locks trailers during iteration

[owen-d]

I was recently debugging a panic in our queriers and found out we acquire a lock for mutating this slice but don't acquire the lock for reading. I'm guessing this was an oversight as the lock was only used in one place prior (the addTrailer method).

panic: runtime error: invalid memory address or nil pointer dereference

goroutine 34388532 [running]:
github.com/grafana/loki/pkg/util/server.onPanic(0x2072aa0, 0x3787ff0, 0x3787ff0, 0x0)
/src/loki/pkg/util/server/recovery.go:41 +0x7b
github.com/grafana/loki/pkg/util/server.glob..func1.1.1(0x2834870, 0xc004b2e8c0)
/src/loki/pkg/util/server/recovery.go:29 +0x55
panic(0x2072aa0, 0x3787ff0)
/usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/grafana/loki/pkg/logql/stats.decodeTrailer(0x283eea8, 0xc054461d10, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/src/loki/pkg/logql/stats/grpc.go:115 +0xc8
github.com/grafana/loki/pkg/logql/stats.decodeTrailers(0x283eea8, 0xc054461d10, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/src/loki/pkg/logql/stats/grpc.go:95 +0x117
github.com/grafana/loki/pkg/logql/stats.Snapshot(0x283eea8, 0xc054461d10, 0x12b0ec7, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/src/loki/pkg/logql/stats/context.go:149 +0x71
github.com/grafana/loki/pkg/logql.(*query).Exec(0xc02dc7e190, 0x283ee70, 0xc054461d10, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/src/loki/pkg/logql/engine.go:155 +0x3a5
github.com/grafana/loki/pkg/querier.(*Querier).RangeQueryHandler(0xc000502a10, 0x2834870, 0xc004b2e8c0, 0xc0266f1500)
/src/loki/pkg/querier/http.go:61 +0x362

[cyriltovena]

LGTM

It wasn't an oversight we never call decode trailers in // as opposed to encode but I don't think it can hurt to have this perf wise.

So careful I wouldn't assume this was the cause of your panic.