[lldb-dev] gdb-remote lldb prompt race condition

Matthew Gardiner mg11 at csr.com
Wed Aug 20 01:42:47 PDT 2014

Hi folks

I have been seeing another issue with the display of the lldb prompt. 
This time it's when I do "target create elf-file", then "gdb-remote 
port-number". After the "gdb-remote" command I see the fact that my 
process is stopped, e.g.

Process 1 stopped

on the screen. But no (lldb) prompt.

Some investigation revealed that what's *probably* happening is the main 
thread after processing the "gdb-remote" returns to it's IOHandler, 
which then prints (lldb). However, the inferior's state changes seem to 
delivered to stdout via a different thread, basically one which sits in 
Debugger::DefaultEventHandler. This subsequent output then, I think, 
overwrites the previous (lldb) prompt.

Now in my (and presumably other people's) situation, this issue is 
compounded by the speed of the TCP/IP connection to the gdbserver stub, 
the "poll the hardware" nature of my stub, and the fact the hardware is 
actually simulated - yes over a TCP/IP socket.

FWIW, I resolved this by a horrible (POSIX only) hack, of sleeping for 
300ms at the bottom of the CommandInterpreter::HandleCommand function.

@@ -9,6 +9,8 @@

  #include "lldb/lldb-python.h"

+#include <poll.h> // MG for prompt bugs
  #include <string>
  #include <vector>
  #include <stdlib.h>
@@ -1916,6 +1918,9 @@
      if (log)
        log->Printf ("HandleCommand, command %s", (result.Succeeded() ? 
"succeeded" : "did not succeed"));

+    // MG wait for remote connects etc. to complete
+    poll(0,0,300);
      return result.Succeeded();

But this is horrid. Given this, and other prompt issues, I'm wondering 
whether whether we need some brave soul, to redesign the current lldb IO 
handling mechanisms.


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