Relax tenant verification for credentials having optional tenant IDs (Azure#23951)

Author: chlowell

sdk/azidentity/CHANGELOG.md

@@ -9,6 +9,9 @@
 ### Bugs Fixed
 * User credential types inconsistently log access token scopes
 * `DefaultAzureCredential` skips managed identity in Azure Container Instances
+* Credentials having optional tenant IDs such as `AzureCLICredential` and
+  `InteractiveBrowserCredential` require setting `AdditionallyAllowedTenants`
+  when used with some clients
 
 ### Other Changes
 * `ChainedTokenCredential` and `DefaultAzureCredential` continue to their next

sdk/azidentity/azidentity.go

@@ -105,7 +105,16 @@ func resolveAdditionalTenants(tenants []string) []string {
 	return cp
 }
 
-// resolveTenant returns the correct tenant for a token request
+// resolveTenant returns the correct tenant for a token request, or "" when the calling credential doesn't
+// have an explicitly configured tenant and the caller didn't specify a tenant for the token request.
+//
+//   - defaultTenant: tenant set when constructing the credential, if any. "" is valid for credentials
+//     having an optional or implicit tenant such as dev tool and interactive user credentials. Those
+//     default to the tool's configured tenant or the user's home tenant, respectively.
+//   - specified: tenant specified for this token request i.e., TokenRequestOptions.TenantID. May be "".
+//   - credName: name of the calling credential type; for error messages
+//   - additionalTenants: optional allow list of tenants the credential may acquire tokens from in
+//     addition to defaultTenant i.e., the credential's AdditionallyAllowedTenants option
 func resolveTenant(defaultTenant, specified, credName string, additionalTenants []string) (string, error) {
 	if specified == "" || specified == defaultTenant {
 		return defaultTenant, nil
@@ -121,6 +130,17 @@ func resolveTenant(defaultTenant, specified, credName string, additionalTenants
 			return specified, nil
 		}
 	}
+	if len(additionalTenants) == 0 {
+		switch defaultTenant {
+		case "", organizationsTenantID:
+			// The application didn't specify a tenant or allow list when constructing the credential. Allow the
+			// tenant specified for this token request because we have nothing to compare it to (i.e., it vacuously
+			// satisfies the credential's configuration); don't know whether the application is multitenant; and
+			// don't want to return an error in the common case that the specified tenant matches the credential's
+			// default tenant determined elsewhere e.g., in some dev tool's configuration.
+			return specified, nil
+		}
+	}
 	return "", fmt.Errorf(`%s isn't configured to acquire tokens for tenant %q. To enable acquiring tokens for this tenant add it to the AdditionallyAllowedTenants on the credential options, or add "*" to allow acquiring tokens for any tenant`, credName, specified)
 }