Add test for child_err_report_fd conflation with target fds

This tests for glib#2506.

gio/tests/gsubprocess.c

@@ -1819,7 +1819,8 @@ test_pass_fd_inherit_fds (void)
 
 static void
 do_test_fd_conflation (GSubprocessFlags     flags,
-                       GSpawnChildSetupFunc child_setup)
+                       GSpawnChildSetupFunc child_setup,
+                       gboolean             test_child_err_report_fd)
 {
   char success_message[] = "Yay success!";
   GError *error = NULL;
@@ -1829,6 +1830,7 @@ do_test_fd_conflation (GSubprocessFlags     flags,
   GPtrArray *args;
   int unused_pipefds[2];
   int pipefds[2];
+  int fd_to_pass_to_child;
   gsize bytes_written;
   gboolean success;
   char *fd_str;
@@ -1881,18 +1883,26 @@ do_test_fd_conflation (GSubprocessFlags     flags,
    * fds. The primary goal of this test is to ensure this particular conflation
    * issue is not reintroduced. See glib#2503.
    *
+   * This test also has an alternate mode of operation where it instead tests
+   * for conflation with gspawn's child_err_report_fd, glib#2506.
+   *
    * Be aware this test is necessarily extremely fragile. To reproduce these
    * bugs, it relies on internals of gspawn and gmain that will likely change
    * in the future, eventually causing this test to no longer test the bugs
    * it was originally designed to test. That is OK! If the test fails, at
    * least you know *something* is wrong.
    */
+  if (test_child_err_report_fd)
+    fd_to_pass_to_child = pipefds[1] + 2 /* 8 */;
+  else
+    fd_to_pass_to_child = pipefds[1] + 3 /* 9 */;
+
   launcher = g_subprocess_launcher_new (flags);
-  g_subprocess_launcher_take_fd (launcher, pipefds[0] /* 5 */, pipefds[1] + 3 /* 9 */);
+  g_subprocess_launcher_take_fd (launcher, pipefds[0] /* 5 */, fd_to_pass_to_child);
   g_subprocess_launcher_take_fd (launcher, unused_pipefds[0] /* 3 */, pipefds[1] + 1 /* 7 */);
   if (child_setup != NULL)
     g_subprocess_launcher_set_child_setup (launcher, child_setup, NULL, NULL);
-  fd_str = g_strdup_printf ("%d", pipefds[1] + 3);
+  fd_str = g_strdup_printf ("%d", fd_to_pass_to_child);
   args = get_test_subprocess_args ("read-from-fd", fd_str, NULL);
   proc = g_subprocess_launcher_spawnv (launcher, (const gchar * const *) args->pdata, &error);
   g_assert_no_error (error);
@@ -1908,12 +1918,20 @@ do_test_fd_conflation (GSubprocessFlags     flags,
   /* Also close the write end of the unused pipe. */
   close (unused_pipefds[1]);
 
-  /* So now pipefds[0] should be inherited into the subprocess as
+  /* If doing our normal test:
+   *
+   * So now pipefds[0] should be inherited into the subprocess as
    * pipefds[1] + 2, and unused_pipefds[0] should be inherited as
    * pipefds[1] + 1. We will write to pipefds[1] and the subprocess will verify
    * that it reads the expected data. But older broken GIO will accidentally
    * clobber pipefds[1] + 2 with pipefds[1] + 1! This will cause the subprocess
    * to hang trying to read from the wrong pipe.
+   *
+   * If testing conflation with child_err_report_fd:
+   *
+   * We are actually already done. The real test succeeded if we made it this
+   * far without hanging while spawning the child. But let's continue with our
+   * write and read anyway, to ensure things are good.
    */
   output_stream = g_unix_output_stream_new (pipefds[1], TRUE);
   success = g_output_stream_write_all (output_stream,
@@ -1934,7 +1952,7 @@ do_test_fd_conflation (GSubprocessFlags     flags,
 static void
 test_fd_conflation (void)
 {
-  do_test_fd_conflation (G_SUBPROCESS_FLAGS_NONE, NULL);
+  do_test_fd_conflation (G_SUBPROCESS_FLAGS_NONE, NULL, FALSE);
 }
 
 static void
@@ -1943,7 +1961,7 @@ test_fd_conflation_empty_child_setup (void)
   /* Using a child setup function forces gspawn to use fork/exec
    * rather than posix_spawn.
    */
-  do_test_fd_conflation (G_SUBPROCESS_FLAGS_NONE, empty_child_setup);
+  do_test_fd_conflation (G_SUBPROCESS_FLAGS_NONE, empty_child_setup, FALSE);
 }
 
 static void
@@ -1954,7 +1972,16 @@ test_fd_conflation_inherit_fds (void)
    * posix_spawn codepath does not currently handle closing
    * non-inherited fds.
    */
-  do_test_fd_conflation (G_SUBPROCESS_FLAGS_INHERIT_FDS, NULL);
+  do_test_fd_conflation (G_SUBPROCESS_FLAGS_INHERIT_FDS, NULL, FALSE);
+}
+
+static void
+test_fd_conflation_child_err_report_fd (void)
+{
+  /* Using a child setup function forces gspawn to use fork/exec
+   * rather than posix_spawn.
+   */
+  do_test_fd_conflation (G_SUBPROCESS_FLAGS_NONE, empty_child_setup, TRUE);
 }
 
 #endif
@@ -2100,6 +2127,7 @@ main (int argc, char **argv)
   g_test_add_func ("/gsubprocess/fd-conflation/basic", test_fd_conflation);
   g_test_add_func ("/gsubprocess/fd-conflation/empty-child-setup", test_fd_conflation_empty_child_setup);
   g_test_add_func ("/gsubprocess/fd-conflation/inherit-fds", test_fd_conflation_inherit_fds);
+  g_test_add_func ("/gsubprocess/fd-conflation/child-err-report-fd", test_fd_conflation_child_err_report_fd);
 #endif
   g_test_add_func ("/gsubprocess/launcher-environment", test_launcher_environment);