gdb: cleanup of displaced_step_inferior_state::reset/displaced_step_clear

displaced_step_inferior_state::reset and displaced_step_clear appear to
have the same goal, but they don't do the same thing.
displaced_step_inferior_state::reset clears more things than
displaced_step_clear, but it misses free'ing the closure, which
displaced_step_clear does.

This patch replaces displaced_step_clear's implementation with just a call to
displaced_step_inferior_state::reset.  It then changes
displaced_step_inferior_state::step_closure to be a unique_ptr, to indicate the
fact that displaced_step_inferior_state owns the closure (and so that it is
automatically freed when the field is reset).

The test gdb.base/step-over-syscall.exp caught a problem when doing this, which
I consider to be a latent bug which my cleanup exposes.  In
handle_inferior_event, in the TARGET_WAITKIND_FORKED case, if we displaced-step
over a fork syscall, we make sure to restore the memory that we used as a
displaced-stepping buffer in the child.  We do so using the
displaced_step_inferior_state of the parent.  However, we do it after calling
displaced_step_fixup for the parent, which clears the information in the
parent's displaced_step_inferior_state.  It worked fine before, because
displaced_step_clear didn't completely clear the displaced_step_inferior_state
structure, so the required information (in this case the gdbarch) was
still available after clearing.

I fixed it by making GDB restore the child's memory before calling the
displaced_step_fixup on the parent.  This way, the data in the
displaced_step_inferior_state structure is still valid when we use it for the
child.  This is the error you would get in
gdb.base/step-over-syscall.exp without this fix:

    /home/smarchi/src/binutils-gdb/gdb/gdbarch.c:3911: internal-error: ULONGEST gdbarch_max_insn_length(gdbarch*): Assertion `gdbarch != NULL' failed.

gdb/ChangeLog:

	* infrun.c (get_displaced_step_closure_by_addr): Adjust to
	std::unique_ptr.
	(displaced_step_clear): Rename to...
	(displaced_step_reset): ... this.  Just call displaced->reset ().
	(displaced_step_clear_cleanup): Rename to...
	(displaced_step_reset_cleanup): ... this.
	(displaced_step_prepare_throw): Adjust to std::unique_ptr.
	(displaced_step_fixup): Likewise.
	(resume_1): Likewise.
	(handle_inferior_event): Restore child's memory before calling
	displaced_step_fixup on the parent.
	* infrun.h (displaced_step_inferior_state) <reset>: Adjust
	to std::unique_ptr.
	<step_closure>: Change type to std::unique_ptr.

1 files changed, 2 insertions, 2 deletions

diff --git a/gdb/infrun.h b/gdb/infrun.h
index 8040b28f01..c6329c844d 100644
--- a/gdb/infrun.h
+++ b/gdb/infrun.h
@@ -290,7 +290,7 @@ struct displaced_step_inferior_state
     failed_before = 0;
     step_thread = nullptr;
     step_gdbarch = nullptr;
-    step_closure = nullptr;
+    step_closure.reset ();
     step_original = 0;
     step_copy = 0;
     step_saved_copy.clear ();
@@ -310,7 +310,7 @@ struct displaced_step_inferior_state
 
   /* The closure provided gdbarch_displaced_step_copy_insn, to be used
      for post-step cleanup.  */
-  displaced_step_closure *step_closure;
+  std::unique_ptr<displaced_step_closure> step_closure;
 
   /* The address of the original instruction, and the copy we
      made.  */