(Merge 3.4) Issue #21090: io.FileIO.readall() does not ignore I/O errors

anymore. Before, it ignored I/O errors if at least the first C call read()
succeed.

Misc/NEWS

@@ -103,6 +103,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21090: io.FileIO.readall() does not ignore I/O errors anymore. Before,
+  it ignored I/O errors if at least the first C call read() succeed.
+
 - Issue #5800: headers parameter of wsgiref.headers.Headers is now optional.
   Initial patch by Pablo Torres Navarrete and SilentGhost.
 

Modules/_io/fileio.c

@@ -694,9 +694,9 @@ fileio_readall(fileio *self)
                 }
                 continue;
             }
-            if (bytes_read > 0)
-                break;
             if (errno == EAGAIN) {
+                if (bytes_read > 0)
+                    break;
                 Py_DECREF(result);
                 Py_RETURN_NONE;
             }