Fix PR remote/21852: Remote run without specifying a local binary crashes GDB

There is an assertion that is triggering when we start GDB and
instruct it to debug a remote inferior, but don't provide a local
binary, like:

  ./gdb -nx -q --data-directory=data-directory -ex "tar ext :1234" \
    -ex "set remote exec-file /bin/ls" -ex r

In this case, when calling exec_file_locate_attach to locate the
inferior, GDB is incorrectly resetting the breakpoints without a
thread/inferior even running, which causes an assertion to be
triggered:

  binutils-gdb/gdb/thread.c:1609: internal-error: scoped_restore_current_thread::scoped_restore_current_thread(): Assertion `tp != NULL' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n)

This happens because add_current_inferior_and_thread (on remote.c) is
breaking an invariant: making inferior_ptid point to a non-existing
thread and then calling common code, which in this case is
breakpoint_re_set.  The fix is to make sure that inferior_ptid points
to null_ptid if there is no thread present.

A testcase is provided.  Regtested on buildbot.

gdb/ChangeLog:
2017-08-23  Pedro Alves  <palves@redhat.com>

	PR remote/21852
	* remote.c (add_current_inferior_and_thread): Set inferior_ptid
	to null_ptid and switch to thread without reading the registers
	after adding the inferior.

gdb/testsuite/ChangeLog:
2017-08-23  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR remote/21852
	* gdb.server/normal.c: New file, copied from gdb.base.
	* gdb.server/run-without-local-binary.exp: New file.

1 files changed, 9 insertions, 9 deletions

diff --git a/gdb/remote.c b/gdb/remote.c
index ff59a0f..2249533 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -3831,19 +3831,16 @@ add_current_inferior_and_thread (char *wait_status)
 {
   struct remote_state *rs = get_remote_state ();
   int fake_pid_p = 0;
-  ptid_t ptid;
 
   inferior_ptid = null_ptid;
 
   /* Now, if we have thread information, update inferior_ptid.  */
-  ptid = get_current_thread (wait_status);
+  ptid_t curr_ptid = get_current_thread (wait_status);
 
-  if (!ptid_equal (ptid, null_ptid))
+  if (curr_ptid != null_ptid)
     {
       if (!remote_multi_process_p (rs))
 	fake_pid_p = 1;
-
-      inferior_ptid = ptid;
     }
   else
     {
@@ -3851,14 +3848,17 @@ add_current_inferior_and_thread (char *wait_status)
 	 (such as kill) won't work.  This variable serves (at least)
 	 double duty as both the pid of the target process (if it has
 	 such), and as a flag indicating that a target is active.  */
-      inferior_ptid = magic_null_ptid;
+      curr_ptid = magic_null_ptid;
       fake_pid_p = 1;
     }
 
-  remote_add_inferior (fake_pid_p, ptid_get_pid (inferior_ptid), -1, 1);
+  remote_add_inferior (fake_pid_p, ptid_get_pid (curr_ptid), -1, 1);
 
-  /* Add the main thread.  */
-  add_thread_silent (inferior_ptid);
+  /* Add the main thread and switch to it.  Don't try reading
+     registers yet, since we haven't fetched the target description
+     yet.  */
+  thread_info *tp = add_thread_silent (curr_ptid);
+  switch_to_thread_no_regs (tp);
 }
 
 /* Print info about a thread that was found already stopped on