lwp_info: Make the arch code free arch_lwp_info

I have the goal of "poisoning" the XNEW/xfree-family of functions, so
that we catch their usages with non-POD types.  A few things need to be
fixed in the mean time, this is one.

The common lwp code in linux-nat.c and gdbserver/linux-low.c xfrees the
private lwp data of type arch_lwp_info.  However, that type is opaque
from its point of view, as its defined differently in each arch-specific
implementation.  This trips on the std::is_pod<T> check, since the
compiler can't tell whether the type is POD or not if it doesn't know
about it.

My initial patch [1] made a class hierarchy with a virtual destructor.
However, as Pedro pointed out, we only have one native architecture at
the time built in gdb and gdbserver, so that's overkill.  Instead, we
can move the responsibility of free'ing arch_lwp_info to the arch code
(which is also the one that allocated it in the first place).  This is
what this patch does.

Also, I had the concern that if we wanted to use C++ features in these
structures, we would have a problem with the one-definition rule.
However, since a build will only have one version of arch_lwp_info,
that's not a problem.

There are changes in arch-specific files, I was only able to built-test
this patch with the following cross-compilers:

  aarch64-linux-gnu
  alpha-linux-gnu
  arm-linux-gnueabihf
  hppa-linux-gnu
  m68k-linux-gnu
  mips64el-linux-gnuabi64
  powerpc64-linux-gnu
  s390x-linux-gnu
  sh4-linux-gnu
  sparc64-linux-gnu
  x86_64-linux-gnu
  x86_64-w64-mingw32

A buildbot run didn't find any regression.

[1] https://sourceware.org/ml/gdb-patches/2017-08/msg00255.html

gdb/ChangeLog:

	* linux-nat.h (linux_nat_set_delete_thread): New declaration.
	* linux-nat.c (linux_nat_delete_thread): New variable.
	(lwp_free): Invoke linux_nat_delete_thread if set.
	(linux_nat_set_delete_thread): New function.
	* aarch64-linux-nat.c (_initialize_aarch64_linux_nat): Assign
	thread delete callback.
	* arm-linux-nat.c (arm_linux_delete_thread): New function.
	(_initialize_arm_linux_nat): Assign thread delete callback.
	* s390-linux-nat.c (s390_delete_thread): New function.
	(_initialize_s390_nat): Assign thread delete callback.
	* x86-linux-nat.c (x86_linux_add_target): Likewise.
	* nat/aarch64-linux.c (aarch64_linux_delete_thread): New
	function.
	* nat/aarch64-linux.h (aarch64_linux_delete_thread): New
	declaration.
	* nat/x86-linux.c (x86_linux_delete_thread): New function.
	* nat/x86-linux.h (x86_linux_delete_thread): New declaration.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (the_low_target): Add thread delete
	callback.
	* linux-arm-low.c (arm_delete_thread): New function.
	(the_low_target): Add thread delete callback.
	* linux-bfin-low.c (the_low_target): Likewise.
	* linux-crisv32-low.c (the_low_target): Likewise.
	* linux-low.c (delete_lwp): Invoke delete_thread callback if
	set.
	* linux-low.h (struct linux_target_ops) <delete_thread>: New
	field.
	* linux-m32r-low.c (the_low_target): Add thread delete callback.
	* linux-mips-low.c (mips_linux_delete_thread): New function.
	(the_low_target): Add thread delete callback.
	* linux-ppc-low.c (the_low_target): Likewise.
	* linux-s390-low.c (the_low_target): Likewise.
	* linux-sh-low.c (the_low_target): Likewise.
	* linux-tic6x-low.c (the_low_target): Likewise.
	* linux-tile-low.c (the_low_target): Likewise.
	* linux-x86-low.c (the_low_target): Likewise.
	* linux-xtensa-low.c (the_low_target): Likewise.

1 files changed, 20 insertions, 1 deletions

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 6080415..c89303c 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -197,6 +197,9 @@ static struct target_ops linux_ops_saved;
 /* The method to call, if any, when a new thread is attached.  */
 static void (*linux_nat_new_thread) (struct lwp_info *);
 
+/* The method to call, if any, when a thread is destroyed.  */
+static void (*linux_nat_delete_thread) (struct arch_lwp_info *);
+
 /* The method to call, if any, when a new fork is attached.  */
 static linux_nat_new_fork_ftype *linux_nat_new_fork;
 
@@ -839,7 +842,12 @@ static int check_ptrace_stopped_lwp_gone (struct lwp_info *lp);
 static void
 lwp_free (struct lwp_info *lp)
 {
-  xfree (lp->arch_private);
+  /* Let the arch specific bits release arch_lwp_info.  */
+  if (linux_nat_delete_thread != NULL)
+    linux_nat_delete_thread (lp->arch_private);
+  else
+    gdb_assert (lp->arch_private == NULL);
+
   xfree (lp);
 }
 
@@ -4873,6 +4881,17 @@ linux_nat_set_new_thread (struct target_ops *t,
   linux_nat_new_thread = new_thread;
 }
 
+/* Register a method to call whenever a new thread is attached.  */
+void
+linux_nat_set_delete_thread (struct target_ops *t,
+			     void (*delete_thread) (struct arch_lwp_info *))
+{
+  /* Save the pointer.  We only support a single registered instance
+     of the GNU/Linux native target, so we do not need to map this to
+     T.  */
+  linux_nat_delete_thread = delete_thread;
+}
+
 /* See declaration in linux-nat.h.  */
 
 void