Make OTel instanceID work with Cloud Run too

cmd/tesseract/gcp/otel.go

@@ -57,27 +57,7 @@ func initOTel(ctx context.Context, traceFraction float64, origin string, project
 		}
 	}
 
-	instanceID, err := os.Hostname()
-	if err != nil {
-		slog.ErrorContext(ctx, "os.Hostname() failed, setting OTel service instance ID to UUID", slog.Any("error", err))
-		instanceID = uuid.NewString()
-	}
-	attrs := []attribute.KeyValue{
-		semconv.ServiceNamespaceKey.String("tesseract"),
-		semconv.ServiceNameKey.String(origin),
-		semconv.ServiceInstanceIDKey.String(instanceID),
-	}
-	if projectID != "" {
-		// Selects the destination project for the Cloud Telemetry API.
-		attrs = append(attrs, attribute.String("gcp.project_id", projectID))
-	}
-	resources, err := resource.New(ctx,
-		resource.WithTelemetrySDK(),
-		// Add your own custom attributes to identify your application
-		resource.WithAttributes(attrs...),
-		resource.WithFromEnv(), // unpacks OTEL_RESOURCE_ATTRIBUTES
-		resource.WithDetectors(gcp.NewDetector()),
-	)
+	resources, err := detectResources(ctx, origin, projectID, os.Hostname, gcp.NewDetector())
 	if err != nil {
 		fatal(ctx, "Failed to detect resources", slog.Any("error", err))
 	}
@@ -139,3 +119,57 @@ func initOTel(ctx context.Context, traceFraction float64, origin string, project
 	}
 	return shutdown
 }
+
+// detectResources builds the OTel Resource describing this service instance.
+//
+// It only sets service.instance.id when the detectors (e.g. gcp.NewDetector())
+// and environment variables do not already supply faas.instance (Cloud Run),
+// host.id (GCE/GKE), or service.instance.id. When falling back to hostname,
+// a random UUID is used if hostname resolution fails or returns "localhost"
+// (as on Cloud Run gen2 containers).
+func detectResources(ctx context.Context, origin, projectID string, hostname func() (string, error), detectors ...resource.Detector) (*resource.Resource, error) {
+	attrs := []attribute.KeyValue{
+		semconv.ServiceNamespaceKey.String("tesseract"),
+		semconv.ServiceNameKey.String(origin),
+	}
+	if projectID != "" {
+		// Selects the destination project for the Cloud Telemetry API.
+		attrs = append(attrs, attribute.String("gcp.project_id", projectID))
+	}
+	resources, err := resource.New(ctx,
+		resource.WithTelemetrySDK(),
+		resource.WithAttributes(attrs...),
+		resource.WithFromEnv(), // unpacks OTEL_RESOURCE_ATTRIBUTES
+		resource.WithDetectors(detectors...),
+	)
+	if err != nil {
+		return nil, err
+	}
+	// Set an instance ID if the detectors and env extractor above didn't come up with one.
+	if allAttrsEmpty(resources.Set(), semconv.ServiceInstanceIDKey, semconv.FaaSInstanceKey, semconv.HostIDKey) {
+		instanceID, err := hostname()
+		if err != nil {
+			slog.ErrorContext(ctx, "hostname() failed, setting OTel service instance ID to UUID", slog.Any("error", err))
+			instanceID = uuid.NewString()
+		} else if instanceID == "" || instanceID == "localhost" {
+			// localhost isn't very useful, so use a random UUID as a last resort.
+			instanceID = uuid.NewString()
+		}
+		resources, err = resource.Merge(resources, resource.NewSchemaless(semconv.ServiceInstanceIDKey.String(instanceID)))
+		if err != nil {
+			return nil, err
+		}
+	}
+	return resources, nil
+}
+
+// allAttrsEmpty returns true if the provided attribute set does not contain a non-empty value
+// associated with any of the provided keys.
+func allAttrsEmpty(s *attribute.Set, keys ...attribute.Key) bool {
+	for _, k := range keys {
+		if v, ok := s.Value(k); ok && v.AsString() != "" {
+			return false
+		}
+	}
+	return true
+}

cmd/tesseract/gcp/otel_test.go

@@ -0,0 +1,106 @@
+// Copyright 2026 The Tessera authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/google/uuid"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/sdk/resource"
+	semconv "go.opentelemetry.io/otel/semconv/v1.4.0"
+)
+
+type fakeDetector struct {
+	attrs []attribute.KeyValue
+}
+
+func (f fakeDetector) Detect(context.Context) (*resource.Resource, error) {
+	return resource.NewSchemaless(f.attrs...), nil
+}
+
+func TestDetectResources(t *testing.T) {
+	for _, test := range []struct {
+		desc                  string
+		detectorAttrs         []attribute.KeyValue
+		hostname              func() (string, error)
+		wantNoServiceInstance bool
+		wantServiceInstance   string
+		wantUUIDInstance      bool
+	}{
+		{
+			desc: "Cloud Run supplies faas.instance, service.instance.id not set",
+			detectorAttrs: []attribute.KeyValue{
+				semconv.FaaSInstanceKey.String("00bf4bf02da24966c43a39d0c6d3bd884d3b81c4"),
+			},
+			hostname:              func() (string, error) { return "localhost", nil },
+			wantNoServiceInstance: true,
+		},
+		{
+			desc: "GCE supplies host.id, service.instance.id not set",
+			detectorAttrs: []attribute.KeyValue{
+				semconv.HostIDKey.String("1234567890123456789"),
+			},
+			hostname:              func() (string, error) { return "gce-vm-1", nil },
+			wantNoServiceInstance: true,
+		},
+		{
+			desc:                  "fallback to non-localhost hostname when detector supplies no instance ID",
+			hostname:              func() (string, error) { return "workstation-01", nil },
+			wantServiceInstance:   "workstation-01",
+		},
+		{
+			desc:             "fallback to UUID when hostname is localhost",
+			hostname:         func() (string, error) { return "localhost", nil },
+			wantUUIDInstance: true,
+		},
+		{
+			desc:             "fallback to UUID when os.Hostname fails",
+			hostname:         func() (string, error) { return "", errors.New("hostname error") },
+			wantUUIDInstance: true,
+		},
+	} {
+		t.Run(test.desc, func(t *testing.T) {
+			t.Setenv("OTEL_RESOURCE_ATTRIBUTES", "")
+			res, err := detectResources(t.Context(), "example.com/log", "my-project", test.hostname, fakeDetector{attrs: test.detectorAttrs})
+			if err != nil {
+				t.Fatalf("detectResources() unexpected error: %v", err)
+			}
+
+			s := res.Set()
+			val, hasServiceInstance := s.Value(semconv.ServiceInstanceIDKey)
+			if test.wantNoServiceInstance {
+				if hasServiceInstance {
+					t.Errorf("service.instance.id = %q, want unset", val.AsString())
+				}
+				return
+			}
+
+			if !hasServiceInstance {
+				t.Fatalf("service.instance.id not set on resource: %v", res)
+			}
+			got := val.AsString()
+			if test.wantUUIDInstance {
+				if _, err := uuid.Parse(got); err != nil {
+					t.Errorf("service.instance.id = %q, want valid UUID: %v", got, err)
+				}
+			} else if got != test.wantServiceInstance {
+				t.Errorf("service.instance.id = %q, want %q", got, test.wantServiceInstance)
+			}
+		})
+	}
+}