Restore ARM CAE support for azcore beta (Azure#20657)

This reverts commit 9020972.

Author: chlowell

sdk/azcore/arm/runtime/policy_bearer_token.go

@@ -5,6 +5,7 @@ package runtime
 
 import (
 	"context"
+	"encoding/base64"
 	"fmt"
 	"net/http"
 	"strings"
@@ -63,11 +64,28 @@ func NewBearerTokenPolicy(cred azcore.TokenCredential, opts *armpolicy.BearerTok
 	p.scopes = make([]string, len(opts.Scopes))
 	copy(p.scopes, opts.Scopes)
 	p.btp = azruntime.NewBearerTokenPolicy(cred, opts.Scopes, &azpolicy.BearerTokenOptions{
-		AuthorizationHandler: azpolicy.AuthorizationHandler{OnRequest: p.onRequest},
+		AuthorizationHandler: azpolicy.AuthorizationHandler{
+			OnChallenge: p.onChallenge,
+			OnRequest:   p.onRequest,
+		},
 	})
 	return p
 }
 
+func (b *BearerTokenPolicy) onChallenge(req *azpolicy.Request, res *http.Response, authNZ func(azpolicy.TokenRequestOptions) error) error {
+	challenge := res.Header.Get(shared.HeaderWWWAuthenticate)
+	claims, err := parseChallenge(challenge)
+	if err != nil {
+		// the challenge contains claims we can't parse
+		return err
+	} else if claims != "" {
+		// request a new token having the specified claims, send the request again
+		return authNZ(azpolicy.TokenRequestOptions{Claims: claims, Scopes: b.scopes})
+	}
+	// auth challenge didn't include claims, so this is a simple authorization failure
+	return azruntime.NewResponseError(res)
+}
+
 // onRequest authorizes requests with one or more bearer tokens
 func (b *BearerTokenPolicy) onRequest(req *azpolicy.Request, authNZ func(azpolicy.TokenRequestOptions) error) error {
 	// authorize the request with a token for the primary tenant
@@ -97,3 +115,31 @@ func (b *BearerTokenPolicy) onRequest(req *azpolicy.Request, authNZ func(azpolic
 func (b *BearerTokenPolicy) Do(req *azpolicy.Request) (*http.Response, error) {
 	return b.btp.Do(req)
 }
+
+// parseChallenge parses claims from an authentication challenge issued by ARM so a client can request a token
+// that will satisfy conditional access policies. It returns a non-nil error when the given value contains
+// claims it can't parse. If the value contains no claims, it returns an empty string and a nil error.
+func parseChallenge(wwwAuthenticate string) (string, error) {
+	claims := ""
+	var err error
+	for _, param := range strings.Split(wwwAuthenticate, ",") {
+		if _, after, found := strings.Cut(param, "claims="); found {
+			if claims != "" {
+				// The header contains multiple challenges, at least two of which specify claims. The specs allow this
+				// but it's unclear what a client should do in this case and there's as yet no concrete example of it.
+				err = fmt.Errorf("found multiple claims challenges in %q", wwwAuthenticate)
+				break
+			}
+			// trim stuff that would get an error from RawURLEncoding; claims may or may not be padded
+			claims = strings.Trim(after, `\"=`)
+			// we don't return this error because it's something unhelpful like "illegal base64 data at input byte 42"
+			if b, decErr := base64.RawURLEncoding.DecodeString(claims); decErr == nil {
+				claims = string(b)
+			} else {
+				err = fmt.Errorf("failed to parse claims from %q", wwwAuthenticate)
+				break
+			}
+		}
+	}
+	return claims, err
+}

sdk/azcore/arm/runtime/policy_bearer_token_test.go

@@ -203,7 +203,6 @@ func TestAuxiliaryTenants(t *testing.T) {
 }
 
 func TestBearerTokenPolicyChallengeParsing(t *testing.T) {
-	t.Skip("unskip this test after adding back CAE support")
 	for _, test := range []struct {
 		challenge, desc, expectedClaims string
 		err                             error
@@ -262,10 +261,9 @@ func TestBearerTokenPolicyChallengeParsing(t *testing.T) {
 			cred := mockCredential{
 				getTokenImpl: func(ctx context.Context, actual azpolicy.TokenRequestOptions) (azcore.AccessToken, error) {
 					calls += 1
-					// TODO: uncomment after restoring TokenRequestOptions.Claims
-					// if calls == 2 && test.expectedClaims != "" {
-					// require.Equal(t, test.expectedClaims, actual.Claims)
-					// }
+					if calls == 2 && test.expectedClaims != "" {
+						require.Equal(t, test.expectedClaims, actual.Claims)
+					}
 					return azcore.AccessToken{Token: "...", ExpiresOn: time.Now().Add(time.Hour).UTC()}, nil
 				},
 			}

sdk/azcore/internal/exported/exported.go

@@ -51,6 +51,10 @@ type AccessToken struct {
 // TokenRequestOptions contain specific parameter that may be used by credentials types when attempting to get a token.
 // Exported as policy.TokenRequestOptions.
 type TokenRequestOptions struct {
+	// Claims are any additional claims required for the token to satisfy a conditional access policy, such as a
+	// service may return in a claims challenge following an authorization failure. If a service returned the
+	// claims value base64 encoded, it must be decoded before setting this field.
+	Claims string
 	// Scopes contains the list of permission scopes required for the token.
 	Scopes []string