fbsd-nat: Fix several issues with detaching.

- Detach from any child processes implicitly attached to by the kernel
  due to fork following that have not yet been processed by GDB's
  core.

- Delete breakpoints before detaching.

  inf-ptrace::detach does not do this (somewhat surprisingly), so add
  an override to remove breakpoints from a process before detaching
  from it.

  This also requires explicitly draining any pending SIGTRAP events
  for software breakpoints before detaching.  In particular, threads
  may need their PC adjusted due to the software breakpoint before
  being resumed after detach.  On more modern systems using the si_code
  from SIGTRAP to identify software breakpoint traps, the PC is adjusted
  in ::wait_1 as a side effect of parsing the event.  To support older
  kernels, ::detach fixes up the PC for any SIGTRAP stop whose potential
  new PC matches an existing software breakpoint.

Author: bsdjhb

gdb/fbsd-nat.c

@@ -1837,6 +1837,268 @@ fbsd_nat_target::attach (const char *args, int from_tty)
   inf_ptrace_target::attach (args, from_tty);
 }
 
+/* If this thread has a pending fork event, there is a child process
+   GDB is attached to that the core of GDB doesn't know about.
+   Detach from it.  */
+
+void
+fbsd_nat_target::detach_fork_children (thread_info *tp)
+{
+  /* Check in thread_info::pending_waitstatus.  */
+  if (tp->has_pending_waitstatus ())
+    {
+      const target_waitstatus &ws = tp->pending_waitstatus ();
+
+      if (ws.kind () == TARGET_WAITKIND_VFORKED
+	  || ws.kind () == TARGET_WAITKIND_FORKED)
+	{
+	  pid_t pid = ws.child_ptid ().pid ();
+	  fbsd_nat_debug_printf ("detaching from child %d", pid);
+	  (void) ptrace (PT_DETACH, pid, (caddr_t) 1, 0);
+	}
+    }
+
+  /* Check in thread_info::pending_follow.  */
+  if (tp->pending_follow.kind () == TARGET_WAITKIND_VFORKED
+      || tp->pending_follow.kind () == TARGET_WAITKIND_FORKED)
+    {
+      pid_t pid = tp->pending_follow.child_ptid ().pid ();
+      fbsd_nat_debug_printf ("detaching from child %d", pid);
+      (void) ptrace (PT_DETACH, pid, (caddr_t) 1, 0);
+    }
+}
+
+/* Detach from any child processes associated with pending fork events
+   for a stopped process.  Returns true if the process has terminated
+   and false if it is still alive.  */
+
+bool
+fbsd_nat_target::detach_fork_children (inferior *inf)
+{
+  /* Detach any child processes associated with pending fork events in
+     threads belonging to this process.  */
+  for (thread_info *tp : inf->non_exited_threads ())
+    detach_fork_children (tp);
+
+  /* Unwind state associated with any pending events.  Reset
+     fbsd_inf->resumed_lwps so that take_pending_event will harvest
+     events.  */
+  fbsd_inferior *fbsd_inf = get_fbsd_inferior (inf);
+  ptid_t ptid = ptid_t (inf->pid);
+  fbsd_inf->resumed_lwps = ptid;
+
+  while (1)
+    {
+      gdb::optional<pending_event> event = take_pending_event (ptid);
+      if (!event.has_value ())
+	break;
+
+      switch (event->status.kind ())
+	{
+	case TARGET_WAITKIND_EXITED:
+	case TARGET_WAITKIND_SIGNALLED:
+	  return true;
+	case TARGET_WAITKIND_FORKED:
+	case TARGET_WAITKIND_VFORKED:
+	  {
+	    pid_t pid = event->status.child_ptid ().pid ();
+	    fbsd_nat_debug_printf ("detaching from child %d", pid);
+	    (void) ptrace (PT_DETACH, pid, (caddr_t) 1, 0);
+	  }
+	  break;
+	}
+    }
+  return false;
+}
+
+/* Scan all of the threads for a stopped process invoking the supplied
+   callback on the ptrace_lwpinfo object for threads other than the
+   thread which reported the current stop.  The callback can return
+   true to terminate the iteration early.  This function returns true
+   if the callback returned true, otherwise it returns false.  */
+
+typedef bool (ptrace_event_ftype) (const struct ptrace_lwpinfo &pl);
+
+static bool
+iterate_other_ptrace_events (pid_t pid,
+			     gdb::function_view<ptrace_event_ftype> callback)
+{
+  /* Fetch the LWP ID of the thread that just reported the last stop
+     and ignore that LWP in the following loop.  */
+  ptrace_lwpinfo pl;
+  if (ptrace (PT_LWPINFO, pid, (caddr_t) &pl, sizeof (pl)) != 0)
+    perror_with_name (("ptrace (PT_LWPINFO)"));
+  lwpid_t lwpid = pl.pl_lwpid;
+
+  int nlwps = ptrace (PT_GETNUMLWPS, pid, NULL, 0);
+  if (nlwps == -1)
+    perror_with_name (("ptrace (PT_GETLWPLIST)"));
+  if (nlwps == 1)
+    return false;
+
+  gdb::unique_xmalloc_ptr<lwpid_t[]> lwps (XCNEWVEC (lwpid_t, nlwps));
+
+  nlwps = ptrace (PT_GETLWPLIST, pid, (caddr_t) lwps.get (), nlwps);
+  if (nlwps == -1)
+    perror_with_name (("ptrace (PT_GETLWPLIST)"));
+
+  for (int i = 0; i < nlwps; i++)
+    {
+      if (lwps[i] == lwpid)
+	continue;
+
+      if (ptrace (PT_LWPINFO, lwps[i], (caddr_t) &pl, sizeof (pl)) != 0)
+	perror_with_name (("ptrace (PT_LWPINFO)"));
+
+      if (callback (pl))
+	return true;
+    }
+  return false;
+}
+
+/* True if there are any stopped threads with an interesting event.  */
+
+static bool
+pending_ptrace_events (inferior *inf)
+{
+  auto lambda = [] (const struct ptrace_lwpinfo &pl)
+  {
+#if defined(PT_LWP_EVENTS) && __FreeBSD_kernel_version < 1400090
+    if (pl.pl_flags == PL_FLAG_BORN)
+      return true;
+#endif
+#ifdef TDP_RFPPWAIT
+    if (pl.pl_flags & PL_FLAG_FORKED)
+      return true;
+#endif
+    if (pl.pl_event == PL_EVENT_SIGNAL)
+      {
+	if ((pl.pl_flags & PL_FLAG_SI) == 0)
+	  {
+	    /* Not sure which signal, assume it matters.  */
+	    return true;
+	  }
+	if (pl.pl_siginfo.si_signo == SIGTRAP)
+	  return true;
+      }
+    return false;
+  };
+  return iterate_other_ptrace_events (inf->pid,
+				      gdb::make_function_view (lambda));
+}
+
+void
+fbsd_nat_target::detach (inferior *inf, int from_tty)
+{
+  fbsd_nat_debug_start_end ("pid %d", inf->pid);
+
+  stop_process (inf);
+
+  remove_breakpoints_inf (inf);
+
+  if (detach_fork_children (inf)) {
+    /* No need to detach now.  */
+    target_announce_detach (from_tty);
+
+    detach_success (inf);
+    return;
+  }
+
+  /* If there are any pending events (SIGSTOP from stop_process or a
+     breakpoint hit that needs a PC fixup), drain events until the
+     process can be safely detached.  */
+  fbsd_inferior *fbsd_inf = get_fbsd_inferior (inf);
+  ptid_t ptid = ptid_t (inf->pid);
+  if (fbsd_inf->pending_sigstop || pending_ptrace_events (inf))
+    {
+      bool pending_sigstop = fbsd_inf->pending_sigstop;
+      int sig = 0;
+
+      if (pending_sigstop)
+	fbsd_nat_debug_printf ("waiting for SIGSTOP");
+
+      /* Force wait_1 to report the SIGSTOP instead of swallowing it.  */
+      fbsd_inf->pending_sigstop = false;
+
+      /* Report event for all threads from wait_1.  */
+      fbsd_inf->resumed_lwps = ptid;
+
+      do
+	{
+	  if (ptrace (PT_CONTINUE, inf->pid, (caddr_t) 1, sig) != 0)
+	    perror_with_name (("ptrace(PT_CONTINUE)"));
+
+	  target_waitstatus ws;
+	  ptid_t wptid = wait_1 (ptid, &ws, 0);
+
+	  switch (ws.kind ())
+	    {
+	    case TARGET_WAITKIND_EXITED:
+	    case TARGET_WAITKIND_SIGNALLED:
+	      /* No need to detach now.  */
+	      target_announce_detach (from_tty);
+
+	      detach_success (inf);
+	      return;
+	    case TARGET_WAITKIND_FORKED:
+	    case TARGET_WAITKIND_VFORKED:
+	      {
+		pid_t pid = ws.child_ptid ().pid ();
+		fbsd_nat_debug_printf ("detaching from child %d", pid);
+		(void) ptrace (PT_DETACH, pid, (caddr_t) 1, 0);
+		sig = 0;
+	      }
+	      break;
+	    case TARGET_WAITKIND_STOPPED:
+	      sig = gdb_signal_to_host (ws.sig ());
+	      switch (sig)
+		{
+		case SIGSTOP:
+		  if (pending_sigstop)
+		    {
+		      sig = 0;
+		      pending_sigstop = false;
+		    }
+		  break;
+		case SIGTRAP:
+#ifndef USE_SIGTRAP_SIGINFO
+		  {
+		    /* Update PC from software breakpoint hit.  */
+		    struct regcache *regcache = get_thread_regcache (this, wptid);
+		    struct gdbarch *gdbarch = regcache->arch ();
+		    int decr_pc = gdbarch_decr_pc_after_break (gdbarch);
+
+		    if (decr_pc != 0)
+		      {
+			CORE_ADDR pc;
+
+			pc = regcache_read_pc (regcache);
+			if (breakpoint_inserted_here_p (regcache->aspace (),
+							pc - decr_pc))
+			  {
+			    fbsd_nat_debug_printf ("adjusted PC for LWP %ld",
+						   wptid.lwp ());
+			    regcache_write_pc (regcache, pc - decr_pc);
+			  }
+		      }
+		  }
+#endif
+		  sig = 0;
+		  break;
+		}
+	    }
+	}
+      while (pending_sigstop || pending_ptrace_events (inf));
+    }
+
+  target_announce_detach (from_tty);
+
+  if (ptrace (PT_DETACH, inf->pid, (caddr_t) 1, 0) == -1)
+	perror_with_name (("ptrace (PT_DETACH)"));
+
+  detach_success (inf);
+}
+
 void
 fbsd_nat_target::mourn_inferior ()
 {

gdb/fbsd-nat.h

@@ -81,6 +81,8 @@ class fbsd_nat_target : public inf_ptrace_target
 
   void attach (const char *, int) override;
 
+  void detach (inferior *, int) override;
+
   void mourn_inferior () override;
 
   void resume (ptid_t, int, enum gdb_signal) override;
@@ -263,6 +265,18 @@ class fbsd_nat_target : public inf_ptrace_target
   /* List of pending events.  */
 
   std::list<pending_event> m_pending_events;
+
+  /* If this thread has a pending fork event, there is a child process
+     GDB is attached to that the core of GDB doesn't know about.
+     Detach from it.  */
+
+  void detach_fork_children (thread_info *tp);
+
+  /* Detach from any child processes associated with pending fork events
+     for a stopped process.  Returns true if the process has terminated
+     and false if it is still alive.  */
+
+  bool detach_fork_children (inferior *inf);
 };
 
 /* Fetch the signal information for PTID and store it in *SIGINFO.