This is surprisingly memory expensive when Poll gets called a lot, such
as when syncing lots of small files generating itemstarted/itemfinished
events. It's line number three in this heap profile on the
TestBenchmarkManyFiles test:
jb@syno:~/src/github.com/syncthing/syncthing/test (master) $ go tool pprof ../bin/syncthing heap-13194.pprof
Entering interactive mode (type "help" for commands)
(pprof) top
80.91MB of 83.05MB total (97.42%)
Dropped 1024 nodes (cum <= 0.42MB)
Showing top 10 nodes out of 85 (cum >= 1.75MB)
flat flat% sum% cum cum%
32MB 38.53% 38.53% 32.01MB 38.54% github.com/syndtr/goleveldb/leveldb/memdb.New
22.16MB 26.68% 65.21% 22.16MB 26.68% github.com/syndtr/goleveldb/leveldb/util.(*BufferPool).Get
13.02MB 15.68% 80.89% 13.02MB 15.68% time.NewTimer
6.94MB 8.35% 89.24% 6.94MB 8.35% github.com/syndtr/goleveldb/leveldb/memdb.(*DB).Put
3.18MB 3.82% 93.06% 3.18MB 3.82% github.com/calmh/xdr.(*Reader).ReadBytesMaxInto
With this change the allocation is removed entirely.
90a1d99785