[3.9] bpo-41687: Fix sendfile implementation to work with Solaris (GH-22040) (GH-22273)

(cherry picked from commit 8c0be6fd91)

Co-authored-by: Jakub Kulík <Kulikjak@gmail.com>

Lib/test/test_asyncio/test_sendfile.py

@@ -445,6 +445,12 @@ class SendfileMixin(SendfileBase):
         self.assertEqual(srv_proto.data, self.DATA)
         self.assertEqual(self.file.tell(), len(self.DATA))
 
+    # On Solaris, lowering SO_RCVBUF on a TCP connection after it has been
+    # established has no effect. Due to its age, this bug affects both Oracle
+    # Solaris as well as all other OpenSolaris forks (unless they fixed it
+    # themselves).
+    @unittest.skipIf(sys.platform.startswith('sunos'),
+                     "Doesn't work on Solaris")
     def test_sendfile_close_peer_in_the_middle_of_receiving(self):
         srv_proto, cli_proto = self.prepare_sendfile(close_after=1024)
         with self.assertRaises(ConnectionError):

Misc/NEWS.d/next/Library/2020-09-01-15-57-51.bpo-41687.m1b1KA.rst

@@ -0,0 +1 @@
+Fix implementation of sendfile to be compatible with Solaris.

Modules/posixmodule.c

@@ -9453,6 +9453,25 @@ done:
     if (!Py_off_t_converter(offobj, &offset))
         return NULL;
 
+#if defined(__sun) && defined(__SVR4)
+    // On Solaris, sendfile raises EINVAL rather than returning 0
+    // when the offset is equal or bigger than the in_fd size.
+    int res;
+    struct stat st;
+
+    do {
+        Py_BEGIN_ALLOW_THREADS
+        res = fstat(in_fd, &st);
+        Py_END_ALLOW_THREADS
+    } while (res != 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
+    if (ret < 0)
+        return (!async_err) ? posix_error() : NULL;
+
+    if (offset >= st.st_size) {
+        return Py_BuildValue("i", 0);
+    }
+#endif
+
     do {
         Py_BEGIN_ALLOW_THREADS
         ret = sendfile(out_fd, in_fd, &offset, count);