Accommodate Swarming try number into log stream.

milo/appengine/swarming/buildinfo.go

Index: milo/appengine/swarming/buildinfo.go
diff --git a/milo/appengine/swarming/buildinfo.go b/milo/appengine/swarming/buildinfo.go
index abb04252bc85fdde1d6f5514a61f2f37e40eb109..1dd051d9eb2c542efc076ead78e134ca9eea17e1 100644
--- a/milo/appengine/swarming/buildinfo.go
+++ b/milo/appengine/swarming/buildinfo.go
@@ -5,9 +5,12 @@
 package swarming
 
 import (
+	"strconv"
 	"strings"
+	"unicode/utf8"
 
 	swarming "github.com/luci/luci-go/common/api/swarming/swarming/v1"
+	"github.com/luci/luci-go/common/errors"
 	"github.com/luci/luci-go/common/logging"
 	miloProto "github.com/luci/luci-go/common/proto/milo"
 	"github.com/luci/luci-go/grpc/grpcutil"
@@ -86,7 +89,7 @@ func (p *BuildInfoProvider) GetBuildInfo(c context.Context, req *milo.BuildInfoR
 	// Determine the LogDog annotation stream path for this Swarming task.
 	//
 	// On failure, will return a gRPC error.
-	as, err := resolveLogDogAnnotations(c, fr.req, projectHint, host, req.Task)
+	as, err := resolveLogDogAnnotations(c, fr.req, projectHint, host, req.Task, fr.res.TryNumber)
 	if err != nil {
 		logging.WithError(err).Warningf(c, "Failed to get annotation stream parameters.")
 		return nil, err
@@ -96,6 +99,11 @@ func (p *BuildInfoProvider) GetBuildInfo(c context.Context, req *milo.BuildInfoR
 		return nil, grpcutil.Internal
 	}
 
+	logging.Fields{
+		"project": as.Project,
+		"path":    as.Path,
+	}.Infof(c, "Resolved annotation stream.")
+
 	client, err := p.newLogdogClient(c)
 	if err != nil {
 		logging.WithError(err).Errorf(c, "Failed to create LogDog client.")
@@ -136,7 +144,7 @@ func (p *BuildInfoProvider) GetBuildInfo(c context.Context, req *milo.BuildInfoR
 // endpoint, though. All of the nastiness here should be replaced with something
 // more elegant once that becomes available. In the meantime...
 func resolveLogDogAnnotations(c context.Context, sr *swarming.SwarmingRpcsTaskRequest, projectHint cfgtypes.ProjectName,
-	host, taskID string) (*logdog.AnnotationStream, error) {
+	host, taskID string, tryNumber int64) (*logdog.AnnotationStream, error) {
 
 	// If this is a Kitchen command, maybe we can infer our LogDog project from
 	// the command-line.
@@ -165,7 +173,17 @@ func resolveLogDogAnnotations(c context.Context, sr *swarming.SwarmingRpcsTaskRe
 	//
 	// This is an implementation of:
 	// https://chromium.googlesource.com/infra/infra/+/a7032e3e240d4b81a1912bfaf29a20d02f665cc1/go/src/infra/tools/kitchen/cook_logdog.go#129
-	prefix, err := types.MakeStreamName("", "swarm", host, taskID)
+	runID, err := getRunID(taskID, tryNumber)
+	if err != nil {
+		logging.Fields{
+			logging.ErrorKey: err,
+			"taskID":         taskID,
+			"tryNumber":      tryNumber,
+		}.Errorf(c, "Failed to get Run ID for task/try.")
+		return nil, grpcutil.Internal
+	}
+
+	prefix, err := types.MakeStreamName("", "swarm", host, runID)
 	if err != nil {
 		logging.WithError(err).Errorf(c, "Failed to generate Swarming prefix.")
 		return nil, grpcutil.Internal
@@ -197,3 +215,33 @@ func getLogDogProjectFromKitchen(cmd []string) (proj cfgtypes.ProjectName, isKit
 	}
 	return
 }
+
+// getRunID converts a Swarming task ID and try number into a Swarming Run ID.
+//
+// The run ID is a permutation of the last four bits of the Swarming Task ID.
+// Therefore, we chop it off of the string, mutate it, and then add it back.
+//
+// TODO(dnj): Replace this with Swarming API call once finished.
+func getRunID(taskID string, tryNumber int64) (string, error) {
+	// Slice off the last character form the task ID.
+	if len(taskID) == 0 {
+		return "", errors.New("swarming task ID is empty")
+	}
+
+	// Parse "tryNumber" as a hex string.
+	if tryNumber < 0 || tryNumber > 0x0F {
+		return "", errors.Reason("try number %(try)d exceeds 4 bits").
+			D("try", tryNumber).
+			Err()
+	}
+
+	lastChar, lastCharSize := utf8.DecodeLastRuneInString(taskID)
+	v, err := strconv.ParseUint(string(lastChar), 16, 8)
+	if err != nil {
+		return "", errors.Annotate(err).Reason("failed to parse hex from rune: %(rune)r").
+			D("rune", lastChar).
+			Err()
+	}
+
+	return taskID[:len(taskID)-lastCharSize] + strconv.FormatUint((v|uint64(tryNumber)), 16), nil
+}