feat(internal/detect): add helper to detect projectID from env

internal/detect/detect.go

@@ -0,0 +1,44 @@
+// Package detect is used find information from the environment.
+package detect
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+
+	"google.golang.org/api/option"
+	"google.golang.org/api/transport"
+)
+
+const projectIDSentinel = "*detect-project-id*"
+
+// ProjectID tries to detect the project ID from the environment if the sentinel
+// value, "*detect-project-id*", is sent. It looks in the following order:
+//   1. GOOGLE_CLOUD_PROJECT envvar
+//   2. ADC creds.ProjectID
+//   3. A static value if the environment is emulated.
+func ProjectID(ctx context.Context, projectID string, emulatorEnvVar string, opts ...option.ClientOption) (string, error) {
+	if projectID != projectIDSentinel {
+		return projectID, nil
+	}
+	// 1. Try a well known environment variable
+	if id := os.Getenv("GOOGLE_CLOUD_PROJECT"); id != "" {
+		return id, nil
+	}
+	// 2. Try ADC
+	creds, err := transport.Creds(ctx, opts...)
+	if err != nil {
+		return "", fmt.Errorf("fetching creds: %v", err)
+	}
+	// 3. If ADC does not work, and the environment is emulated, return a const value.
+	if creds.ProjectID == "" && emulatorEnvVar != "" && os.Getenv(emulatorEnvVar) != "" {
+		return "emulated-project", nil
+	}
+	// 4. If 1-3 don't work, error out
+	if creds.ProjectID == "" {
+		return "", errors.New("unable to detect projectID, please refer to docs for DetectProjectID")
+	}
+	// Success from ADC
+	return creds.ProjectID, nil
+}