reactor: To add apiException as a prtavie field under SpannerException.

@@ -54,6 +54,7 @@ public String getResourceName() {
 ProtoUtils.keyForProto(RetryInfo.getDefaultInstance());
 
 private final ErrorCode code;
+private final ApiException apiException;
 
 /** Private constructor. Use {@link SpannerExceptionFactory} to create instances. */
 SpannerException(
@@ -62,10 +63,23 @@ public String getResourceName() {
 boolean retryable,
 @Nullable String message,
 @Nullable Throwable cause) {
-super(message, cause, code.getCode(), retryable);
+super(
+message,
+// If the cause is instance of APIException then using its cause to avoid the breaking
+// change, because earlier we were passing APIException's cause to constructor.
+cause instanceof ApiException ? cause.getCause() : cause,
+code.getCode(),
+retryable);
+
 if (token != DoNotConstructDirectly.ALLOWED) {
 throw new AssertionError("Do not construct directly: use SpannerExceptionFactory");
 }
+
+if (cause instanceof ApiException) {
+this.apiException = (ApiException) cause;
+} else {
+this.apiException = null;
+}
 this.code = Preconditions.checkNotNull(code);
 }
 
@@ -108,8 +122,8 @@ static long extractRetryDelay(Throwable cause) {
 * @return the reason of an error.
 */
 public String getReason() {
-if (this.getCause() instanceof ApiException) {
-return ((ApiException) this.getCause()).getReason();
+if (this.apiException != null) {
+return this.apiException.getReason();
 }
 return null;
 }
@@ -123,8 +137,8 @@ public String getReason() {
 * @return the logical grouping to which the "reason" belongs.
 */
 public String getDomain() {
-if (this.getCause() instanceof ApiException) {
-return ((ApiException) this.getCause()).getDomain();
+if (this.apiException != null) {
+return this.apiException.getDomain();
 }
 return null;
 }
@@ -138,8 +152,8 @@ public String getDomain() {
 * @return the map of additional structured details about an error.
 */
 public Map<String, String> getMetadata() {
-if (this.getCause() instanceof ApiException) {
-return ((ApiException) this.getCause()).getMetadata();
+if (this.apiException != null) {
+return this.apiException.getMetadata();
 }
 return null;
 }
@@ -156,8 +170,8 @@ public Map<String, String> getMetadata() {
 * @return An object containing getters for structured objects from error_details.proto.
 */
 public ErrorDetails getErrorDetails() {
-if (this.getCause() instanceof ApiException) {
-return ((ApiException) this.getCause()).getErrorDetails();
+if (this.apiException != null) {
+return this.apiException.getErrorDetails();
 }
 return null;
 }