Current master non-deterministic

[mike]

Hey all,

In my quest to debug issue 2801, I stumbled upon behavior in LilyPond that is 
non-deterministic.  You'll see four attached files: a simple diff to apply to 
current master, two .ly files, and a python script to run in the same directory 
as the LilyPond files.

1) Apply the diff and compile.  The diff prints out the name of a grob and its 
extent to stderr every time Grob::extent is called.
2) Download the files and the python script to the same directory.  Change the 
Lilypond executable in the python script as need be.  The python script 
overwrites log0.txt and log1.txt, so make sure you don't have important files 
named that.
3) Oscillate between foo.ly and bar.ly in the python script.

foo.ly is deterministic whereas bar.ly is not.  bar.ly will usually produce 
non-deterministic behavior before it hits 20 runs.

The only difference between the two files is the presence of a slur.

As a test, I rewound Lilypond to 26e8b94f358244216a83c1a6653d5a72c1a1c5e7 
(about 2 years ago) and did the same thing - it is still not deterministic, but 
at different points in the compilation process.

I realize that LilyPond may be using containers that do not guarantee the order 
of things (i.e. sets) and that the test I've written may not be a good 
reflection of what "deterministic" should mean.  However, for debugging 
purposes, I think it's important that when LilyPond compiles the same score, 
the extents measured and the order in which they're measured remain constant.  
Otherwise, finding changes in extents, offsets, and skylines is difficult.

I think it's important to get 2-3 people brainstorming to figure out where and 
why this is happening.

Cheers,
MS

P.S. I don't believe that the measurement tool I'm using are the cause of the 
non-deterministic behavior, but I'm aware that that is always a problem.  The 
.diff is minimally invasive.

simplelogger.py
Description: Text Data

bar.ly
Description: Binary data

foo.ly
Description: Binary data

simple.diff
Description: Binary data