Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is

raised when the wrapped raw file is non-blocking and the write would block.
Previous code assumed that the raw write() would raise BlockingIOError, but
RawIOBase.write() is defined to returned None when the call would block.
Patch by sbt.

Lib/_pyio.py

@@ -6,6 +6,7 @@ import os
 import abc
 import codecs
 import warnings
+import errno
 # Import _thread instead of threading to reduce startup cost
 try:
     from _thread import allocate_lock as Lock
@@ -717,8 +718,11 @@ class _BufferedIOMixin(BufferedIOBase):
 
     def close(self):
         if self.raw is not None and not self.closed:
-            self.flush()
+            try:
-            self.raw.close()
+                # may raise BlockingIOError or BrokenPipeError etc
+                self.flush()
+            finally:
+                self.raw.close()
 
     def detach(self):
         if self.raw is None:
@@ -1077,13 +1081,9 @@ class BufferedWriter(_BufferedIOMixin):
             # XXX we can implement some more tricks to try and avoid
             # partial writes
             if len(self._write_buf) > self.buffer_size:
-                # We're full, so let's pre-flush the buffer
+                # We're full, so let's pre-flush the buffer.  (This may
-                try:
+                # raise BlockingIOError with characters_written == 0.)
-                    self._flush_unlocked()
+                self._flush_unlocked()
-                except BlockingIOError as e:
-                    # We can't accept anything else.
-                    # XXX Why not just let the exception pass through?
-                    raise BlockingIOError(e.errno, e.strerror, 0)
             before = len(self._write_buf)
             self._write_buf.extend(b)
             written = len(self._write_buf) - before
@@ -1114,22 +1114,21 @@ class BufferedWriter(_BufferedIOMixin):
     def _flush_unlocked(self):
         if self.closed:
             raise ValueError("flush of closed file")
-        written = 0
+        while self._write_buf:
-        try:
+            try:
-            while self._write_buf:
+                n = self.raw.write(self._write_buf)
-                try:
+            except InterruptedError:
-                    n = self.raw.write(self._write_buf)
+                continue
-                except InterruptedError:
+            except BlockingIOError:
-                    continue
+                raise RuntimeError("self.raw should implement RawIOBase: it "
-                if n > len(self._write_buf) or n < 0:
+                                   "should not raise BlockingIOError")
-                    raise IOError("write() returned incorrect number of bytes")
+            if n is None:
-                del self._write_buf[:n]
+                raise BlockingIOError(
-                written += n
+                    errno.EAGAIN,
-        except BlockingIOError as e:
+                    "write could not complete without blocking", 0)
-            n = e.characters_written
+            if n > len(self._write_buf) or n < 0:
+                raise IOError("write() returned incorrect number of bytes")
             del self._write_buf[:n]
-            written += n
-            raise BlockingIOError(e.errno, e.strerror, written)
 
     def tell(self):
         return _BufferedIOMixin.tell(self) + len(self._write_buf)

Lib/test/test_io.py

@@ -42,7 +42,10 @@ try:
     import threading
 except ImportError:
     threading = None
-
+try:
+    import fcntl
+except ImportError:
+    fcntl = None
 
 def _default_chunk_size():
     """Get the default TextIOWrapper chunk size"""
@@ -242,9 +245,14 @@ class MockNonBlockWriterIO:
             except ValueError:
                 pass
             else:
-                self._blocker_char = None
+                if n > 0:
-                self._write_stack.append(b[:n])
+                    # write data up to the first blocker
-                raise self.BlockingIOError(0, "test blocking", n)
+                    self._write_stack.append(b[:n])
+                    return n
+                else:
+                    # cancel blocker and indicate would block
+                    self._blocker_char = None
+                    return None
         self._write_stack.append(b)
         return len(b)
 
@@ -2753,6 +2761,70 @@ class MiscIOTest(unittest.TestCase):
                 with self.open(support.TESTFN, **kwargs) as f:
                     self.assertRaises(TypeError, pickle.dumps, f, protocol)
 
+    @unittest.skipUnless(fcntl, 'fcntl required for this test')
+    def test_nonblock_pipe_write_bigbuf(self):
+        self._test_nonblock_pipe_write(16*1024)
+
+    @unittest.skipUnless(fcntl, 'fcntl required for this test')
+    def test_nonblock_pipe_write_smallbuf(self):
+        self._test_nonblock_pipe_write(1024)
+
+    def _set_non_blocking(self, fd):
+        flags = fcntl.fcntl(fd, fcntl.F_GETFL)
+        self.assertNotEqual(flags, -1)
+        res = fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
+        self.assertEqual(res, 0)
+
+    def _test_nonblock_pipe_write(self, bufsize):
+        sent = []
+        received = []
+        r, w = os.pipe()
+        self._set_non_blocking(r)
+        self._set_non_blocking(w)
+
+        # To exercise all code paths in the C implementation we need
+        # to play with buffer sizes.  For instance, if we choose a
+        # buffer size less than or equal to _PIPE_BUF (4096 on Linux)
+        # then we will never get a partial write of the buffer.
+        rf = self.open(r, mode='rb', closefd=True, buffering=bufsize)
+        wf = self.open(w, mode='wb', closefd=True, buffering=bufsize)
+
+        with rf, wf:
+            for N in 9999, 73, 7574:
+                try:
+                    i = 0
+                    while True:
+                        msg = bytes([i % 26 + 97]) * N
+                        sent.append(msg)
+                        wf.write(msg)
+                        i += 1
+
+                except self.BlockingIOError as e:
+                    self.assertEqual(e.args[0], errno.EAGAIN)
+                    self.assertEqual(e.args[2], e.characters_written)
+                    sent[-1] = sent[-1][:e.characters_written]
+                    received.append(rf.read())
+                    msg = b'BLOCKED'
+                    wf.write(msg)
+                    sent.append(msg)
+
+            while True:
+                try:
+                    wf.flush()
+                    break
+                except self.BlockingIOError as e:
+                    self.assertEqual(e.args[0], errno.EAGAIN)
+                    self.assertEqual(e.args[2], e.characters_written)
+                    self.assertEqual(e.characters_written, 0)
+                    received.append(rf.read())
+
+            received += iter(rf.read, None)
+
+        sent, received = b''.join(sent), b''.join(received)
+        self.assertTrue(sent == received)
+        self.assertTrue(wf.closed)
+        self.assertTrue(rf.closed)
+
 class CMiscIOTest(MiscIOTest):
     io = io
 

Misc/ACKS

@@ -860,6 +860,7 @@ Ilya Sandler
 Mark Sapiro
 Ty Sarna
 Ben Sayer
+sbt
 Andrew Schaaf
 Michael Scharf
 Andreas Schawo

Misc/NEWS

@@ -384,6 +384,12 @@ Core and Builtins
 Library
 -------
 
+- Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is
+  raised when the wrapped raw file is non-blocking and the write would block.
+  Previous code assumed that the raw write() would raise BlockingIOError, but
+  RawIOBase.write() is defined to returned None when the call would block.
+  Patch by sbt.
+
 - Issue #13358: HTMLParser now calls handle_data only once for each CDATA.
 
 - Issue #4147: minidom's toprettyxml no longer adds whitespace around a text

Modules/_io/bufferedio.c

@@ -596,7 +596,7 @@ buffered_getstate(buffered *self, PyObject *args)
 
 /* Forward decls */
 static PyObject *
-_bufferedwriter_flush_unlocked(buffered *, int);
+_bufferedwriter_flush_unlocked(buffered *);
 static Py_ssize_t
 _bufferedreader_fill_buffer(buffered *self);
 static void
@@ -618,6 +618,18 @@ _bufferedreader_raw_read(buffered *self, char *start, Py_ssize_t len);
  * Helpers
  */
 
+/* Sets the current error to BlockingIOError */
+static void
+_set_BlockingIOError(char *msg, Py_ssize_t written)
+{
+    PyObject *err;
+    err = PyObject_CallFunction(PyExc_BlockingIOError, "isn",
+                                errno, msg, written);
+    if (err)
+        PyErr_SetObject(PyExc_BlockingIOError, err);
+    Py_XDECREF(err);
+}
+
 /* Returns the address of the `written` member if a BlockingIOError was
    raised, NULL otherwise. The error is always re-raised. */
 static Py_ssize_t *
@@ -772,7 +784,7 @@ buffered_flush_and_rewind_unlocked(buffered *self)
 {
     PyObject *res;
 
-    res = _bufferedwriter_flush_unlocked(self, 0);
+    res = _bufferedwriter_flush_unlocked(self);
     if (res == NULL)
         return NULL;
     Py_DECREF(res);
@@ -1188,7 +1200,7 @@ buffered_seek(buffered *self, PyObject *args)
 
     /* Fallback: invoke raw seek() method and clear buffer */
     if (self->writable) {
-        res = _bufferedwriter_flush_unlocked(self, 0);
+        res = _bufferedwriter_flush_unlocked(self);
         if (res == NULL)
             goto end;
         Py_CLEAR(res);
@@ -1807,6 +1819,7 @@ _bufferedwriter_raw_write(buffered *self, char *start, Py_ssize_t len)
     Py_buffer buf;
     PyObject *memobj, *res;
     Py_ssize_t n;
+    int errnum;
     /* NOTE: the buffer needn't be released as its object is NULL. */
     if (PyBuffer_FillInfo(&buf, NULL, start, len, 1, PyBUF_CONTIG_RO) == -1)
         return -1;
@@ -1819,11 +1832,21 @@ _bufferedwriter_raw_write(buffered *self, char *start, Py_ssize_t len)
        raised (see issue #10956).
     */
     do {
+        errno = 0;
         res = PyObject_CallMethodObjArgs(self->raw, _PyIO_str_write, memobj, NULL);
+        errnum = errno;
     } while (res == NULL && _trap_eintr());
     Py_DECREF(memobj);
     if (res == NULL)
         return -1;
+    if (res == Py_None) {
+        /* Non-blocking stream would have blocked. Special return code!
+           Being paranoid we reset errno in case it is changed by code
+           triggered by a decref.  errno is used by _set_BlockingIOError(). */
+        Py_DECREF(res);
+        errno = errnum;
+        return -2;
+    }
     n = PyNumber_AsSsize_t(res, PyExc_ValueError);
     Py_DECREF(res);
     if (n < 0 || n > len) {
@@ -1840,7 +1863,7 @@ _bufferedwriter_raw_write(buffered *self, char *start, Py_ssize_t len)
 /* `restore_pos` is 1 if we need to restore the raw stream position at
    the end, 0 otherwise. */
 static PyObject *
-_bufferedwriter_flush_unlocked(buffered *self, int restore_pos)
+_bufferedwriter_flush_unlocked(buffered *self)
 {
     Py_ssize_t written = 0;
     Py_off_t n, rewind;
@@ -1862,14 +1885,11 @@ _bufferedwriter_flush_unlocked(buffered *self, int restore_pos)
             Py_SAFE_DOWNCAST(self->write_end - self->write_pos,
                              Py_off_t, Py_ssize_t));
         if (n == -1) {
-            Py_ssize_t *w = _buffered_check_blocking_error();
+            goto error;
-            if (w == NULL)
+        }
-                goto error;
+        else if (n == -2) {
-            self->write_pos += *w;
+            _set_BlockingIOError("write could not complete without blocking",
-            self->raw_pos = self->write_pos;
+                                 0);
-            written += *w;
-            *w = written;
-            /* Already re-raised */
             goto error;
         }
         self->write_pos += n;
@@ -1882,16 +1902,6 @@ _bufferedwriter_flush_unlocked(buffered *self, int restore_pos)
             goto error;
     }
 
-    if (restore_pos) {
-        Py_off_t forward = rewind - written;
-        if (forward != 0) {
-            n = _buffered_raw_seek(self, forward, 1);
-            if (n < 0) {
-                goto error;
-            }
-            self->raw_pos += forward;
-        }
-    }
     _bufferedwriter_reset_buf(self);
 
 end:
@@ -1944,7 +1954,7 @@ bufferedwriter_write(buffered *self, PyObject *args)
     }
 
     /* First write the current buffer */
-    res = _bufferedwriter_flush_unlocked(self, 0);
+    res = _bufferedwriter_flush_unlocked(self);
     if (res == NULL) {
         Py_ssize_t *w = _buffered_check_blocking_error();
         if (w == NULL)
@@ -1967,14 +1977,19 @@ bufferedwriter_write(buffered *self, PyObject *args)
             PyErr_Clear();
             memcpy(self->buffer + self->write_end, buf.buf, buf.len);
             self->write_end += buf.len;
+            self->pos += buf.len;
             written = buf.len;
             goto end;
         }
         /* Buffer as much as possible. */
         memcpy(self->buffer + self->write_end, buf.buf, avail);
         self->write_end += avail;
-        /* Already re-raised */
+        self->pos += avail;
-        *w = avail;
+        /* XXX Modifying the existing exception e using the pointer w
+           will change e.characters_written but not e.args[2].
+           Therefore we just replace with a new error. */
+        _set_BlockingIOError("write could not complete without blocking",
+                             avail);
         goto error;
     }
     Py_CLEAR(res);
@@ -1999,11 +2014,9 @@ bufferedwriter_write(buffered *self, PyObject *args)
         Py_ssize_t n = _bufferedwriter_raw_write(
             self, (char *) buf.buf + written, buf.len - written);
         if (n == -1) {
-            Py_ssize_t *w = _buffered_check_blocking_error();
+            goto error;
-            if (w == NULL)
+        } else if (n == -2) {
-                goto error;
+            /* Write failed because raw file is non-blocking */
-            written += *w;
-            remaining -= *w;
             if (remaining > self->buffer_size) {
                 /* Can't buffer everything, still buffer as much as possible */
                 memcpy(self->buffer,
@@ -2011,8 +2024,9 @@ bufferedwriter_write(buffered *self, PyObject *args)
                 self->raw_pos = 0;
                 ADJUST_POSITION(self, self->buffer_size);
                 self->write_end = self->buffer_size;
-                *w = written + self->buffer_size;
+                written += self->buffer_size;
-                /* Already re-raised */
+                _set_BlockingIOError("write could not complete without "
+                                     "blocking", written);
                 goto error;
             }
             PyErr_Clear();