[chore] kineto: stub MaxEvents in cap test, cut verify-test by ~390s

docs/rfcs/0012-kineto-receiver-scope.md

@@ -236,7 +236,7 @@ Every error path calls `selftel.IncError(kind)` exactly once per file (not per e
 
 ### Resource bounds
 
-Pinned in `pkg/kineto/decoder.go` package-level constants:
+Pinned in `pkg/kineto/decoder.go` as package-level bounds:
 
 | Bound | Default | Derivation |
 |---|---|---|
@@ -245,7 +245,7 @@ Pinned in `pkg/kineto/decoder.go` package-level constants:
 | `MaxStringBytes` | `1<<20` (1 MiB) | Per-field cap; matches the M11 precedent; defends against malformed `args.Stream` strings |
 | `MaxDepth` | `64` | Chrome-trace structure is flat; no nesting beyond `args.{}` |
 
-Arithmetic for `MaxEvents`: `2.2e9 bytes / 140 bytes/event ≈ 15.7e6 events`, rounded up to the nearest power of two (`1<<24 = 16.78e6`). Exceed any bound triggers `ErrLimitExceeded`, aborts the file, bumps `IncError(KindLimitExceeded)`, and continues to the next file.
+`MaxBytes` and `MaxEvents` are declared as `var` (not `const`) so tests can lower them to exercise the cap path on a tiny input. Production code does not mutate either; `MaxStringBytes` and `MaxDepth` remain `const`. Arithmetic for `MaxEvents`: `2.2e9 bytes / 140 bytes/event ≈ 15.7e6 events`, rounded up to the nearest power of two (`1<<24 = 16.78e6`). Exceed any bound triggers `ErrLimitExceeded`, aborts the file, bumps `IncError(KindLimitExceeded)`, and continues to the next file.
 
 ### `safe.Call` boundary
 

pkg/kineto/decoder.go

@@ -16,9 +16,14 @@ import (
 // See RFC-0012 §Resource bounds.
 var MaxBytes int64 = 1 << 32 // 4 GiB
 
-// Per-event and per-field resource bounds. See RFC-0012 §Resource bounds.
+// MaxEvents caps the number of traceEvents we will parse. Package-level
+// var (not const); mirrors MaxBytes so tests can lower it to exercise
+// the cap path without allocating a 16M-event input. Production code does
+// not mutate it. See RFC-0012 §Resource bounds.
+var MaxEvents = 1 << 24 // ~16.7M; derived from 2.2 GB / ~140 bytes-per-event median (pytorch#130622)
+
+// Per-field resource bounds. See RFC-0012 §Resource bounds.
 const (
-	MaxEvents      = 1 << 24 // ~16.7M; derived from 2.2 GB / ~140 bytes-per-event median (pytorch#130622)
 	MaxStringBytes = 1 << 20 // 1 MiB per string field (Name/Cat/Ph/Args.ExternalID)
 	MaxDepth       = 64      // Chrome-trace structure is flat; 64 is generous
 )

pkg/kineto/decoder_test.go

@@ -81,12 +81,20 @@ func TestParse_VisitErrorPropagates(t *testing.T) {
 }
 
 func TestParse_MaxEventsCap(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skipping MaxEvents stress under -short")
-	}
+	// Stub MaxEvents down to ~100 to exercise the cap path on a tiny
+	// input. Override pattern mirrors the MaxBytes test below
+	// (TestParse_MaxBytesCap_DistinguishedFromTruncated). The naive form
+	// (build MaxEvents+1 = 16M events as a ~1.1 GB JSON string and run
+	// the parser under -race) took ~400s and dominated verify-test
+	// wallclock; this preserves the assertion at sub-millisecond cost.
+	const stubCap = 100
+	orig := MaxEvents
+	MaxEvents = stubCap
+	t.Cleanup(func() { MaxEvents = orig })
+
 	var sb strings.Builder
 	sb.WriteString(`{"traceEvents":[`)
-	for i := 0; i < MaxEvents+1; i++ {
+	for i := 0; i < stubCap+1; i++ {
 		if i > 0 {
 			sb.WriteString(",")
 		}