vocdoni / vocdoni-node / 6616580774

63%

push

github

db/pebbledb: make WriteTx.Discard safe to call again

The interface always documented that Discard was safe to call
after Commit, and presumably after Discard too, and it's a good idea.
However, the pebbledb implementation lead to a data race,
as explained in the added Discard method comment.

I suspect that this race existed for some time, but it was rather rare.
It seems to have surfaced recently on October 10th with commit
19c6d6851,
which added more Discard calls in the form of:

        v.tx.Discard()
    +   v.store.NoStateWriteTx.Discard()

It turns out that, if NoStateWriteTx is simply a wrapper over
the other tx we were already discarding, this would trigger the race
with relatively high probability.

Rather than trying to keep track of which transactions have already
been discarded or committed, which is very easy prone to mistakes,
make Discard properly safe to call at any point.

While here, make Commit loudly error if called after itself or Discard.
That's definitely a bug that we should surface quickly.

22 of 24 new or added lines in 1 file covered. (91.67%)

14256 of 22920 relevant lines covered (62.2%)

32462.44 hits per line