[binutils-gdb] Avoid GDB SIGTTOU on catch exec + set follow-exec-mode new (PR 23368)

[Simon Marchi]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=35ed81d4f45855b98ea0c517b396662c3ae2a8c5

commit 35ed81d4f45855b98ea0c517b396662c3ae2a8c5
Author: Simon Marchi <simon.marchi@polymtl.ca>
Date:   Tue Oct 23 17:00:33 2018 -0400

    Avoid GDB SIGTTOU on catch exec + set follow-exec-mode new (PR 23368)
    
    Here's a summary of PR 23368:
    
      #include <unistd.h>
      int main (void)
      {
        char *exec_args[] = { "/bin/ls", NULL };
        execve (exec_args[0], exec_args, NULL);
      }
    
    $ gdb -nx t -ex "catch exec" -ex "set follow-exec-mode new" -ex run
    ...
    [1]  + 13146 suspended (tty output)  gdb -q -nx t -ex "catch exec" -ex "set follow-exec-mode new" -ex run
    $
    
    Here's what happens: when the inferior execs with "follow-exec-mode
    new", we first "mourn" it before creating the new one.  This ends up
    calling inflow_inferior_exit, which sets the per-inferior terminal state
    to "is_ours":
    
      inf->terminal_state = target_terminal_state::is_ours;
    
    At this point, the inferior's terminal_state is is_ours, while the
    "reality", tracked by gdb_tty_state, is is_inferior (GDB doesn't own the
    terminal).
    
    Later, we continue processing the exec inferior event and decide we want
    to stop (because of the "catch exec") and call target_terminal::ours to
    make sure we own the terminal.  However, we don't actually go to the
    target backend to change the settings, because the core thinks that no
    inferior owns the terminal (inf->terminal_state is
    target_terminal_state::is_ours, as checked in
    target_terminal_is_ours_kind, for both inferiors).  When something in
    readline tries to mess with the terminal settings, it generates a
    SIGTTOU.
    
    This patch fixes this by tranferring the state of the terminal from the
    old inferior to the new inferior.
    
    gdb/ChangeLog:
    
    	PR gdb/23368
    	* infrun.c (follow_exec): In the follow_exec_mode_new case,
    	transfer terminal state from old new new inferior.
    	* terminal.h (swap_terminal_info): New function.
    	* inflow.c (swap_terminal_info): New function.

Diff:
---
 gdb/ChangeLog  |  8 ++++++++
 gdb/inflow.c   | 16 ++++++++++++++++
 gdb/infrun.c   | 10 ++++++----
 gdb/terminal.h |  3 +++
 4 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 19a87f4..5ef217d 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2018-10-23  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	PR gdb/23368
+	* infrun.c (follow_exec): In the follow_exec_mode_new case,
+	transfer terminal state from old new new inferior.
+	* terminal.h (swap_terminal_info): New function.
+	* inflow.c (swap_terminal_info): New function.
+
 2018-10-23  Tom Tromey  <tom@tromey.com>
 
 	* record-btrace.c (get_thread_current_frame_id): Rename from
diff --git a/gdb/inflow.c b/gdb/inflow.c
index caff646..a0c1c7d 100644
--- a/gdb/inflow.c
+++ b/gdb/inflow.c
@@ -702,6 +702,22 @@ copy_terminal_info (struct inferior *to, struct inferior *from)
   to->terminal_state = from->terminal_state;
 }
 
+/* See terminal.h.  */
+
+void
+swap_terminal_info (inferior *a, inferior *b)
+{
+  terminal_info *info_a
+    = (terminal_info *) inferior_data (a, inflow_inferior_data);
+  terminal_info *info_b
+    = (terminal_info *) inferior_data (a, inflow_inferior_data);
+
+  set_inferior_data (a, inflow_inferior_data, info_b);
+  set_inferior_data (b, inflow_inferior_data, info_a);
+
+  std::swap (a->terminal_state, b->terminal_state);
+}
+
 void
 info_terminal_command (const char *arg, int from_tty)
 {
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 72e2496..9473d1f 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1189,12 +1189,14 @@ follow_exec (ptid_t ptid, char *exec_file_target)
       /* The user wants to keep the old inferior and program spaces
 	 around.  Create a new fresh one, and switch to it.  */
 
-      /* Do exit processing for the original inferior before adding
-	 the new inferior so we don't have two active inferiors with
-	 the same ptid, which can confuse find_inferior_ptid.  */
+      /* Do exit processing for the original inferior before setting the new
+	 inferior's pid.  Having two inferiors with the same pid would confuse
+	 find_inferior_p(t)id.  Transfer the terminal state and info from the
+	  old to the new inferior.  */
+      inf = add_inferior_with_spaces ();
+      swap_terminal_info (inf, current_inferior ());
       exit_inferior_silent (current_inferior ());
 
-      inf = add_inferior_with_spaces ();
       inf->pid = pid;
       target_follow_exec (inf, exec_file_target);
 
diff --git a/gdb/terminal.h b/gdb/terminal.h
index 7774eef..da27e46 100644
--- a/gdb/terminal.h
+++ b/gdb/terminal.h
@@ -29,6 +29,9 @@ extern void new_tty_postfork (void);
 
 extern void copy_terminal_info (struct inferior *to, struct inferior *from);
 
+/* Exchange the terminal info and state between inferiors A and B.  */
+extern void swap_terminal_info (inferior *a, inferior *b);
+
 extern pid_t create_tty_session (void);
 
 /* Set up a serial structure describing standard input.  In inflow.c.  */