chore(spanner): add comments

Author: harshachinta

google-cloud-spanner/src/main/java/com/google/cloud/spanner/MultiplexedSessionDatabaseClient.java

@@ -53,6 +53,11 @@
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReference;
 
+/**
+ * {@link TransactionRunner} that automatically handles "UNIMPLEMENTED" errors with the message
+ * "Transaction type read_write not supported with multiplexed sessions" by switching from a
+ * multiplexed session to a regular session and then restarts the transaction.
+ */
 class MultiplexedSessionTransactionRunner implements TransactionRunner {
   private final SessionPool sessionPool;
   private final TransactionRunnerImpl transactionRunnerForMultiplexedSession;
@@ -91,8 +96,9 @@ public <T> T run(TransactionCallable<T> callable) {
       try {
         return getRunner().run(callable);
       } catch (SpannerException e) {
-        if (e.getErrorCode() == ErrorCode.UNIMPLEMENTED) {
-          this.isUsingMultiplexedSession = false;
+        if (e.getErrorCode() == ErrorCode.UNIMPLEMENTED
+            && verifyUnimplementedErrorMessageForRWMux(e)) {
+          this.isUsingMultiplexedSession = false; // Fallback to regular session
         } else {
           throw e; // Other errors propagate
         }
@@ -115,6 +121,19 @@ public TransactionRunner allowNestedTransaction() {
     getRunner().allowNestedTransaction();
     return this;
   }
+
+  private boolean verifyUnimplementedErrorMessageForRWMux(SpannerException spannerException) {
+    if (spannerException.getCause() == null) {
+      return false;
+    }
+    if (spannerException.getCause().getMessage() == null) {
+      return false;
+    }
+    return spannerException
+        .getCause()
+        .getMessage()
+        .contains("Transaction type read_write not supported with multiplexed sessions");
+  }
 }
 
 /**

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java

@@ -333,6 +333,9 @@ DatabaseClientImpl createDatabaseClient(
       boolean useMultiplexedSessionForRW,
       Attributes commonAttributes) {
     if (multiplexedSessionClient != null) {
+      // Set the session pool in the multiplexed session client.
+      // This is required to handle fallback to regular sessions for in-progress transactions that
+      // use multiplexed sessions but fail with UNIMPLEMENTED errors.
       multiplexedSessionClient.setPool(pool);
     }
     return new DatabaseClientImpl(