internal/trace: emit a Sync event even if the next generation is broken

Since CL 644215 each Sync event now represents the coming generation,
with a final Sync event emitted even when there's nothing ahead. This
change however failed to emit a Sync event at the end of a completely
valid generation when the next generation was invalid, causing the
runtime test TestCrashWhileTracing to start failing.

Fix this by emitting a final Sync event even when the next generation is
broken. We hold onto the error in parsing the next generation and emit
it after that final Sync event.

(Should these "final" Sync events distinguish themselves in some way?)

Fixes #71615.

Change-Id: I1f8abee5abaa39e1219e6fa05e9f82f1478db4c9
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/648195
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: mknyszek

src/internal/trace/reader.go

@@ -152,12 +152,10 @@ func (r *Reader) ReadEvent() (e Event, err error) {
 			return syncEvent(nil, r.lastTs, r.syncs), nil
 		}
 		// Read the next generation.
-		var err error
-		r.gen, r.spill, err = readGeneration(r.r, r.spill)
+		r.gen, r.spill, r.spillErr = readGeneration(r.r, r.spill)
 		if r.gen == nil {
-			return Event{}, err
+			return syncEvent(nil, r.lastTs, r.syncs), nil
 		}
-		r.spillErr = err
 
 		// Reset CPU samples cursor.
 		r.cpuSamples = r.gen.cpuSamples