Update doc for how to re-run an action with another RBE instance.

go/README.md

@@ -12,6 +12,14 @@ Flags that describe the RBE instance used and the authentication method are comm
 
 `--alsologtostderr` and `-v VERBOSITY_LEVEL` can be used to tune the verbosity and location of the output, e.g. `--alsologtosrderr -v 1`.
 
+For example, the common flags could be:
+
+```
+    --service remotebuildexecution.googleapis.com:443 \
+    --use_application_default_credentials=true \
+    --alsologtostderr \
+    --v 1
+```
 #### Downloading an action's inputs and metadata
 ```
 bazelisk run //go/cmd/remotetool -- \
@@ -42,6 +50,36 @@ bazelisk run //go/cmd/remotetool -- \
 - An action digest formatted as `"{hash}/{size}"` can be provided directly using the `--digest` flag, or a previously downloaded action can be used with `--action_root`. The `--action_root` path should point to the `--path` of a previous `download_action` invocation. Specifically, the `--action_root` folder must contain a `cmd.textproto` and `ac.textproto` files.
 - `--path` is the destination where the outputs of the action will be downloaded
 
+#### Re-running a downloaded action remotely on a different RBE instance
+
+If you want to download an action from an RBE instance `FOO`, and remotely 
+execute it on another RBE instance `BAR` (probably because you only have the 
+permission to download actions from `FOO`, but don't have the permission to
+execute that instance). In this case, you want to download the action first, and
+then re-run it on another RBE instance with
+`--instance` and `--action_root` flags. 
+
+1. download the action from instance `FOO` to dir `/tmp/out/downloaded_action`
+
+```
+bazelisk run //go/cmd/remotetool -- \
+    --operation download_action \
+    --instance=FOO \
+    --digest=DIGEST
+    COMMON_FLAGS
+    --path /tmp/out/downloaded_action
+```
+2. run the downloaded action with another RBE instance `BAR`
+```
+bazelisk run //go/cmd/remotetool -- \
+    --operation execute_action \
+    --instance=BAR \
+    --action_root /tmp/out/downloaded_action \
+    COMMON_FLAGS
+    --path /tmp/output_file_path
+```
+
+
 #### Running a modified version of a downloaded action
 
 1. Download an action to a given `PATH` following the [instructions for downloading an action](#downloading-an-actions-inputs-and-metadata),

go/pkg/tool/embeddedtool.go

@@ -27,10 +27,10 @@ var (
 
 // RegisterFlags registers the flags necessary for the embedded tool to work.
 func RegisterFlags() {
-	flag.StringVar(&inputDigest, "digest", "", "Digest in <digest/size_bytes> format.")
+	flag.StringVar(&inputDigest, "digest", "", "Digest in <digest/size_bytes> format. This flag should not be provided if action_root is set.")
 	flag.StringVar(&pathPrefix, "path", "", "Path to which outputs should be downloaded to.")
 	flag.BoolVar(&overwrite, "overwrite", false, "Overwrite the output path if it already exist.")
-	flag.StringVar(&actionRoot, "action_root", "", "For execute_action: the root of the action spec, containing ac.textproto (Action proto), cmd.textproto (Command proto), and input/ (root of the input tree).")
+	flag.StringVar(&actionRoot, "action_root", "", "For execute_action: the root of the action spec, containing ac.textproto (Action proto), cmd.textproto (Command proto), and input/ (root of the input tree). This flag should not be provided if digest is set.")
 	flag.IntVar(&execAttempts, "exec_attempts", 10, "For check_determinism: the number of times to remotely execute the action and check for mismatches.")
 	flag.StringVar(&jsonOutput, "json", "", "Path to output operation result as JSON. Currently supported for \"upload_dir\", and includes various upload metadata (see UploadStats).")
 }
@@ -99,12 +99,14 @@ var RemoteToolOperations = map[OpType]func(ctx context.Context, c *Client){
 		fmt.Printf("Action downloaded to %v\n", getPathFlag())
 	},
 	executeAction: func(ctx context.Context, c *Client) {
-		if _, err := c.ExecuteAction(ctx, getDigestFlag(), actionRoot, getPathFlag(), outerr.SystemOutErr); err != nil {
+		validateActionRootAndDg()
+		if _, err := c.ExecuteAction(ctx, inputDigest, actionRoot, getPathFlag(), outerr.SystemOutErr); err != nil {
 			log.Exitf("error executing action: %v", err)
 		}
 	},
 	checkDeterminism: func(ctx context.Context, c *Client) {
-		if err := c.CheckDeterminism(ctx, getDigestFlag(), actionRoot, execAttempts); err != nil {
+		validateActionRootAndDg()
+		if err := c.CheckDeterminism(ctx, inputDigest, actionRoot, execAttempts); err != nil {
 			log.Exitf("error checking determinism: %v", err)
 		}
 	},
@@ -157,3 +159,12 @@ func getPathFlag() string {
 	}
 	return pathPrefix
 }
+
+func validateActionRootAndDg() {
+	if inputDigest != "" && actionRoot != "" {
+		log.Exitf("either specify --digest or --action_root, should not set both of them together.")
+	}
+	if inputDigest == "" && actionRoot == "" {
+		log.Exitf("either specify --digest or --action_root, one of these flags must be set, but not both.")
+	}
+}

go/pkg/tool/tool.go

@@ -6,14 +6,14 @@ import (
 	"bufio"
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
 	"time"
 
-	"errors"
 	log "github.com/golang/glog"
 	"golang.org/x/sync/errgroup"
 	"google.golang.org/protobuf/encoding/prototext"
@@ -98,7 +98,7 @@ func (c *Client) prepCommand(ctx context.Context, client *rexec.Client, actionDi
 		return nil, err
 	}
 
-	log.Infof("Reading command from action digest..")
+	log.Infof("Reading command from action digest: %v", acDg)
 	if _, err := c.GrpcClient.ReadProto(ctx, cmdDg, commandProto); err != nil {
 		return nil, err
 	}
@@ -656,6 +656,7 @@ func (c *Client) ExecuteAction(ctx context.Context, actionDigest, actionRoot, ou
 	case command.LocalErrorResultStatus:
 		oe.WriteErr([]byte(fmt.Sprintf("Local error: %v.\n", ec.Result.Err)))
 	}
+	fmt.Printf("Result: %+v\n", ec.Result)
 	if ec.Result.Err == nil && outDir != "" {
 		ec.DownloadOutputs(outDir)
 		fmt.Printf("Output written to %v\n", outDir)