gh-129205: Add os.readinto() API for reading data into a caller provided buffer (#129211)

Add a new OS API which will read data directly into a caller provided
writeable buffer protocol object.

Co-authored-by: Bénédikt Tran <[email protected]>
Co-authored-by: Victor Stinner <[email protected]>

Author: 3 people

Doc/library/os.rst

@@ -1659,6 +1659,33 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
       :exc:`InterruptedError` exception (see :pep:`475` for the rationale).
 
 
+.. function:: readinto(fd, buffer, /)
+
+   Read from a file descriptor *fd* into a mutable
+   :ref:`buffer object <bufferobjects>` *buffer*.
+
+   The *buffer* should be mutable and :term:`bytes-like <bytes-like object>`. On
+   success, returns the number of bytes read. Less bytes may be read than the
+   size of the buffer. The underlying system call will be retried when
+   interrupted by a signal, unless the signal handler raises an exception.
+   Other errors will not be retried and an error will be raised.
+
+   Returns 0 if *fd* is at end of file or if the provided *buffer* has
+   length 0 (which can be used to check for errors without reading data).
+   Never returns negative.
+
+   .. note::
+
+      This function is intended for low-level I/O and must be applied to a file
+      descriptor as returned by :func:`os.open` or :func:`os.pipe`.  To read a
+      "file object" returned by the built-in function :func:`open`, or
+      :data:`sys.stdin`, use its member functions, for example
+      :meth:`io.BufferedIOBase.readinto`, :meth:`io.BufferedIOBase.read`, or
+      :meth:`io.TextIOBase.read`
+
+   .. versionadded:: next
+
+
 .. function:: sendfile(out_fd, in_fd, offset, count)
               sendfile(out_fd, in_fd, offset, count, headers=(), trailers=(), flags=0)
 

Doc/whatsnew/3.14.rst

@@ -561,6 +561,10 @@ os
   to the :mod:`os` module.
   (Contributed by James Roy in :gh:`127688`.)
 
+* Add the :func:`os.readinto` function to read into a
+  :ref:`buffer object <bufferobjects>` from a file descriptor.
+  (Contributed by Cody Maloney in :gh:`129205`.)
+
 
 pathlib
 -------

Lib/test/_test_eintr.py

@@ -152,6 +152,37 @@ def test_read(self):
                 self.assertEqual(data, os.read(rd, len(data)))
             self.assertEqual(proc.wait(), 0)
 
+    def test_readinto(self):
+        rd, wr = os.pipe()
+        self.addCleanup(os.close, rd)
+        # wr closed explicitly by parent
+
+        # the payload below are smaller than PIPE_BUF, hence the writes will be
+        # atomic
+        datas = [b"hello", b"world", b"spam"]
+
+        code = '\n'.join((
+            'import os, sys, time',
+            '',
+            'wr = int(sys.argv[1])',
+            'datas = %r' % datas,
+            'sleep_time = %r' % self.sleep_time,
+            '',
+            'for data in datas:',
+            '    # let the parent block on read()',
+            '    time.sleep(sleep_time)',
+            '    os.write(wr, data)',
+        ))
+
+        proc = self.subprocess(code, str(wr), pass_fds=[wr])
+        with kill_on_error(proc):
+            os.close(wr)
+            for data in datas:
+                buffer = bytearray(len(data))
+                self.assertEqual(os.readinto(rd, buffer), len(data))
+                self.assertEqual(buffer, data)
+            self.assertEqual(proc.wait(), 0)
+
     def test_write(self):
         rd, wr = os.pipe()
         self.addCleanup(os.close, wr)

Lib/test/test_os.py

@@ -230,6 +230,93 @@ def test_read(self):
             self.assertEqual(type(s), bytes)
             self.assertEqual(s, b"spam")
 
+    def test_readinto(self):
+        with open(os_helper.TESTFN, "w+b") as fobj:
+            fobj.write(b"spam")
+            fobj.flush()
+            fd = fobj.fileno()
+            os.lseek(fd, 0, 0)
+            # Oversized so readinto without hitting end.
+            buffer = bytearray(7)
+            s = os.readinto(fd, buffer)
+            self.assertEqual(type(s), int)
+            self.assertEqual(s, 4)
+            # Should overwrite the first 4 bytes of the buffer.
+            self.assertEqual(buffer[:4], b"spam")
+
+            # Readinto at EOF should return 0 and not touch buffer.
+            buffer[:] = b"notspam"
+            s = os.readinto(fd, buffer)
+            self.assertEqual(type(s), int)
+            self.assertEqual(s, 0)
+            self.assertEqual(bytes(buffer), b"notspam")
+            s = os.readinto(fd, buffer)
+            self.assertEqual(s, 0)
+            self.assertEqual(bytes(buffer), b"notspam")
+
+            # Readinto a 0 length bytearray when at EOF should return 0
+            self.assertEqual(os.readinto(fd, bytearray()), 0)
+
+            # Readinto a 0 length bytearray with data available should return 0.
+            os.lseek(fd, 0, 0)
+            self.assertEqual(os.readinto(fd, bytearray()), 0)
+
+    @unittest.skipUnless(hasattr(os, 'get_blocking'),
+                     'needs os.get_blocking() and os.set_blocking()')
+    @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
+    def test_readinto_non_blocking(self):
+        # Verify behavior of a readinto which would block on a non-blocking fd.
+        r, w = os.pipe()
+        try:
+            os.set_blocking(r, False)
+            with self.assertRaises(BlockingIOError):
+                os.readinto(r, bytearray(5))
+
+            # Pass some data through
+            os.write(w, b"spam")
+            self.assertEqual(os.readinto(r, bytearray(4)), 4)
+
+            # Still don't block or return 0.
+            with self.assertRaises(BlockingIOError):
+                os.readinto(r, bytearray(5))
+
+            # At EOF should return size 0
+            os.close(w)
+            w = None
+            self.assertEqual(os.readinto(r, bytearray(5)), 0)
+            self.assertEqual(os.readinto(r, bytearray(5)), 0)  # Still EOF
+
+        finally:
+            os.close(r)
+            if w is not None:
+                os.close(w)
+
+    def test_readinto_badarg(self):
+        with open(os_helper.TESTFN, "w+b") as fobj:
+            fobj.write(b"spam")
+            fobj.flush()
+            fd = fobj.fileno()
+            os.lseek(fd, 0, 0)
+
+            for bad_arg in ("test", bytes(), 14):
+                with self.subTest(f"bad buffer {type(bad_arg)}"):
+                    with self.assertRaises(TypeError):
+                        os.readinto(fd, bad_arg)
+
+            with self.subTest("doesn't work on file objects"):
+                with self.assertRaises(TypeError):
+                    os.readinto(fobj, bytearray(5))
+
+            # takes two args
+            with self.assertRaises(TypeError):
+                os.readinto(fd)
+
+            # No data should have been read with the bad arguments.
+            buffer = bytearray(4)
+            s = os.readinto(fd, buffer)
+            self.assertEqual(s, 4)
+            self.assertEqual(buffer, b"spam")
+
     @support.cpython_only
     # Skip the test on 32-bit platforms: the number of bytes must fit in a
     # Py_ssize_t type
@@ -249,6 +336,29 @@ def test_large_read(self, size):
         # operating system is free to return less bytes than requested.
         self.assertEqual(data, b'test')
 
+
+    @support.cpython_only
+    # Skip the test on 32-bit platforms: the number of bytes must fit in a
+    # Py_ssize_t type
+    @unittest.skipUnless(INT_MAX < PY_SSIZE_T_MAX,
+                         "needs INT_MAX < PY_SSIZE_T_MAX")
+    @support.bigmemtest(size=INT_MAX + 10, memuse=1, dry_run=False)
+    def test_large_readinto(self, size):
+        self.addCleanup(os_helper.unlink, os_helper.TESTFN)
+        create_file(os_helper.TESTFN, b'test')
+
+        # Issue #21932: For readinto the buffer contains the length rather than
+        # a length being passed explicitly to read, should still get capped to a
+        # valid size / not raise an OverflowError for sizes larger than INT_MAX.
+        buffer = bytearray(INT_MAX + 10)
+        with open(os_helper.TESTFN, "rb") as fp:
+            length = os.readinto(fp.fileno(), buffer)
+
+        # The test does not try to read more than 2 GiB at once because the
+        # operating system is free to return less bytes than requested.
+        self.assertEqual(length, 4)
+        self.assertEqual(buffer[:4], b'test')
+
     def test_write(self):
         # os.write() accepts bytes- and buffer-like objects but not strings
         fd = os.open(os_helper.TESTFN, os.O_CREAT | os.O_WRONLY)
@@ -2467,6 +2577,10 @@ def test_lseek(self):
     def test_read(self):
         self.check(os.read, 1)
 
+    @unittest.skipUnless(hasattr(os, 'readinto'), 'test needs os.readinto()')
+    def test_readinto(self):
+        self.check(os.readinto, bytearray(5))
+
     @unittest.skipUnless(hasattr(os, 'readv'), 'test needs os.readv()')
     def test_readv(self):
         buf = bytearray(10)

Misc/NEWS.d/next/Library/2025-01-22-16-54-25.gh-issue-129205.FMqrUt.rst

@@ -0,0 +1 @@
+Add :func:`os.readinto` to read into a :ref:`buffer object <bufferobjects>` from a file descriptor.

Modules/clinic/posixmodule.c.h

@@ -11433,6 +11433,38 @@ os_read_impl(PyObject *module, int fd, Py_ssize_t length)
     return buffer;
 }
 
+/*[clinic input]
+os.readinto -> Py_ssize_t
+    fd: int
+    buffer: Py_buffer(accept={rwbuffer})
+    /
+
+Read into a buffer object from a file descriptor.
+
+The buffer should be mutable and bytes-like. On success, returns the number of
+bytes read. Less bytes may be read than the size of the buffer. The underlying
+system call will be retried when interrupted by a signal, unless the signal
+handler raises an exception. Other errors will not be retried and an error will
+be raised.
+
+Returns 0 if *fd* is at end of file or if the provided *buffer* has length 0
+(which can be used to check for errors without reading data). Never returns
+negative.
+[clinic start generated code]*/
+
+static Py_ssize_t
+os_readinto_impl(PyObject *module, int fd, Py_buffer *buffer)
+/*[clinic end generated code: output=8091a3513c683a80 input=d40074d0a68de575]*/
+{
+    assert(buffer->len >= 0);
+    Py_ssize_t result = _Py_read(fd, buffer->buf, buffer->len);
+    /* Ensure negative is never returned without an error. Simplifies calling
+        code. _Py_read should succeed, possibly reading 0 bytes, _or_ set an
+        error. */
+    assert(result >= 0 || (result == -1 && PyErr_Occurred()));
+    return result;
+}
+
 #if (defined(HAVE_SENDFILE) && (defined(__FreeBSD__) || defined(__DragonFly__) \
                                 || defined(__APPLE__))) \
     || defined(HAVE_READV) || defined(HAVE_PREADV) || defined (HAVE_PREADV2) \
@@ -16973,6 +17005,7 @@ static PyMethodDef posix_methods[] = {
     OS_LOCKF_METHODDEF
     OS_LSEEK_METHODDEF
     OS_READ_METHODDEF
+    OS_READINTO_METHODDEF
     OS_READV_METHODDEF
     OS_PREAD_METHODDEF
     OS_PREADV_METHODDEF