[llvm-bugs] [Bug 37294] New: lldb-server can deadlock in ack-mode

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37294

            Bug ID: 37294
           Summary: lldb-server can deadlock in ack-mode
           Product: lldb
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: lldb-dev at lists.llvm.org
          Reporter: labath at google.com
                CC: llvm-bugs at lists.llvm.org

lldb-server is a single-threaded event-driven app and uses the MainLoop class
to manage events and their actions. It's two main sources of events are a
network socket (requests from the client) and SEGV signals (events in the
inferior).

Normally everything works fine, if we have a very fast client (or a heavily
loaded system) which tries to communicate with lldb-server in ack-mode, things
can break down. The sequence of events if approximately like this:
1. inferior stops for some reason (event A)
2. lldb-server sends a stop-reply packet
3. lldb-server waits for acknowledgement from the client.
- now if the client is really fast, he will manage to also send a regular
packet in addition to a acknowledgement
4. GetAck will read both of these things, but process only the acknowledgement,
and store the rest in a side buffer
5. event A processing done, mainloop calls ppoll()
- at this point, we should wake up and process the client request, however,
ppoll blocks because as far as it is concerned, the event has already been
processed
- so server blocks waiting for data, and client blocks waiting for the server
to respond - deadlock.

I have a couple of ideas how to fix this, but none of them seemed completely
appealing. This issue only affects the lldb-server gtests, because they are the
only ones running in ack-mode. Given that this was more of an omission than
explicit intent, I am going to switch the tests to noack-mode as a stop-gap
measure, to make sure the tests are not flaky.

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