
  mpl_PyFile_DupClose, which calls PyObject_AsFileDescriptor... which,
  on Py3, calls the fileno() method on the file object.
- PyObject_AsFileDescriptor fails because the originally set TypeError
  has not been cleared, so mpl_PyFile_DupClose likewise fails, causing
  close_file_callback to throw a C++ exception.

Instead, carefully stash and restore the current exception state, both
in mpl_PyFile_DupClose, and likewise below, in mpl_PyFile_CloseFile (the
latter is needed because otherwise PyObject_CallMethod will clear the
exception, cf comment in CPython's Objects/abstract.c:

    /* PyObject_Call() must not be called with an exception set,
    because it may clear it (directly or indirectly) and so the
    caller loses its exception */

Note that if an exception *actually* gets raised by mpl_PyFile_DupClose
or mpl_PyFile_CloseFile, it will overwrite the TypeError.  Strictly
speaking, on Py3, it may be preferrable to chain the exceptions, but
this seems a bit overkill (mostly, I just want to avoid aborting the
whole Python process).
diff --git a/src/file_compat.h b/src/file_compat.h
@@ -126,6 +126,9 @@ static NPY_INLINE FILE *mpl_PyFile_Dup(PyObject *file, char *mode, mpl_off_t *or
  */
 static NPY_INLINE int mpl_PyFile_DupClose(PyObject *file, FILE *handle, mpl_off_t orig_pos)
 {
+    PyObject *type = NULL, *value = NULL, *tb = NULL;
+    PyErr_Fetch(&type, &value, &tb);
+
     int fd;
     PyObject *ret;
     mpl_off_t position;
@@ -136,25 +139,33 @@ static NPY_INLINE int mpl_PyFile_DupClose(PyObject *file, FILE *handle, mpl_off_
     fclose(handle);
 
     /* Restore original file handle position, in order to not confuse
-       Python-side data structures */
+       Python-side data structures.  Note that this would fail if an exception
+       is currently set, which can happen as this function is called in cleanup
+       code, so we need to carefully fetch and restore the exception state. */
     fd = PyObject_AsFileDescriptor(file);
     if (fd == -1) {
-        return -1;
+        goto fail;
     }
     if (mpl_lseek(fd, orig_pos, SEEK_SET) != -1) {
         if (position == -1) {
             PyErr_SetString(PyExc_IOError, "obtaining file position failed");
-            return -1;
+            goto fail;
         }
 
         /* Seek Python-side handle to the FILE* handle position */
         ret = PyObject_CallMethod(file, (char *)"seek", (char *)(MPL_OFF_T_PYFMT "i"), position, 0);
         if (ret == NULL) {
-            return -1;
+            goto fail;
         }
         Py_DECREF(ret);
     }
+    PyErr_Restore(type, value, tb);
     return 0;
+fail:
+    Py_XDECREF(type);
+    Py_XDECREF(value);
+    Py_XDECREF(tb);
+    return -1;
 }
 
 static NPY_INLINE int mpl_PyFile_Check(PyObject *file)
@@ -200,14 +211,23 @@ static NPY_INLINE PyObject *mpl_PyFile_OpenFile(PyObject *filename, const char *
 
 static NPY_INLINE int mpl_PyFile_CloseFile(PyObject *file)
 {
+    PyObject *type, *value, *tb;
+    PyErr_Fetch(&type, &value, &tb);
+
     PyObject *ret;
 
     ret = PyObject_CallMethod(file, (char *)"close", NULL);
     if (ret == NULL) {
-        return -1;
+        goto fail;
     }
     Py_DECREF(ret);
+    PyErr_Restore(type, value, tb);
     return 0;
+fail:
+    Py_XDECREF(type);
+    Py_XDECREF(value);
+    Py_XDECREF(tb);
+    return -1;
 }
 
 #ifdef __cplusplus
