feat(auth): add detect package (#8491)

The purpose of this package is to source credentials via ADC. It
is analogous to the oauth2/google package but is more limited,
currently. It does not provide helper constructors for creating
things like a compute credential and instead has a single
constructor that does it all. Because of this it is more has more
extensible options.

Purposefully missing from this package is support for external
accounts, but some documentation is there. This will come up in a
separate PR as that has a lot of extra code to review and I tried
to shrink the size of this commit as much as possible.

There are some TODOs in this commit that will be cleaned up in a
future commit that requires more refactoring.

auth/auth.go

@@ -18,7 +18,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"io"
 	"net/http"
 	"net/url"
 	"strings"
@@ -300,7 +299,7 @@ func (tp tokenProvider2LO) Token(ctx context.Context) (*Token, error) {
 		return nil, fmt.Errorf("auth: cannot fetch token: %w", err)
 	}
 	defer resp.Body.Close()
-	body, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	body, err := internal.ReadAll(resp.Body)
 	if err != nil {
 		return nil, fmt.Errorf("auth: cannot fetch token: %w", err)
 	}

auth/auth_test.go

@@ -21,12 +21,12 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
-	"reflect"
 	"strings"
 	"testing"
 	"time"
 
 	"cloud.google.com/go/auth/internal/jwt"
+	"github.com/google/go-cmp/cmp"
 )
 
 var fakePrivateKey = []byte(`-----BEGIN RSA PRIVATE KEY-----
@@ -427,7 +427,7 @@ func TestConfigJWT2LO_AssertionPayload(t *testing.T) {
 				}
 				m := got.(map[string]interface{})
 				for v, k := range opts.PrivateClaims {
-					if !reflect.DeepEqual(m[v], k) {
+					if !cmp.Equal(m[v], k) {
 						t.Errorf("payload private claims key = %q: got %#v; want %#v", v, m[v], k)
 					}
 				}

auth/detect/compute.go

@@ -0,0 +1,87 @@
+// Copyright 2023 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
+//
+//      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 detect
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/url"
+	"strings"
+	"time"
+
+	"cloud.google.com/go/auth"
+	"cloud.google.com/go/compute/metadata"
+)
+
+var (
+	computeTokenMetadata = map[string]interface{}{
+		"auth.google.tokenSource":    "compute-metadata",
+		"auth.google.serviceAccount": "default",
+	}
+	computeTokenURI = "instance/service-accounts/default/token"
+)
+
+// computeTokenProvider creates a [cloud.google.com/go/auth.TokenProvider] that
+// uses the metadata service to retrieve tokens.
+func computeTokenProvider(earlyExpiry time.Duration, scope ...string) auth.TokenProvider {
+	return auth.NewCachedTokenProvider(computeProvider{scopes: scope}, &auth.CachedTokenProviderOptions{
+		ExpireEarly: earlyExpiry,
+	})
+}
+
+// computeProvider fetches tokens from the google cloud metadata service.
+type computeProvider struct {
+	scopes []string
+}
+
+type metadataTokenResp struct {
+	AccessToken  string `json:"access_token"`
+	ExpiresInSec int    `json:"expires_in"`
+	TokenType    string `json:"token_type"`
+}
+
+func (cs computeProvider) Token(ctx context.Context) (*auth.Token, error) {
+	// TODO(codyoss): account may need to be configurable if we have a constructor for
+	// the provider.
+	tokenURI, err := url.Parse(computeTokenURI)
+	if err != nil {
+		return nil, err
+	}
+	if len(cs.scopes) > 0 {
+		v := url.Values{}
+		v.Set("scopes", strings.Join(cs.scopes, ","))
+		tokenURI.RawQuery = v.Encode()
+	}
+	tokenJSON, err := metadata.Get(tokenURI.String())
+	if err != nil {
+		return nil, err
+	}
+	var res metadataTokenResp
+	if err := json.NewDecoder(strings.NewReader(tokenJSON)).Decode(&res); err != nil {
+		return nil, fmt.Errorf("detect: invalid token JSON from metadata: %w", err)
+	}
+	if res.ExpiresInSec == 0 || res.AccessToken == "" {
+		return nil, errors.New("detect: incomplete token received from metadata")
+	}
+	return &auth.Token{
+		Value:    res.AccessToken,
+		Type:     res.TokenType,
+		Expiry:   time.Now().Add(time.Duration(res.ExpiresInSec) * time.Second),
+		Metadata: computeTokenMetadata,
+	}, nil
+
+}

auth/detect/compute_test.go

@@ -0,0 +1,51 @@
+// Copyright 2023 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
+//
+//      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 detect
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+const computeMetadataEnvVar = "GCE_METADATA_HOST"
+
+func TestComputeTokenProvider(t *testing.T) {
+	scope := "https://www.googleapis.com/auth/bigquery"
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if strings.HasSuffix(r.URL.String(), computeTokenURI) {
+			t.Errorf("got %q, want %q", r.URL.String(), computeTokenURI)
+		}
+		if r.URL.Query().Get("scopes") != scope {
+			t.Errorf("got %q, want %q", r.URL.Query().Get("scopes"), scope)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"access_token": "90d64460d14870c08c81352a05dedd3465940a7c", "token_type": "bearer", "expires_in": 86400}`))
+	}))
+	t.Setenv(computeMetadataEnvVar, strings.TrimPrefix(ts.URL, "http://"))
+	tp := computeTokenProvider(0, scope)
+	tok, err := tp.Token(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := "90d64460d14870c08c81352a05dedd3465940a7c"; tok.Value != want {
+		t.Errorf("got %q, want %q", tok.Value, want)
+	}
+	if want := "bearer"; tok.Type != want {
+		t.Errorf("got %q, want %q", tok.Value, want)
+	}
+}