[main-] fix rare duplication of replay cmds

[midichef]

When replaying command files, in rare circumstances, some commands can be run twice.

The cause is that reload() is called twice for the command file. Once inside eval_vd(), and again after eval_vd() finishes:

visidata/visidata/main.py

Line 49 in 3ad7a3d

vs.reload()

visidata/visidata/main.py

Line 352 in 3ad7a3d

vd.sync(vs.reload())

Both reload() calls can run at the same time in two different threads, because TableSheet.reload() has the @asyncthread decorator. Both threads add to self.rows via self.addRow(r) in TableSheet.loader(), so command rows can get added twice.

The specific events that must happen to trigger the bug are:

1. the first reload() call must run TableSheet.loader(), past the line of self.rows = []

   visidata/visidata/sheets.py

   Line 282 in 3ad7a3d

   self.rows = []

2. before that reload() finishes adding rows, the second reload() call must run TableSheet.loader(), past the line of self.rows = []
3. before the second reload() call finishes adding rows, the first call continues adding rows

I made a pathological loader that triggers these conditions every time, by loading only two command rows per second : 5c97a84. To test with that loader, do touch row_duplication.slow; vd -p dupes.slow, and see that the dupes_vd sheet contains more than 5 commands. The loader only generates 5 commands, but in my tests, the demo code duplicates all of them, for a total of 10 commands.

The affected versions of Visidata could be multiple versions since Sep 2019, as of 330117f. But on my system, I have not yet found a way to trigger it for replay files that are short (under 100 lines). For much longer files, it does happen every time. For example, it happens if I add ~5000 lines of comments to a short .vdj file. But for short files with no comments (~10 lines), I couldn't trigger it at all by adding CPU/IO/disk stress via the Ubuntu stress binary, in ~500 tries.

[saulpw]

Thank you for this fix, and the corresponding analysis. This makes sense to me.