cmd/trace: SWEEP events' swept/reclaimed bytes info is not emitted

[hyangah]

https://golang.org/cl/92375 changed emitted ViewerEvents to be processed in a streaming fashion - traceContext.emit, called as soon as a ViewerEvent is produced, encodes and writes the ViewerEvent as a json object. This helped use creating a big slice of ViewerEvent.

This doesn't work for trace.EvGCSweepStart which depends on modification of the ViewerEvent object after the traceContext.emit call.

case trace.EvGCSweepStart:
   slice := ctx.emitSlice(ev, "SWEEP")
   if done := ev.Link; done != nil && done.Args[0] != 0 {
                                slice.Arg = struct {
                                        Swept     uint64 `json:"Swept bytes"`
                                        Reclaimed uint64 `json:"Reclaimed bytes"`
                                }{done.Args[0], done.Args[1]}
  }

This caused to drop the useful information about the sweep process in Arg field.

@aclements

[gopherbot]

Change https://golang.org/cl/135775 mentions this issue: cmd/trace: don't drop sweep slice details

[aclements]

@gopherbot, please backport to 1.11: this is a regression from Go 1.9.

[gopherbot]

Backport issue(s) opened: #27716 (for 1.10), #27717 (for 1.11).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[gopherbot]

Change https://golang.org/cl/146698 mentions this issue: [release-branch.go1.11] cmd/trace: don't drop sweep slice details