Fix Ravenscar "process" resume

A coworker noticed that gdb would send the wrong vCont packet to qemu
when debugging a Ravenscar program:

    > (gdb) thread 2
    > [Switching to thread 2 (Thread 1.2)]
    > #0  0x0000000000001000 in ?? ()

    > (gdb) c
    [...]
    > Sending packet: $vCont;c:p1.1#e2...Ack

Here, we've switched to thread 2, but the packet says to resume thread
1.

This turned out to be a bug in ravenscar_thread_target::resume, which
did not properly handle the case of a "process" resume.  In
particular, the resume method would be passed a ptid of (1, 0, 0) --
but then rewrite this to its saved ptid.

This patch fixes the problem by recognizing this case in the resume
method.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::resume): Handle
	"is_pid" case.

2 files changed, 11 insertions, 1 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e67b386..0d443a8 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2020-08-07  Tom Tromey  <tromey@adacore.com>
 
+	* ravenscar-thread.c (ravenscar_thread_target::resume): Handle
+	"is_pid" case.
+
+2020-08-07  Tom Tromey  <tromey@adacore.com>
+
 	* ravenscar-thread.c (xfer_partial, enable_btrace, add_thread):
 	New methods.
 	(ravenscar_thread_target::get_thread_base_cpu): Check m_cpu_map
diff --git a/gdb/ravenscar-thread.c b/gdb/ravenscar-thread.c
index a67a5e9..91f0920 100644
--- a/gdb/ravenscar-thread.c
+++ b/gdb/ravenscar-thread.c
@@ -363,7 +363,12 @@ ravenscar_thread_target::resume (ptid_t ptid, int step,
   /* If we see a wildcard resume, we simply pass that on.  Otherwise,
      arrange to resume the base ptid.  */
   inferior_ptid = m_base_ptid;
-  if (ptid != minus_one_ptid)
+  if (ptid.is_pid ())
+    {
+      /* We only have one process, so resume all threads of it.  */
+      ptid = minus_one_ptid;
+    }
+  else if (ptid != minus_one_ptid)
     ptid = m_base_ptid;
   beneath ()->resume (ptid, step, siggnal);
 }