cmd/trace/v2: tolerate traces with broken tails

This change modifies cmd/trace/v2 to tolerate traces with
incomplete/broken generations at the tail. These broken tails can be
created if a program crashes while a trace is being produced. Although
the runtime tries to flush the trace on some panics, it may still
produce some extra trace data that is incomplete.

This change modifies cmd/trace/v2 to still work on any complete
generations, even if there are incomplete/broken generations at the tail
end of the trace. Basically, the tool now just tracks when the last good
generation ended (via Sync events) and truncates the trace to that point
when it encounters an error.

This change also revamps the text output of the tool to emit regular
progress notifications as well as warnings as to how much of the trace
data was lost.

Fixes #65316.

Change-Id: I877d39993bc02a81eebe647db9c2be17635bcec8
Reviewed-on: https://go-review.googlesource.com/c/go/+/580135
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/trace/v2/jsontrace_test.go b/src/cmd/trace/v2/jsontrace_test.go
index 65ce041c4fa..e1a53669b72 100644
--- a/src/cmd/trace/v2/jsontrace_test.go
+++ b/src/cmd/trace/v2/jsontrace_test.go
@@ -283,7 +283,7 @@ func getTestTrace(t *testing.T, testPath string) *parsedTrace {
 	}
 
 	// Parse the test trace.
-	parsed, err := parseTrace(&trace)
+	parsed, err := parseTrace(&trace, int64(trace.Len()))
 	if err != nil {
 		t.Fatalf("failed to parse trace: %v", err)
 	}