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GUI <--> interpreter threading issues

From: John W. Eaton
Subject: GUI <--> interpreter threading issues
Date: Mon, 23 Sep 2019 17:25:09 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0

After this change


I think all calls from the GUI to the interpreter are now done in a thread-safe way.

We now consistently use the following pattern to call the interpreter from the GUI:

  emit interpreter_event
    ([CAPTURES] (interpreter& interp)
       // Code to run in the interpreter thread.

Note that the use of lambda expressions allows us to write functions in the GUI sources that call code from the interpreter (and that will be executed in the interpreter thread) but that may also emit Qt signals directly in order to call back to the GUI thread for further action. Doing this job without lambda expressions would be somewhat more difficult and less clear.

The most complicated case I think I came across is the code in file-editor-tab.cc to save a file, because it checks whether the file that is being saved is currently being executed in the debugger, and if so, displays a confirmation dialog (running in the GUI thread), then jumps back to the interpreter thread to clear the function and the back to the GUI thread to save the file.

After making these changes, I attempted to audit the code to determine that I had actually removed all unsafe calls across threads. To do that, I commented out all the code inside the lambda expressions passed to the interpreter_event signals and all include statements that include header files from the interpreter, then compiled Octave and examined error messages about symbols that were used without declarations. By doing that, I found the places where we still call some interpreter functions directly from the GUI, but as far as I can tell, those functions are thread safe.

I'm not sure of any better way to make this assessment about thread safety. If anyone has ideas, please share!

In the future, we should be careful to follow the patterns now used in the interpreter_event signals and to avoid calling any interpreter functions directly from the GUI unless they are known to be thread safe.

I'll post a separate message about the current state of the event_manager class (formerly known as octave_link).


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