GUI <--> interpreter threading issues

[John W. Eaton]

After this change

  http://hg.savannah.gnu.org/hgweb/octave/rev/a3ec8c75ece3

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).

jwe