docs: document the freeBlk/reset invariant used by dispatch tests

Add a comment on bulkT.reset noting that only freeBlk should call it,
since TestDrainAndFreeAbortedBlkResponse relies on that invariant to
use reset as an observable proxy for freeBlk having run.

Also split the two concerns in the test comment — synchronous execution
(for race-free reads) and the reset-as-proxy assertion — onto separate
lines so they're not conflated.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ycombinator

internal/pkg/bulk/block.go

@@ -67,6 +67,9 @@ func (a actionT) String() string {
 	return actionStrings[a]
 }
 
+// reset zeros blk's fields so it can be reused from the pool. Only freeBlk
+// should call this; tests rely on this invariant to use reset as a proxy
+// for observing that freeBlk ran.
 func (blk *bulkT) reset() {
 	blk.action = 0
 	blk.flags = 0

internal/pkg/bulk/dispatch_test.go

@@ -92,8 +92,10 @@ func TestDrainAndFreeAbortedBlkResponse(t *testing.T) {
 	blk.ch <- respT{}
 
 	// Run synchronously so blk is only touched by this goroutine, making
-	// the post-drain field reads race-free. blk.reset() is only called
-	// from freeBlk, so observing the reset proves freeBlk ran.
+	// the post-drain field reads race-free.
+	//
+	// blk.reset() is only called from freeBlk, so observing the reset
+	// below proves freeBlk ran.
 	b.drainAndFreeAbortedBlk(blk)
 
 	require.Zero(t, blk.action)