kv: gRPC Unavailable errors are ambiguous

This error code is used for fail-fast errors (which can be retried
unambiguously), but it is also used in other cases (such as a server
draining) in which we cannot assume that the previous attempt was not
completed. (It's unclear whether this assumption was once true and
changed or if it's always been incorrect. The specific source of
ambiguous Unavailable errors we're seeing is grpc/grpc-go#1147)

This is expected to increase prevalence of AmbiguousResultErrors; this
will be fixed in a follow-up change.

Fixes cockroachdb#17491

pkg/kv/dist_sender.go

@@ -19,9 +19,6 @@ import (
 	"sync/atomic"
 	"unsafe"
 
-	"google.golang.org/grpc"
-	"google.golang.org/grpc/codes"
-
 	"golang.org/x/net/context"
 
 	"github.com/cockroachdb/cockroach/pkg/base"
@@ -1162,10 +1159,9 @@ func (ds *DistSender) sendToReplicas(
 
 		case call := <-done:
 			if err := call.Err; err != nil {
-				// All connection errors except for an unavailable node (this
-				// is GRPC's fail-fast error), may mean that the request
-				// succeeded on the remote server, but we were unable to
-				// receive the reply. Set the ambiguous commit flag.
+				// All connection errors may mean that the request succeeded
+				// on the remote server, but we were unable to receive the
+				// reply. Set the ambiguous commit flag.
 				//
 				// We retry ambiguous commit batches to avoid returning the
 				// unrecoverable AmbiguousResultError. This is safe because
@@ -1176,14 +1172,7 @@ func (ds *DistSender) sendToReplicas(
 				// leader). If the original attempt merely timed out or was
 				// lost, then the batch will succeed and we can be assured the
 				// commit was applied just once.
-				//
-				// The Unavailable code is used by GRPC to indicate that a
-				// request fails fast and is not sent, so we can be sure there
-				// is no ambiguity on these errors. Note that these are common
-				// if a node is down.
-				// See https://github.com/grpc/grpc-go/blob/52f6504dc290bd928a8139ba94e3ab32ed9a6273/call.go#L182
-				// See https://github.com/grpc/grpc-go/blob/52f6504dc290bd928a8139ba94e3ab32ed9a6273/stream.go#L158
-				if haveCommit && grpc.Code(err) != codes.Unavailable {
+				if haveCommit {
 					ambiguousError = err
 				}
 				log.ErrEventf(ctx, "RPC error: %s", err)