Hand controlling TTY to child, if we have one

dumb-init.c

@@ -15,6 +15,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/ioctl.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ -250,6 +251,15 @@ int main(int argc, char *argv[]) {
     for (i = 1; i <= MAXSIG; i++)
         signal(i, dummy);
 
+    /* detach dumb-init from controlling tty */
+    if (use_setsid && ioctl(STDIN_FILENO, TIOCNOTTY) == -1) {
+        DEBUG(
+            "Unable to detach from controlling tty (errno=%d %s).\n",
+            errno,
+            strerror(errno)
+        );
+    }
+
     child_pid = fork();
     if (child_pid < 0) {
         PRINTERR("Unable to fork. Exiting.\n");
@@ -266,6 +276,14 @@ int main(int argc, char *argv[]) {
                 );
                 exit(1);
             }
+
+            if (ioctl(STDIN_FILENO, TIOCSCTTY, 0) == -1) {
+                DEBUG(
+                    "Unable to attach to controlling tty (errno=%d %s).\n",
+                    errno,
+                    strerror(errno)
+                );
+            }
             DEBUG("setsid complete.\n");
         }
         execvp(cmd[0], &cmd[0]);

tests/tty_test.py

@@ -1,5 +1,6 @@
 import os
 import pty
+import re
 import termios
 
 import pytest
@@ -35,24 +36,48 @@ def readall(fd):
             result += chunk
 
 
-def _test(fd):
-    """write to tac via the pty and verify its output"""
-    ttyflags(fd)
-    assert os.write(fd, b'1\n2\n3\n') == 6
-    assert os.write(fd, EOF * 2) == 2
-    output = readall(fd)
-    assert output == b'3\n2\n1\n', repr(output)
-    print('PASS')
-
-
 # disable debug output so it doesn't break our assertion
 @pytest.mark.usefixtures('debug_disabled')
 def test_tty():
-    """
-    Ensure processes wrapped by dumb-init can write successfully, given a tty
-    """
+    """Ensure processes under dumb-init can write successfully, given a tty."""
     pid, fd = pty.fork()
     if pid == 0:
         os.execvp('dumb-init', ('dumb-init', 'tac'))
     else:
-        _test(fd)
+        # write to tac via the pty and verify its output
+        ttyflags(fd)
+        assert os.write(fd, b'1\n2\n3\n') == 6
+        assert os.write(fd, EOF * 2) == 2
+        output = readall(fd)
+        assert os.waitpid(pid, 0) == (pid, 0)
+
+        assert output == b'3\n2\n1\n', repr(output)
+
+
+@pytest.mark.usefixtures('both_debug_modes')
+@pytest.mark.usefixtures('both_setsid_modes')
+def test_child_gets_controlling_tty_if_we_had_one():
+    """If dumb-init has a controlling TTY, it should give it to the child.
+
+    To test this, we make a new TTY then exec "dumb-init bash" and ensure that
+    the shell has working job control.
+    """
+    pid, sfd = pty.fork()
+    if pid == 0:
+        os.execvpe('./dumb-init', ('dumb-init', 'bash', '-m'), {})
+    else:
+        ttyflags(sfd)
+
+        # We might get lots of extra output from the shell, so print something
+        # we can match on easily.
+        assert os.write(sfd, b'echo "flags are: [[$-]]"\n') == 25
+        assert os.write(sfd, b'exit 0\n') == 7
+        output = readall(sfd)
+        assert os.waitpid(pid, 0) == (pid, 0), output
+
+        m = re.search(b'flags are: \[\[([a-zA-Z]+)\]\]\n', output)
+        assert m, output
+
+        # "m" is job control
+        flags = m.group(1)
+        assert b'm' in flags