[binutils-gdb] Don't call "detach_inferior" on "remote_follow_fork"

[Sergio Durigan Junior <sergiodj at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=69ab5edb4d601611ba7b4d05e56689d4b60ca3b1

commit 69ab5edb4d601611ba7b4d05e56689d4b60ca3b1
Author: Sergio Durigan Junior <sergiodj@redhat.com>
Date:   Mon Jan 29 12:29:21 2018 -0500

    Don't call "detach_inferior" on "remote_follow_fork"
    
    This patch fixes a regression that has been introduced by:
    
      commit bc09b0c14fb713a9aec25e09b78499f3bc2441b5
      Date:   Fri Jan 19 11:48:11 2018 -0500
    
          Make linux_nat_detach/thread_db_detach use the inferior parameter
    
    It is possible to trigger this failure with gdb.base/foll-fork.exp (in
    which case a bunch of ERROR's will be printed), but one can also use
    the test below.
    
    Consider the following example program:
    
      #include <unistd.h>
    
      int
      main (int argc, char *argv[])
      {
        fork ();
    
        return 0;
      }
    
    When running it under gdbserver:
    
      # ./gdb/gdbserver/gdbserver --multi --once :2345
    
    And debugging it under GDB, we see a segmentation fault:
    
      # ./gdb/gdb -q -batch -ex 'set remote exec-file ./a.out' -ex 'tar extended-remote :2345' -ex r ./a.out
      Starting program:
      ...
      [Detaching after fork from child process 16102.]
      Segmentation fault (core dumped)
    
    The problem happens on inferior.c:detach_inferior:
    
      void
      detach_inferior (inferior *inf)
      {
        /* Save the pid, since exit_inferior_1 will reset it.  */
        int pid = inf->pid;
                  ^^^^^^^^^
    
        exit_inferior_1 (inf, 0);
    
        if (print_inferior_events)
          printf_unfiltered (_("[Inferior %d detached]\n"), pid);
      }
    
    When this code is called from remote.c:remote_follow_fork, the PID is
    valid but there is no 'inferior' associated with it, which means that
    'inf == NULL'.
    
    The proper fix here is to not call "detach_inferior" when doing remote
    follow-fork, because we don't have an inferior to detach on the host
    side.
    
    Before bc09b0c1, that call was already a nop (exit_inferior_1 bails
    out early if you pass it a NULL inferior), except that it printed
    "Inferior PID detached" when "set print inferior-events" is on.  Since
    native debugging doesn't call detach_inferior in this case, removing
    the call from remote aligns remote debugging output with native
    debugging output further.
    
    This has been regtested using BuildBot and no regressions were found.
    
    gdb/ChangeLog:
    2018-01-29  Sergio Durigan Junior  <sergiodj@redhat.com>
    
    	* remote.c (remote_follow_fork): Don't call "detach_inferior".

Diff:
---
 gdb/ChangeLog | 4 ++++
 gdb/remote.c  | 1 -
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c422005..12c1874 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-01-29  Sergio Durigan Junior  <sergiodj@redhat.com>
+
+	* remote.c (remote_follow_fork): Don't call "detach_inferior".
+
 2018-01-28  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* dwarf2read.c (free_dwo_files): Add forward-declaration.
diff --git a/gdb/remote.c b/gdb/remote.c
index 5ac84df..74d18f7 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -5206,7 +5206,6 @@ remote_follow_fork (struct target_ops *ops, int follow_child,
 	  child_pid = ptid_get_pid (child_ptid);
 
 	  remote_detach_pid (child_pid);
-	  detach_inferior (child_pid);
 	}
     }
   return 0;