roachprod: reuse start script if start is called with --restart=true

Previously, irrespective of the restart flag value we
ended up overwriting the startup script which led to
defaulting the values of other flags passed previously
to the start command. This was inadequate because when restarting,
i.e., `restart=true`, it makes more sense that we would reuse the
existing startup script by default. This patch changes this behaviour.

Epic: none
Fixes: #125085
Release note: None

pkg/roachprod/install/cockroach.go

@@ -139,7 +139,7 @@ type StartOpts struct {
 	StorageCluster         *SyncedCluster
 
 	// IsRestart allows skipping steps that are used during initial start like
-	// initialization and sequential node starts.
+	// initialization and sequential node starts and also reuses the previous start script.
 	IsRestart bool
 
 	// EnableFluentSink determines whether to enable the fluent-servers attribute
@@ -762,30 +762,35 @@ func (c *SyncedCluster) ExecSQL(
 func (c *SyncedCluster) startNodeWithResult(
 	ctx context.Context, l *logger.Logger, node Node, startOpts StartOpts,
 ) (*RunResultDetails, error) {
-	startCmd, err := c.generateStartCmd(ctx, l, node, startOpts)
-	if err != nil {
-		return newRunResultDetails(node, err), err
-	}
-	var uploadCmd string
-	if c.IsLocal() {
-		uploadCmd = fmt.Sprintf(`cd %s ; `, c.localVMDir(node))
-	}
 	startScriptPath := StartScriptPath(startOpts.VirtualClusterName, startOpts.SQLInstance)
-	uploadCmd += fmt.Sprintf(`cat > %[1]s && chmod +x %[1]s`, startScriptPath)
-
-	var res = &RunResultDetails{}
-	uploadOpts := defaultCmdOpts("upload-start-script")
-	uploadOpts.stdin = strings.NewReader(startCmd)
-	res, err = c.runCmdOnSingleNode(ctx, l, node, uploadCmd, uploadOpts)
-	if err != nil || res.Err != nil {
-		return res, err
-	}
-
 	var runScriptCmd string
 	if c.IsLocal() {
 		runScriptCmd = fmt.Sprintf(`cd %s ; `, c.localVMDir(node))
 	}
 	runScriptCmd += "./" + startScriptPath
+
+	// If we are performing a restart, the start script should already
+	// exist, and we are going to reuse it.
+	if !startOpts.IsRestart {
+		startCmd, err := c.generateStartCmd(ctx, l, node, startOpts)
+		if err != nil {
+			return newRunResultDetails(node, err), err
+		}
+		var uploadCmd string
+		if c.IsLocal() {
+			uploadCmd = fmt.Sprintf(`cd %s ; `, c.localVMDir(node))
+		}
+		uploadCmd += fmt.Sprintf(`cat > %[1]s && chmod +x %[1]s`, startScriptPath)
+
+		var res = &RunResultDetails{}
+		uploadOpts := defaultCmdOpts("upload-start-script")
+		uploadOpts.stdin = strings.NewReader(startCmd)
+		res, err = c.runCmdOnSingleNode(ctx, l, node, uploadCmd, uploadOpts)
+		if err != nil || res.Err != nil {
+			return res, err
+		}
+	}
+
 	return c.runCmdOnSingleNode(ctx, l, node, runScriptCmd, defaultCmdOpts("run-start-script"))
 }