Issue #24402: Factor out PtyTests.run_child() in input() tests

This reuses existing code to hopefully make the new test_input_no_stdout_
fileno() test work. It is hanging Free BSD 9 and OS X Tiger buildbots, and I
don't know why.

Lib/test/test_builtin.py

@@ -1493,21 +1493,14 @@ class PtyTests(unittest.TestCase):
     """Tests that use a pseudo terminal to guarantee stdin and stdout are
     terminals in the test environment"""
 
-    def fork(self):
+    def run_child(self, child, terminal_input):
+        r, w = os.pipe()  # Pipe test results from child back to parent
         try:
-            return pty.fork()
+            pid, fd = pty.fork()
         except (OSError, AttributeError) as e:
-            self.skipTest("pty.fork() raised {}".format(e))
-
-    def check_input_tty(self, prompt, terminal_input, stdio_encoding=None):
-        if not sys.stdin.isatty() or not sys.stdout.isatty():
-            self.skipTest("stdin and stdout must be ttys")
-        r, w = os.pipe()
-        try:
-            pid, fd = self.fork()
-        except:
             os.close(r)
             os.close(w)
+            self.skipTest("pty.fork() raised {}".format(e))
             raise
         if pid == 0:
             # Child
@@ -1515,17 +1508,8 @@ class PtyTests(unittest.TestCase):
                 # Make sure we don't get stuck if there's a problem
                 signal.alarm(2)
                 os.close(r)
-                # Check the error handlers are accounted for
-                if stdio_encoding:
-                    sys.stdin = io.TextIOWrapper(sys.stdin.detach(),
-                                                 encoding=stdio_encoding,
-                                                 errors='surrogateescape')
-                    sys.stdout = io.TextIOWrapper(sys.stdout.detach(),
-                                                  encoding=stdio_encoding,
-                                                  errors='replace')
                 with open(w, "w") as wpipe:
-                    print("tty =", sys.stdin.isatty() and sys.stdout.isatty(), file=wpipe)
-                    print(ascii(input(prompt)), file=wpipe)
+                    child(wpipe)
             except:
                 traceback.print_exc()
             finally:
@@ -1533,7 +1517,7 @@ class PtyTests(unittest.TestCase):
                 os._exit(0)
         # Parent
         os.close(w)
-        os.write(fd, terminal_input + b"\r\n")
+        os.write(fd, terminal_input)
         # Get results from the pipe
         with open(r, "r") as rpipe:
             lines = []
@@ -1546,10 +1530,38 @@ class PtyTests(unittest.TestCase):
         # Check the result was got and corresponds to the user's terminal input
         if len(lines) != 2:
             # Something went wrong, try to get at stderr
-            with open(fd, "r", encoding="ascii", errors="ignore") as child_output:
+            # Beware of Linux raising EIO when the slave is closed
+            child_output = bytearray()
+            while True:
+                try:
+                    chunk = os.read(fd, 3000)
+                except OSError:  # Assume EIO
+                    break
+                if not chunk:
+                    break
+                child_output.extend(chunk)
+            os.close(fd)
+            child_output = child_output.decode("ascii", "ignore")
             self.fail("got %d lines in pipe but expected 2, child output was:\n%s"
-                          % (len(lines), child_output.read()))
+                      % (len(lines), child_output))
         os.close(fd)
+        return lines
+
+    def check_input_tty(self, prompt, terminal_input, stdio_encoding=None):
+        if not sys.stdin.isatty() or not sys.stdout.isatty():
+            self.skipTest("stdin and stdout must be ttys")
+        def child(wpipe):
+            # Check the error handlers are accounted for
+            if stdio_encoding:
+                sys.stdin = io.TextIOWrapper(sys.stdin.detach(),
+                                             encoding=stdio_encoding,
+                                             errors='surrogateescape')
+                sys.stdout = io.TextIOWrapper(sys.stdout.detach(),
+                                              encoding=stdio_encoding,
+                                              errors='replace')
+            print("tty =", sys.stdin.isatty() and sys.stdout.isatty(), file=wpipe)
+            print(ascii(input(prompt)), file=wpipe)
+        lines = self.run_child(child, terminal_input + b"\r\n")
         # Check we did exercise the GNU readline path
         self.assertIn(lines[0], {'tty = True', 'tty = False'})
         if lines[0] != 'tty = True':
@@ -1577,26 +1589,17 @@ class PtyTests(unittest.TestCase):
     def test_input_no_stdout_fileno(self):
         # Issue #24402: If stdin is the original terminal but stdout.fileno()
         # fails, do not use the original stdout file descriptor
-        pid, pty = self.fork()
-        if pid:  # Parent process
-            # Ideally this should read and write concurrently using select()
-            # or similar, to avoid the possibility of a deadlock.
-            os.write(pty, b"quux\r")
-            _, status = os.waitpid(pid, 0)
-            output = os.read(pty, 3000).decode("ascii", "backslashreplace")
-            os.close(pty)
-            self.assertEqual(status, 0, output)
-        else:  # Child process
-            try:
-                self.assertTrue(sys.stdin.isatty(), "stdin not a terminal")
+        def child(wpipe):
+            print("stdin.isatty():", sys.stdin.isatty(), file=wpipe)
             sys.stdout = io.StringIO()  # Does not support fileno()
             input("prompt")
-                self.assertEqual(sys.stdout.getvalue(), "prompt")
-                os._exit(0)  # Success!
-            except:
-                sys.excepthook(*sys.exc_info())
-            finally:
-                os._exit(1)  # Failure
+            print("captured:", ascii(sys.stdout.getvalue()), file=wpipe)
+        lines = self.run_child(child, b"quux\r")
+        expected = (
+            "stdin.isatty(): True",
+            "captured: 'prompt'",
+        )
+        self.assertSequenceEqual(lines, expected)
 
 class TestSorted(unittest.TestCase):