Revert "kvstreamer: reuse incomplete Get requests on resume batches"

This reverts commit 21f2390.

Previously, I didn't realize that the KV layer would modify all requests
included into the BatchRequest in `txnSeqNumAllocator`, so we cannot
reuse even incomplete GetRequests. It is unfortunate, but not a big
deal.

Release note: None

pkg/kv/kvclient/kvstreamer/streamer.go

@@ -1567,6 +1567,14 @@ func buildResumeSingleRangeBatch(
 	// requests with the ResumeSpans.
 	resumeReq.reqsReservedBytes = fp.resumeReqsMemUsage
 	resumeReq.overheadAccountedFor = req.overheadAccountedFor
+	// Note that due to limitations of the KV layer (#75452) we cannot reuse
+	// original requests because the KV doesn't allow mutability (and all
+	// requests are modified by txnSeqNumAllocator, even if they are not
+	// evaluated due to TargetBytes limit).
+	gets := make([]struct {
+		req   roachpb.GetRequest
+		union roachpb.RequestUnion_Get
+	}, fp.numIncompleteGets)
 	scans := make([]struct {
 		req   roachpb.ScanRequest
 		union roachpb.RequestUnion_Scan
@@ -1583,18 +1591,14 @@ func buildResumeSingleRangeBatch(
 				emptyResponse = false
 				continue
 			}
-			// This Get wasn't completed - include it into the batch again (we
-			// can just reuse the original request since it hasn't been
-			// modified which is also asserted below).
-			if buildutil.CrdbTestBuild {
-				if origSpan := req.reqs[i].GetInner().Header().Span(); !get.ResumeSpan.Equal(origSpan) {
-					panic(errors.AssertionFailedf(
-						"unexpectedly the ResumeSpan %s on the GetResponse is different from the original span %s",
-						get.ResumeSpan, origSpan,
-					))
-				}
-			}
-			resumeReq.reqs[resumeReqIdx] = req.reqs[i]
+			// This Get wasn't completed - create a new request according to the
+			// ResumeSpan and include it into the batch.
+			newGet := gets[0]
+			gets = gets[1:]
+			newGet.req.SetSpan(*get.ResumeSpan)
+			newGet.req.KeyLocking = s.keyLocking
+			newGet.union.Get = &newGet.req
+			resumeReq.reqs[resumeReqIdx].Value = &newGet.union
 			resumeReq.positions = append(resumeReq.positions, position)
 			if req.subRequestIdx != nil {
 				resumeReq.subRequestIdx = append(resumeReq.subRequestIdx, req.subRequestIdx[i])