gdb: make post_startup_inferior a virtual method on inf_ptrace_target

While working on a later patch that required me to understand how GDB
starts up inferiors, I was confused by the
target_ops::post_startup_inferior method.

The post_startup_inferior target function is only called from
inf_ptrace_target::create_inferior.

Part of the target class hierarchy looks like this:

  inf_child_target
     |
     '-- inf_ptrace_target
            |
            |-- linux_nat_target
            |
            |-- fbsd_nat_target
            |
            |-- nbsd_nat_target
            |
            |-- obsd_nat_target
            |
            '-- rs6000_nat_target

Every sub-class of inf_ptrace_target, except rs6000_nat_target,
implements ::post_startup_inferior.  The rs6000_nat_target picks up
the implementation of ::post_startup_inferior not from
inf_ptrace_target, but from inf_child_target.

No descendent of inf_child_target, outside the inf_ptrace_target
sub-tree, implements ::post_startup_inferior, which isn't really
surprising, as they would never see the method called (remember, the
method is only called from inf_ptrace_target::create_inferior).

What I find confusing is the role inf_child_target plays in
implementing, what is really a helper function for just one of its
descendents.

In this commit I propose that we formally make ::post_startup_inferior
a helper function of inf_ptrace_target.  To do this I will remove the
::post_startup_inferior from the target_ops API, and instead make this
a protected, pure virtual function on inf_ptrace_target.

I'll remove the empty implementation of ::post_startup_inferior from
the inf_child_target class, and add a new empty implementation to the
rs6000_nat_target class.

All the other descendents of inf_ptrace_target already provide an
implementation of this method and so don't need to change beyond
making the method protected within their class declarations.

To me, this makes much more sense now.  The helper function, which is
only called from within the inf_ptrace_target class, is now a part of
the inf_ptrace_target class.

The only way in which this change is visible to a user is if the user
turns on 'set debug target 1'.  With this debug flag on, prior to this
patch the user would see something like:

  -> native->post_startup_inferior (...)
  <- native->post_startup_inferior (2588939)

After this patch these lines are no longer present, as the
post_startup_inferior is no longer a top level target method.  For me,
this is an acceptable change.

1 files changed, 11 insertions, 0 deletions

diff --git a/gdb/inf-ptrace.h b/gdb/inf-ptrace.h
index 8aded9b..a4ad096 100644
--- a/gdb/inf-ptrace.h
+++ b/gdb/inf-ptrace.h
@@ -60,6 +60,17 @@ struct inf_ptrace_target : public inf_child_target
 protected:
   /* Cleanup the inferior after a successful ptrace detach.  */
   void detach_success (inferior *inf);
+
+  /* Some targets don't allow us to request notification of inferior events
+     such as fork and vfork immediately after the inferior is created.
+     (This is because of how gdb creates inferiors via invoking a shell to
+     do it.  In such a scenario, if the shell init file has commands in it,
+     the shell will fork and exec for each of those commands, and we will
+     see each such fork event.  Very bad.)
+
+     Such targets will supply an appropriate definition for this
+     function.  */
+  virtual void post_startup_inferior (ptid_t ptid) = 0;
 };
 
 #ifndef __NetBSD__