gdb/infrun: switch the context before 'displaced_step_restore'

In infrun.c's 'displaced_step_fixup', as part of the 'finish_step_over'
flow, switch to the eventing thread *before* calling
'displaced_step_restore', because down in the flow ptid-dependent
memory accesses are used via current_inferior() and current_top_target().

Without this patch, the problem is exposed with the scenario below:

   $ gdb -q
   (gdb) maint set target-non-stop on
   (gdb) file a.out
   Reading symbols from a.out...
   (gdb) set remote exec-file a.out
   (gdb) target extended-remote | gdbserver --once --multi -
   ...
   (gdb) add-inferior
   [New inferior 2]
   Added inferior 2 on connection 1 (extended-remote ...)
   (gdb) inferior 2
   [Switching to inferior 2 [<null>] (<noexec>)]
   (gdb) file a.out
   Reading symbols from a.out...
   (gdb) set remote exec-file a.out
   (gdb) run
   ...
   Cannot access memory at address 0x555555555042
   (gdb)

The problem is, down inside 'displaced_step_restore', GDB wants to
access the memory for inferior 2 because of an internal breakpoint.
However, the current inferior and inferior_ptid are out of sync.
While inferior_ptid correctly points to the process of inf 2 that was
just started, current_inferior points to inf 1.  Then, the attempt to
access the memory fails, because target_has_execution results in false
since inf 1 was not started.  I was not able to simplify the failing
scenario, but it shows the problem.

After this patch, we get

  ... same steps above...
  (gdb) run
  ...
  [Inferior 2 (process 28652) exited normally]
  (gdb)

Regression-tested on X86_64 Linux with `make check`s default board file
and also `--target_board=native-extended-gdbserver`.  In fact, the bug
fixed by this patch was exposed when using the native-extended-gdbserver
board file.

gdb/ChangeLog:
2020-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infrun.c (displaced_step_fixup): Switch to the event_thread
	before calling displaced_step_restore, not after.

gdb/testsuite/ChangeLog:
2020-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.multi/run-only-second-inf.c: New file.
	* gdb.multi/run-only-second-inf.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
+
+	* infrun.c (displaced_step_fixup): Switch to the event_thread
+	before calling displaced_step_restore, not after.
+
 2020-04-21  Markus Metzger  <markus.t.metzger@intel.com>
 
 	* record-btrace.c (record_btrace_enable_warn): Ignore thread if

gdb/infrun.c

@@ -1884,15 +1884,16 @@ displaced_step_fixup (thread_info *event_thread, enum gdb_signal signal)
   if (displaced->step_thread != event_thread)
     return 0;
 
+  /* Fixup may need to read memory/registers.  Switch to the thread
+     that we're fixing up.  Also, target_stopped_by_watchpoint checks
+     the current thread, and displaced_step_restore performs ptid-dependent
+     memory accesses using current_inferior() and current_top_target().  */
+  switch_to_thread (event_thread);
+
   displaced_step_reset_cleanup cleanup (displaced);
 
   displaced_step_restore (displaced, displaced->step_thread->ptid);
 
-  /* Fixup may need to read memory/registers.  Switch to the thread
-     that we're fixing up.  Also, target_stopped_by_watchpoint checks
-     the current thread.  */
-  switch_to_thread (event_thread);
-
   /* Did the instruction complete successfully?  */
   if (signal == GDB_SIGNAL_TRAP
       && !(target_stopped_by_watchpoint ()

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
+
+	* gdb.multi/run-only-second-inf.c: New file.
+	* gdb.multi/run-only-second-inf.exp: New file.
+
 2020-04-21  Markus Metzger  <markus.t.metzger@intel.com>
 
 	* gdb.btrace/multi-inferior.c: New test.

gdb/testsuite/gdb.multi/run-only-second-inf.c

@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   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/>.  */
+
+int
+main ()
+{
+  return 0;
+}

gdb/testsuite/gdb.multi/run-only-second-inf.exp

@@ -0,0 +1,50 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2020 Free Software Foundation, Inc.
+
+# 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/>.
+
+# Create two inferiors that are not running.  Then run only the second
+# one.  Expect it to start normally.
+
+standard_testfile
+
+if {[use_gdb_stub]} {
+    return 0
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug}]} {
+    return -1
+}
+
+# Setting the target to non-stop mode revealed a bug where the context
+# was not being switched before making ptid-dependent memory access.
+# So, start GDB with this setting.
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " -ex \"maint set target-non-stop on\""
+    clean_restart ${binfile}
+}
+
+# Add and start the second inferior.
+gdb_test "add-inferior" "Added inferior 2.*" \
+    "add empty inferior 2"
+gdb_test "inferior 2" "Switching to inferior 2.*" \
+    "switch to inferior 2"
+gdb_load $binfile
+
+if {[gdb_start_cmd] < 0} {
+    fail "start the second inf"
+} else {
+    gdb_test "" ".*reakpoint ., main .*${srcfile}.*" "start the second inf"
+}