Kitchen: Generate LogDog prefix from Swarming.

go/src/infra/tools/kitchen/cook_logdog.go

Index: go/src/infra/tools/kitchen/cook_logdog.go
diff --git a/go/src/infra/tools/kitchen/cook_logdog.go b/go/src/infra/tools/kitchen/cook_logdog.go
index 48ea4ea9c274db4a94b4d3dfd18c030842625945..7c20db9d83fcde72d8dc1f5cdae56a176144b01b 100644
--- a/go/src/infra/tools/kitchen/cook_logdog.go
+++ b/go/src/infra/tools/kitchen/cook_logdog.go
@@ -23,12 +23,32 @@ import (
 	"github.com/luci/luci-go/common/ctxcmd"
 	"github.com/luci/luci-go/common/environ"
 	"github.com/luci/luci-go/common/errors"
+	"github.com/luci/luci-go/common/logdog/types"
 	log "github.com/luci/luci-go/common/logging"
 
 	"github.com/golang/protobuf/proto"
 	"golang.org/x/net/context"
 )
 
+// buildLogdogPrefix constructs a LogDog Prefix string from Swarming task
+// parameters.
+func buildLogdogPrefix(host, taskID string) (types.StreamName, error) {
+	return types.MakeStreamName("", "swarm", host, taskID)
+}
+
+func getLogdogPrefix(env environ.Env) (types.StreamName, error) {
+	// Construct our LogDog prefix from the Swarming task parameters.
+	host, taskID, err := getSwarmingTaskParameters(env)
+	if err != nil {
+		return "", errors.Annotate(err).Reason("failed to get swarming task parameters").Err()
+	}
+	prefix, err := buildLogdogPrefix(host, taskID)
+	if err != nil {
+		return "", errors.Annotate(err).Reason("failed to build LogDog prefix.").Err()
+	}
+	return prefix, nil
+}
+
 // runWithLogdogButler rus the supplied command through the a LogDog Butler
 // engine instance. This involves:
 //	- Determine a LogDog Prefix.
@@ -39,7 +59,25 @@ import (
 //	  - Otherwise, wait for the process to finish.
 //	- Shut down the Butler instance.
 func (c *cookRun) runWithLogdogButler(ctx context.Context, cmd *exec.Cmd) (rc int, err error) {
-	_ = auth.Authenticator{}
+	// Get our task's environment.
+	var env environ.Env
+	if cmd.Env != nil {
+		env = environ.New(cmd.Env)
+	} else {
+		env = environ.System()
+	}
+
+	prefix := c.logdog.prefix
+	if prefix == "" {
+		var err error
+		prefix, err = getLogdogPrefix(env)
+		if err != nil {
+			return 0, errors.Annotate(err).Err()
+		}
+		log.Fields{
+			"prefix": prefix,
+		}.Infof(ctx, "Generated LogDog prefix [%q] from Swarming parameters.", prefix)
+	}
 
 	authenticator := auth.NewAuthenticator(ctx, auth.SilentLogin, auth.Options{
 		Scopes: out.Scopes(),
@@ -52,7 +90,7 @@ func (c *cookRun) runWithLogdogButler(ctx context.Context, cmd *exec.Cmd) (rc in
 			Auth:    authenticator,
 			Host:    c.logdog.host,
 			Project: config.ProjectName(c.logdog.project),
-			Prefix:  c.logdog.prefix,
+			Prefix:  prefix,
 			SourceInfo: []string{
 				"Kitchen",
 			},
@@ -101,13 +139,6 @@ func (c *cookRun) runWithLogdogButler(ctx context.Context, cmd *exec.Cmd) (rc in
 		Cmd: cmd,
 	}
 
-	var env environ.Env
-	if proc.Env != nil {
-		env = environ.New(proc.Env)
-	} else {
-		env = environ.System()
-	}
-
 	// Augment our environment with Butler parameters.
 	bsEnv := bootstrap.Environment{
 		Project: config.ProjectName(c.logdog.project),