login_tty tests: Be more verbose when the test fails

[Bruno Haible <bruno@clisp.org>]

On Android 11, when I run the unit tests of a testdir:
  - './test-login_tty' yields "login_tty failed".
  - 'make check TESTS=test-login_tty' yields "Aborted".
To get more information about the second case, I'm making this change.
"tcgetsid(0) = -1 and errno = 13" is more information than nothing.


2023-01-13  Bruno Haible  <bruno@clisp.org>

	login_tty tests: Be more verbose when the test fails.
	* tests/test-login_tty.c (main): When the test fails, write a message
	into a file.

diff --git a/tests/test-login_tty.c b/tests/test-login_tty.c
index 7ed09f048b..294b46d63c 100644
--- a/tests/test-login_tty.c
+++ b/tests/test-login_tty.c
@@ -53,7 +53,8 @@ main ()
   }
 
   /* From here on, we cannot use stderr for error messages any more.
-     If a test fails, just abort.  */
+     If a test fails, write error information into a file named 'err',
+     then abort.  */
 
   /* Check that fd = 0, 1, 2 are now open to the controlling terminal for the
      current process and that it is a session of its own.  */
@@ -61,12 +62,38 @@ main ()
     int fd;
     for (fd = 0; fd < 3; fd++)
       if (!(tcgetpgrp (fd) == getpid ()))
-        abort ();
+        {
+          freopen ("err", "w+", stderr);
+          fprintf (stderr, "tcgetpgrp(%d) = %ld whereas getpid() = %ld\n",
+                   fd, (long) tcgetpgrp (fd), (long) getpid ());
+          fflush (stderr);
+          abort ();
+        }
     for (fd = 0; fd < 3; fd++)
       {
         pid_t sid = tcgetsid (fd);
-        if (!(sid == -1 ? errno == ENOSYS : sid == getpid ()))
-          abort ();
+        if (sid == -1)
+          {
+            if (!(errno == ENOSYS))
+              {
+                freopen ("err", "w+", stderr);
+                fprintf (stderr, "tcgetsid(%d) = -1 and errno = %d\n",
+                         fd, errno);
+                fflush (stderr);
+                abort ();
+              }
+          }
+        else
+          {
+            if (!(sid == getpid ()))
+              {
+                freopen ("err", "w+", stderr);
+                fprintf (stderr, "tcgetsid(%d) = %ld whereas getpid() = %ld\n",
+                         fd, (long) tcgetsid (fd), (long) getpid ());
+                fflush (stderr);
+                abort ();
+              }
+          }
       }
   }