[lldb] Synchronize the debuggers output & error streams

This patch improves the synchronization of the debugger's output and error
streams using two new abstractions: `LockableStreamFile` and
`LockedStreamFile`.

 - `LockableStreamFile` is a wrapper around a `StreamFile` and a mutex. Client
   cannot use the `StreamFile` without calling `Lock`, which returns a
   `LockedStreamFile`.

 - `LockedStreamFile` is an RAII object that locks the stream for the duration
   of its existence.  As long as you hold on to the returned object you are
   permitted to write to the stream. The destruction of the object
   automatically flush the output stream.

1 files changed, 6 insertions, 4 deletions

diff --git a/lldb/source/Commands/CommandObjectBreakpointCommand.cpp b/lldb/source/Commands/CommandObjectBreakpointCommand.cpp
index ac2db59..a913ed5 100644
--- a/lldb/source/Commands/CommandObjectBreakpointCommand.cpp
+++ b/lldb/source/Commands/CommandObjectBreakpointCommand.cpp
@@ -193,10 +193,12 @@ are no syntax errors may indicate that a function was declared but never called.
   Options *GetOptions() override { return &m_all_options; }
 
   void IOHandlerActivated(IOHandler &io_handler, bool interactive) override {
-    StreamFileSP output_sp(io_handler.GetOutputStreamFileSP());
-    if (output_sp && interactive) {
-      output_sp->PutCString(g_reader_instructions);
-      output_sp->Flush();
+    if (interactive) {
+      if (lldb::LockableStreamFileSP output_sp =
+              io_handler.GetOutputStreamFileSP()) {
+        LockedStreamFile locked_stream = output_sp->Lock();
+        locked_stream.PutCString(g_reader_instructions);
+      }
     }
   }