[PATCH 1/3] Windows gdb: Show CONTEXT_EXCEPTION_REQUEST info in "info threads"

[Pedro Alves <pedro@palves.net>]

This patch makes the Windows native target expose
CONTEXT_EXCEPTION_REQUEST information in "info threads" extra info,
like so:

 (gdb) info threads
   Id   Target Id                                     Frame
   1    Thread 15648.0x4338 "sleeper"                 main () at sleeper.c:5
   2    Thread 15648.0x3d58 (in syscall)              0x00007ffde37057b4 in ntdll!ZwWaitForWorkViaWorkerFactory () from C:/WINDOWS/SYSTEM32/ntdll.dll
   3    Thread 15648.0x27e0 "sig" (in syscall)        0x00007ffde3701bc4 in ntdll!ZwReadFile () from C:/WINDOWS/SYSTEM32/ntdll.dll
   4    Thread 15648.0x26f0 (in syscall)              0x00007ffde37057b4 in ntdll!ZwWaitForWorkViaWorkerFactory () from C:/WINDOWS/SYSTEM32/ntdll.dll
 * 5    Thread 15648.0xff8 (in exception)             0x00007ffde00aa464 in KERNELBASE!CtrlRoutine () from C:/WINDOWS/System32/KERNELBASE.dll
 (gdb)

Above, we can see that thread 1 is running user space code, threads 2
to 4 are in some system call, and thread 5 raised an exception (a
Ctrl-C).

This is useful information to see, as system calls are not
interruptible on Windows.  E.g. an infcall on a thread that is blocked
in a system call will appear to hang, until the system call returns on
its own.

Change-Id: I04221f123eef81d59b5cc1c9fbb298f7a33fa001
commit-id:d93544e4
---
 gdb/nat/windows-nat.h |  3 ++-
 gdb/windows-nat.c     | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/gdb/nat/windows-nat.h b/gdb/nat/windows-nat.h
index 52378765438..d66d5ec0ed3 100644
--- a/gdb/nat/windows-nat.h
+++ b/gdb/nat/windows-nat.h
@@ -527,7 +527,8 @@ struct WindowsContext<CONTEXT *>
 				     | CONTEXT_SEGMENTS
 #endif
 				     | CONTEXT_DEBUG_REGISTERS
-				     | CONTEXT_EXTENDED_REGISTERS);
+				     | CONTEXT_EXTENDED_REGISTERS
+				     | CONTEXT_EXCEPTION_REQUEST);
 };
 
 #ifdef __x86_64__
diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index a9647e90bb8..d01fbc8e4ba 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -3363,6 +3363,28 @@ windows_nat_target::extra_thread_info (thread_info *info)
 	   || th->last_event.dwDebugEventCode == EXIT_PROCESS_DEBUG_EVENT)
     return "exiting process";
 
+  /* Specifying CONTEXT_EXCEPTION_REQUEST in ContextFlags as input
+     (which we do), asks Windows to report back why the thread entered
+     kernel mode.  If GetThreadContext returns a context with
+     CONTEXT_EXCEPTION_REPORTING set, it means that it understood the
+     request.  */
+  windows_process->fill_thread_context (th);
+  DWORD context_flags = *windows_process->context_flags_ptr (th);
+  if ((context_flags & CONTEXT_EXCEPTION_REPORTING) != 0)
+    {
+      /* The thread was running user space code which raised an
+	 exception, which we intercepted.  */
+      if ((context_flags & CONTEXT_EXCEPTION_ACTIVE) != 0)
+	return "in exception";
+
+      /* The thread was running a system call.  */
+      if ((context_flags & CONTEXT_SERVICE_ACTIVE) != 0)
+	return "in syscall";
+
+      /* Otherwise, the thread was simply suspended while running
+	 user space code.  */
+    }
+
   return nullptr;
 }
 
-- 
2.53.0