feat(internal/detect): add helper to detect projectID from env (#4582)

This is meant to be used by certain veneers so that we can detect the projectID in a consistent manner. Future PRs will use this logic.
Fixes: #1294

internal/detect/detect.go

@@ -0,0 +1,67 @@
+// Copyright 2021 Google LLC
+//
+// 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
+//
+//     https://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 detect is used find information from the environment.
+package detect
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+
+	"golang.org/x/oauth2/google"
+	"google.golang.org/api/option"
+	"google.golang.org/api/transport"
+)
+
+const (
+	projectIDSentinel = "*detect-project-id*"
+	envProjectID      = "GOOGLE_CLOUD_PROJECT"
+)
+
+var (
+	adcLookupFunc func(context.Context, ...option.ClientOption) (*google.Credentials, error) = transport.Creds
+	envLookupFunc func(string) string                                                        = os.Getenv
+)
+
+// 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 := envLookupFunc(envProjectID); id != "" {
+		return id, nil
+	}
+	// 2. Try ADC
+	creds, err := adcLookupFunc(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 != "" && envLookupFunc(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
+}

internal/detect/detect_test.go

@@ -0,0 +1,80 @@
+// Copyright 2021 Google LLC
+//
+// 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
+//
+//     https://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 detect
+
+import (
+	"context"
+	"testing"
+
+	"golang.org/x/oauth2/google"
+	"google.golang.org/api/option"
+)
+
+func TestIt(t *testing.T) {
+	tests := []struct {
+		name         string
+		projectID    string
+		env          map[string]string
+		adcProjectID string
+		want         string
+	}{
+		{
+			name:      "noop",
+			projectID: "noop",
+			want:      "noop",
+		},
+		{
+			name:      "environment project id",
+			projectID: projectIDSentinel,
+			env:       map[string]string{envProjectID: "environment-project-id"},
+			want:      "environment-project-id",
+		},
+		{
+			name:         "adc project id",
+			projectID:    projectIDSentinel,
+			adcProjectID: "adc-project-id",
+			want:         "adc-project-id",
+		},
+		{
+			name:      "emulator project id",
+			projectID: projectIDSentinel,
+			env:       map[string]string{"EMULATOR_HOST": "something"},
+			want:      "emulated-project",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			envLookupFunc = func(k string) string {
+				if tc.env == nil {
+					return ""
+				}
+				return tc.env[k]
+			}
+			adcLookupFunc = func(context.Context, ...option.ClientOption) (*google.Credentials, error) {
+				return &google.Credentials{ProjectID: tc.adcProjectID}, nil
+			}
+
+			got, err := ProjectID(context.Background(), tc.projectID, "EMULATOR_HOST")
+			if err != nil {
+				t.Fatalf("unexpected error from ProjectID(): %v", err)
+			}
+			if got != tc.want {
+				t.Fatalf("ProjectID() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+
+}