demo: fix --global option

The --global option broke somewhere during the 20.2 cycle. This patch
revives it. --global simulates geo-latencies between nodes. Best used
with `--global --geo-partitioned-replicas`.

Release note (cli change): fix cockroach demo --global from crashing
with "didn't get expected magic bytes header".

pkg/cli/interactive_tests/test_demo_global.tcl

@@ -2,10 +2,13 @@
 
 source [file join [file dirname $argv0] common.tcl]
 
+# Set a larger timeout since we are going global.
+set timeout 90
+
 start_test "Check --global flag runs as expected"
 
 # Start a demo with --global set
-spawn $argv demo movr --global
+spawn $argv demo movr --nodes 9 --global
 
 # Ensure db is movr.
 eexpect "movr>"

pkg/rpc/context.go

@@ -775,6 +775,23 @@ func (ctx *Context) grpcDialOptions(
 			dialOpts = append(dialOpts, grpc.WithStreamInterceptor(testingStreamInterceptor))
 		}
 	}
+	if ctx.Knobs.ArtificialLatencyMap != nil {
+		dialerFunc := func(ctx context.Context, target string) (net.Conn, error) {
+			dialer := net.Dialer{
+				LocalAddr: sourceAddr,
+			}
+			return dialer.DialContext(ctx, "tcp", target)
+		}
+		latency := ctx.Knobs.ArtificialLatencyMap[target]
+		log.VEventf(ctx.masterCtx, 1, "connecting to node %s with simulated latency %dms", target, latency)
+		dialer := artificialLatencyDialer{
+			dialerFunc: dialerFunc,
+			latencyMS:  latency,
+		}
+		dialerFunc = dialer.dial
+		dialOpts = append(dialOpts, grpc.WithContextDialer(dialerFunc))
+	}
+
 	return dialOpts, nil
 }