Always switch fork child to the main UI

The following scenario:

 - gdb started in normal CLI mode.

 - separate MI channel created with new-ui

 - inferior output redirected with the "set inferior-tty" command.

 - use -exec-run in the MI channel to run the inferior

is presently mishandled.

When we create the inferior, in fork-child.c, right after vfork, we'll
close all the file descriptors in the vfork child, and then dup the
tty to file descriptors 0/1/2, create a session, etc.  Note that when
we close all descriptors, we close the file descriptors behind
gdb_stdin/gdb_stdout/gdb_stderr of all secondary UIs...  So if
anything goes wrong in the child and it calls warning/error, it'll end
up writting to the current UI's stdout/stderr streams, which are
backed by file descriptors that have since been closed.  Because this
happens in a vfork region, the corresponding stdin/stdout/stderr in
the parent/gdb end up corrupted.

The fix is to switch to the main UI right after the vfork, so that
gdb_stdin/gdb_stdout/gdb_stderr are correctly mapped to
stdin/stdout/stderr (and thus to file descriptors 0/1/2), so this code
works as it has always worked.

(Technically, we're doing a lot of stuff we shouldn't be doing after a
vfork, while we should only be calling async-signal-safe functions.)

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* fork-child.c (fork_inferior): Switch the child to the main UI
	right after vfork.  Save/restore the current UI in the parent.
	Flush outputs of the main UI instead of the current UI.

gdb/testsuite/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* gdb.mi/mi-exec-run.exp: New file.

1 files changed, 20 insertions, 2 deletions

diff --git a/gdb/fork-child.c b/gdb/fork-child.c
index 204b7cf..8ac3bef 100644
--- a/gdb/fork-child.c
+++ b/gdb/fork-child.c
@@ -31,6 +31,7 @@
 #include "gdbcmd.h"
 #include "solib.h"
 #include "filestuff.h"
+#include "top.h"
 
 #include <signal.h>
 
@@ -141,6 +142,7 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
   struct inferior *inf;
   int i;
   int save_errno;
+  struct ui *save_ui;
 
   /* If no exec file handed to us, get it from the exec-file command
      -- with a good, common error message if none is specified.  */
@@ -275,6 +277,9 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
      restore it.  */
   save_our_env = environ;
 
+  /* Likewise the current UI.  */
+  save_ui = current_ui;
+
   /* Tell the terminal handling subsystem what tty we plan to run on;
      it will just record the information for later.  */
   new_tty_prefork (inferior_io_terminal);
@@ -282,8 +287,8 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
   /* It is generally good practice to flush any possible pending stdio
      output prior to doing a fork, to avoid the possibility of both
      the parent and child flushing the same data after the fork.  */
-  gdb_flush (gdb_stdout);
-  gdb_flush (gdb_stderr);
+  gdb_flush (main_ui->m_gdb_stdout);
+  gdb_flush (main_ui->m_gdb_stderr);
 
   /* If there's any initialization of the target layers that must
      happen to prepare to handle the child we're about fork, do it
@@ -312,6 +317,16 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
 
   if (pid == 0)
     {
+      /* Switch to the main UI, so that gdb_std{in/out/err} in the
+	 child are mapped to std{in/out/err}.  This makes it possible
+	 to use fprintf_unfiltered/warning/error/etc. in the child
+	 from here on.  */
+      current_ui = main_ui;
+
+      /* Close all file descriptors except those that gdb inherited
+	 (usually 0/1/2), so they don't leak to the inferior.  Note
+	 that this closes the file descriptors of all secondary
+	 UIs.  */
       close_most_fds ();
 
       if (debug_fork)
@@ -378,6 +393,9 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
   /* Restore our environment in case a vforked child clob'd it.  */
   environ = save_our_env;
 
+  /* Likewise the current UI.  */
+  current_ui = save_ui;
+
   if (!have_inferiors ())
     init_thread_list ();