azureblob: fix server side copy error "requires exactly one scope"

Before this change, if not using shared key or SAS URL authentication
for the source, rclone gave this error

    ManagedIdentityCredential.GetToken() requires exactly one scope

when doing server side copies.

This was introduced in:

3a5ddfcd3c azureblob: implement multipart server side copy

This fixes the problem by creating a temporary SAS URL using user
delegation to read the source blob when copying.

Fixes #8662

backend/azureblob/azureblob.go

@@ -72,6 +72,7 @@ const (
 	emulatorAccount      = "devstoreaccount1"
 	emulatorAccountKey   = "Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw=="
 	emulatorBlobEndpoint = "http://127.0.0.1:10000/devstoreaccount1"
+	sasCopyValidity      = time.Hour // how long SAS should last when doing server side copy
 )
 
 var (
@@ -559,6 +560,11 @@ type Fs struct {
 	pacer         *fs.Pacer                    // To pace and retry the API calls
 	uploadToken   *pacer.TokenDispenser        // control concurrency
 	publicAccess  container.PublicAccessType   // Container Public Access Level
+
+	// user delegation cache
+	userDelegationMu     sync.Mutex
+	userDelegation       *service.UserDelegationCredential
+	userDelegationExpiry time.Time
 }
 
 // Object describes an azure object
@@ -1720,6 +1726,38 @@ func (f *Fs) Purge(ctx context.Context, dir string) error {
 	return f.deleteContainer(ctx, container)
 }
 
+// Get a user delegation which is valid for at least sasCopyValidity
+//
+// This value is cached in f
+func (f *Fs) getUserDelegation(ctx context.Context) (*service.UserDelegationCredential, error) {
+	f.userDelegationMu.Lock()
+	defer f.userDelegationMu.Unlock()
+
+	if f.userDelegation != nil && time.Until(f.userDelegationExpiry) > sasCopyValidity {
+		return f.userDelegation, nil
+	}
+
+	// Validity window
+	start := time.Now().UTC()
+	expiry := start.Add(2 * sasCopyValidity)
+	startStr := start.Format(time.RFC3339)
+	expiryStr := expiry.Format(time.RFC3339)
+
+	// Acquire user delegation key from the service client
+	info := service.KeyInfo{
+		Start:  &startStr,
+		Expiry: &expiryStr,
+	}
+	userDelegationKey, err := f.svc.GetUserDelegationCredential(ctx, info, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get user delegation key: %w", err)
+	}
+
+	f.userDelegation = userDelegationKey
+	f.userDelegationExpiry = expiry
+	return f.userDelegation, nil
+}
+
 // getAuth gets auth to copy o.
 //
 // tokenOK is used to signal that token based auth (Microsoft Entra
@@ -1731,7 +1769,7 @@ func (f *Fs) Purge(ctx context.Context, dir string) error {
 // URL (not a SAS) and token will be empty.
 //
 // If tokenOK is true it may also return a token for the auth.
-func (o *Object) getAuth(ctx context.Context, tokenOK bool, noAuth bool) (srcURL string, token *string, err error) {
+func (o *Object) getAuth(ctx context.Context, noAuth bool) (srcURL string, err error) {
 	f := o.fs
 	srcBlobSVC := o.getBlobSVC()
 	srcURL = srcBlobSVC.URL()
@@ -1740,29 +1778,47 @@ func (o *Object) getAuth(ctx context.Context, tokenOK bool, noAuth bool) (srcURL
 	case noAuth:
 		// If same storage account then no auth needed
 	case f.cred != nil:
-		if !tokenOK {
+		// Generate a User Delegation SAS URL using Azure AD credentials
-			return srcURL, token, errors.New("not supported: Microsoft Entra ID")
+		userDelegationKey, err := f.getUserDelegation(ctx)
-		}
-		options := policy.TokenRequestOptions{}
-		accessToken, err := f.cred.GetToken(ctx, options)
 		if err != nil {
-			return srcURL, token, fmt.Errorf("failed to create access token: %w", err)
+			return "", fmt.Errorf("sas creation: %w", err)
 		}
-		token = &accessToken.Token
+
+		// Build the SAS values
+		perms := sas.BlobPermissions{Read: true}
+		container, containerPath := o.split()
+		start := time.Now().UTC()
+		expiry := start.Add(sasCopyValidity)
+		vals := sas.BlobSignatureValues{
+			StartTime:     start,
+			ExpiryTime:    expiry,
+			Permissions:   perms.String(),
+			ContainerName: container,
+			BlobName:      containerPath,
+		}
+
+		// Sign with the delegation key
+		queryParameters, err := vals.SignWithUserDelegation(userDelegationKey)
+		if err != nil {
+			return "", fmt.Errorf("signing SAS with user delegation failed: %w", err)
+		}
+
+		// Append the SAS to the URL
+		srcURL = srcBlobSVC.URL() + "?" + queryParameters.Encode()
 	case f.sharedKeyCred != nil:
 		// Generate a short lived SAS URL if using shared key credentials
-		expiry := time.Now().Add(time.Hour)
+		expiry := time.Now().Add(sasCopyValidity)
 		sasOptions := blob.GetSASURLOptions{}
 		srcURL, err = srcBlobSVC.GetSASURL(sas.BlobPermissions{Read: true}, expiry, &sasOptions)
 		if err != nil {
-			return srcURL, token, fmt.Errorf("failed to create SAS URL: %w", err)
+			return srcURL, fmt.Errorf("failed to create SAS URL: %w", err)
 		}
 	case f.anonymous || f.opt.SASURL != "":
 		// If using a SASURL or anonymous, no need for any extra auth
 	default:
-		return srcURL, token, errors.New("unknown authentication type")
+		return srcURL, errors.New("unknown authentication type")
 	}
-	return srcURL, token, nil
+	return srcURL, nil
 }
 
 // Do multipart parallel copy.
@@ -1783,7 +1839,7 @@ func (f *Fs) copyMultipart(ctx context.Context, remote, dstContainer, dstPath st
 	o.fs = f
 	o.remote = remote
 
-	srcURL, token, err := src.getAuth(ctx, true, false)
+	srcURL, err := src.getAuth(ctx, false)
 	if err != nil {
 		return nil, fmt.Errorf("multipart copy: %w", err)
 	}
@@ -1827,7 +1883,8 @@ func (f *Fs) copyMultipart(ctx context.Context, remote, dstContainer, dstPath st
 					Count:  partSize,
 				},
 				// Specifies the authorization scheme and signature for the copy source.
-				CopySourceAuthorization: token,
+				// We use SAS URLs as this doesn't seem to work always
+				// CopySourceAuthorization: token,
 				// CPKInfo *blob.CPKInfo
 				// CPKScopeInfo *blob.CPKScopeInfo
 			}
@@ -1897,7 +1954,7 @@ func (f *Fs) copySinglepart(ctx context.Context, remote, dstContainer, dstPath s
 	dstBlobSVC := f.getBlobSVC(dstContainer, dstPath)
 
 	// Get the source auth - none needed for same storage account
-	srcURL, _, err := src.getAuth(ctx, false, f == src.fs)
+	srcURL, err := src.getAuth(ctx, f == src.fs)
 	if err != nil {
 		return nil, fmt.Errorf("single part copy: source auth: %w", err)
 	}