bpo-32186: Release the GIL during fstat and lseek calls

In fileio, there were 3 fstat() calls and one lseek() call that did not
release the GIL during the call. This can cause all threads to hang for
unlimited time when using io.FileIO with inaccessible NFS server.

Author: nirs

Modules/_io/fileio.c

@@ -146,9 +146,15 @@ dircheck(fileio* self, PyObject *nameobj)
 {
 #if defined(HAVE_FSTAT) && defined(S_IFDIR) && defined(EISDIR)
     struct stat buf;
+    int res;
     if (self->fd < 0)
         return 0;
-    if (fstat(self->fd, &buf) == 0 && S_ISDIR(buf.st_mode)) {
+
+    Py_BEGIN_ALLOW_THREADS
+    res = fstat(self->fd, &buf);
+    Py_END_ALLOW_THREADS
+
+    if (res == 0 && S_ISDIR(buf.st_mode)) {
         errno = EISDIR;
         PyErr_SetFromErrnoWithFilenameObject(PyExc_IOError, nameobj);
         return -1;
@@ -162,17 +168,34 @@ check_fd(int fd)
 {
 #if defined(HAVE_FSTAT)
     struct stat buf;
-    if (!_PyVerify_fd(fd) || (fstat(fd, &buf) < 0 && errno == EBADF)) {
-        PyObject *exc;
-        char *msg = strerror(EBADF);
-        exc = PyObject_CallFunction(PyExc_OSError, "(is)",
-                                    EBADF, msg);
-        PyErr_SetObject(PyExc_OSError, exc);
-        Py_XDECREF(exc);
-        return -1;
+    int res;
+    PyObject *exc;
+    char *msg;
+
+    if (!_PyVerify_fd(fd)) {
+        goto badfd;
     }
-#endif
+
+    Py_BEGIN_ALLOW_THREADS
+    res = fstat(fd, &buf);
+    Py_END_ALLOW_THREADS
+
+    if (res < 0 && errno == EBADF) {
+        goto badfd;
+    }
+
     return 0;
+
+badfd:
+    msg = strerror(EBADF);
+    exc = PyObject_CallFunction(PyExc_OSError, "(is)",
+                                EBADF, msg);
+    PyErr_SetObject(PyExc_OSError, exc);
+    Py_XDECREF(exc);
+    return -1;
+#else
+    return 0;
+#endif
 }
 
 
@@ -519,9 +542,19 @@ new_buffersize(fileio *self, size_t currentsize)
 #ifdef HAVE_FSTAT
     off_t pos, end;
     struct stat st;
-    if (fstat(self->fd, &st) == 0) {
+    int res;
+
+    Py_BEGIN_ALLOW_THREADS
+    res = fstat(self->fd, &st);
+    Py_END_ALLOW_THREADS
+
+    if (res == 0) {
         end = st.st_size;
+
+        Py_BEGIN_ALLOW_THREADS
         pos = lseek(self->fd, 0L, SEEK_CUR);
+        Py_END_ALLOW_THREADS
+
         /* Files claiming a size smaller than SMALLCHUNK may
            actually be streaming pseudo-files. In this case, we
            apply the more aggressive algorithm below.