Fix "maint selftest" regression, add struct scoped_mock_context

This commit:

 commit 3922b302645fda04da42a5279399578ae2f6206c
 Author:     Pedro Alves <palves@redhat.com>
 AuthorDate: Thu Jun 18 21:28:37 2020 +0100

    Decouple inferior_ptid/inferior_thread(); dup ptids in thread list (PR 25412)

caused a regression for gdb.gdb/unittest.exp when GDB is configured
with --enable-targets=all.  The failure is:

  gdb/thread.c:95: internal-error: thread_info* inferior_thread(): Assertion `current_thread_ != nullptr' failed.

The problem is in this line in regcache.c:cooked_read_test:

  /* Switch to the mock thread.  */
  scoped_restore restore_inferior_ptid
    = make_scoped_restore (&inferior_ptid, mock_ptid);

Both gdbarch-selftest.c and regcache.c set up a similar mock context,
but the series the patch above belongs to only updated the
gdbarch-selftest.c context to not write to inferior_ptid directly, and
missed updating regcache.c's.

Instead of copying the fix over to regcache.c, share the mock context
setup code in a new RAII class, based on gdbarch-selftest.c's version.

Also remove the "target already pushed" error from regcache.c, like it
had been removed from gdbarch-selftest.c in the multi-target series.
That check is unnecessary because each inferior now has its own target
stack, and the unit test pushes a target on a separate (mock)
inferior, not the current inferior on entry.

gdb/ChangeLog:
2020-06-23  Pedro Alves  <palves@redhat.com>

	* gdbarch-selftests.c: Don't include inferior.h, gdbthread.h or
	progspace-and-thread.h.  Include scoped-mock-context.h instead.
	(register_to_value_test): Use scoped_mock_context.
	* regcache.c: Include "scoped-mock-context.h".
	(cooked_read_test): Don't error out if a target is already pushed.
	Use scoped_mock_context.  Adjust.
	* scoped-mock-context.h: New file.

1 files changed, 82 insertions, 0 deletions

diff --git a/gdb/scoped-mock-context.h b/gdb/scoped-mock-context.h
new file mode 100644
index 0000000..461c2a3
--- /dev/null
+++ b/gdb/scoped-mock-context.h
@@ -0,0 +1,82 @@
+/* RAII type to create a temporary mock context.
+
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef SCOPED_MOCK_CONTEXT_H
+#define SCOPED_MOCK_CONTEXT_H
+
+#include "inferior.h"
+#include "gdbthread.h"
+#include "progspace.h"
+#include "progspace-and-thread.h"
+
+#if GDB_SELF_TEST
+namespace selftests {
+
+/* RAII type to create (and switch to) a temporary mock context.  An
+   inferior with a thread, with a process_stratum target pushed.  */
+
+template<typename Target>
+struct scoped_mock_context
+{
+  /* Order here is important.  */
+
+  Target mock_target;
+  ptid_t mock_ptid {1, 1};
+  program_space mock_pspace {new_address_space ()};
+  inferior mock_inferior {mock_ptid.pid ()};
+  thread_info mock_thread {&mock_inferior, mock_ptid};
+
+  scoped_restore_current_pspace_and_thread restore_pspace_thread;
+
+  scoped_restore_tmpl<thread_info *> restore_thread_list
+    {&mock_inferior.thread_list, &mock_thread};
+
+  /* Add the mock inferior to the inferior list so that look ups by
+     target+ptid can find it.  */
+  scoped_restore_tmpl<inferior *> restore_inferior_list
+    {&inferior_list, &mock_inferior};
+
+  explicit scoped_mock_context (gdbarch *gdbarch)
+  {
+    mock_inferior.gdbarch = gdbarch;
+    mock_inferior.aspace = mock_pspace.aspace;
+    mock_inferior.pspace = &mock_pspace;
+
+    /* Switch to the mock inferior.  */
+    switch_to_inferior_no_thread (&mock_inferior);
+
+    /* Push the process_stratum target so we can mock accessing
+       registers.  */
+    gdb_assert (mock_target.stratum () == process_stratum);
+    push_target (&mock_target);
+
+    /* Switch to the mock thread.  */
+    switch_to_thread (&mock_thread);
+  }
+
+  ~scoped_mock_context ()
+  {
+    pop_all_targets_at_and_above (process_stratum);
+  }
+};
+
+} // namespace selftests
+#endif /* GDB_SELF_TEST */
+
+#endif /* !defined (SCOPED_MOCK_CONTEXT_H) */