[draft] storage: test and document pushing intent across epochs

I haven't looked at this code in a while, but while my assumption was
that if a ResolveIntent comes in at a newer epoch, we would keep the
existing intent and rewrite it to that epoch, but apparently we "just"
remove it.
This behavior might make sense in practice (now that our concurrency
control is cooperative), but it was not what I remembered, and plays
a prominent role in the correctness bug cockroachdb#69414.

Still need to figure out what to do with this. Comments definitely
refer to the behavior I remember. It's possible that we changed this
by accident. Will investigate more.

Release justification: testing improvement related to release blocker cockroachdb#69414.
Release note: None

pkg/storage/mvcc_history_test.go

@@ -548,8 +548,26 @@ func (e *evalCtx) tryWrapForIntentPrinting(rw ReadWriter) ReadWriter {
 	return rw
 }
 
+func (e *evalCtx) getEpoch() enginepb.TxnEpoch {
+	const key = "epoch"
+	e.t.Helper()
+	if !e.hasArg(key) {
+		return 0
+	}
+	var epoch int
+	e.scanArg(key, &epoch)
+	return enginepb.TxnEpoch(epoch)
+}
+
 func cmdResolveIntent(e *evalCtx) error {
-	txn := e.getTxn(mandatory)
+	txn := e.getTxn(mandatory).Clone()
+	if epoch := e.getEpoch(); epoch > 0 {
+		txn.Epoch = epoch
+	}
+	if ts := e.getTs(nil /* txn */); ts != (hlc.Timestamp{}) {
+		txn.WriteTimestamp = ts
+	}
+
 	key := e.getKey()
 	status := e.getTxnStatus()
 	return e.resolveIntent(e.tryWrapForIntentPrinting(e.engine), key, txn, status)

pkg/storage/testdata/mvcc_histories/push_from_old_epoch

@@ -0,0 +1,49 @@
+# Show that intents are removed when pushed across an epoch change.
+
+# Start txn A and lay down an intent at epoch 2.
+run ok
+with t=A k=k
+  txn_begin ts=100
+  txn_restart
+  txn_restart
+  put  k=k v=v
+----
+>> at end:
+txn: "A" meta={id=00000000 key="k" pri=0.00000000 epo=2 ts=100.000000000,0 min=0,0 seq=0} lock=true stat=PENDING rts=100.000000000,0 wto=false gul=0,0
+meta: "k"/0,0 -> txn={id=00000000 key="k" pri=0.00000000 epo=2 ts=100.000000000,0 min=0,0 seq=0} ts=100.000000000,0 del=false klen=12 vlen=6
+data: "k"/100.000000000,0 -> /BYTES/v
+
+# A pusher who knows that this txn is now in epoch three pushes the intent.
+# The result will be that the intent is removed. Note that we alternatively
+# could also let the intent survive, but we currently do not do that.
+run ok
+with t=A k=k epoch=3 ts=101 status=PENDING
+  resolve_intent
+----
+>> at end:
+<no data>
+
+# Repeat the above scenario with a txn B, the only difference being that when
+# we push the intent, we use a timestamp that is behind the txn's timestamp,
+# meaning that it internally has a chance of hitting a different code path
+# as we wouldn't actually move the intent forward in time if we decided to
+# keep it.
+
+run ok
+with t=B k=k
+  txn_begin ts=100
+  txn_restart
+  txn_restart
+  put  k=k v=v
+----
+>> at end:
+txn: "B" meta={id=00000000 key="k" pri=0.00000000 epo=2 ts=100.000000000,0 min=0,0 seq=0} lock=true stat=PENDING rts=100.000000000,0 wto=false gul=0,0
+meta: "k"/0,0 -> txn={id=00000000 key="k" pri=0.00000000 epo=2 ts=100.000000000,0 min=0,0 seq=0} ts=100.000000000,0 del=false klen=12 vlen=6
+data: "k"/100.000000000,0 -> /BYTES/v
+
+run ok
+with t=B k=k epoch=3 ts=99 status=PENDING
+  resolve_intent
+----
+>> at end:
+<no data>