feat: support PG show transaction isolation level (#1777)

* feat: support PG show transaction isolation level

Adds support for the PostgreSQL statement SHOW TRANSACTION ISOLATION
LEVEL. This statement is used by the PG JDBC driver to get the isolation
level of a connection. The statement will always return the static
string 'serializable', as that is the only isolation level Spangres
supports.

* build: force rebuild

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionStatementExecutor.java

@@ -109,4 +109,6 @@ StatementResult statementSetPgSessionCharacteristicsTransactionMode(
   StatementResult statementSetRPCPriority(Priority priority);
 
   StatementResult statementShowRPCPriority();
+
+  StatementResult statementShowTransactionIsolationLevel();
 }

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionStatementExecutorImpl.java

@@ -49,6 +49,7 @@
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_RPC_PRIORITY;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_STATEMENT_TAG;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_STATEMENT_TIMEOUT;
+import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_TRANSACTION_ISOLATION_LEVEL;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_TRANSACTION_TAG;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.START_BATCH_DDL;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.START_BATCH_DML;
@@ -436,4 +437,9 @@ public StatementResult statementShowRPCPriority() {
             : getConnection().getRPCPriority(),
         SHOW_RPC_PRIORITY);
   }
+
+  @Override
+  public StatementResult statementShowTransactionIsolationLevel() {
+    return resultSet("transaction_isolation", "serializable", SHOW_TRANSACTION_ISOLATION_LEVEL);
+  }
 }

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/StatementResult.java

@@ -82,7 +82,8 @@ enum ClientSideStatementType {
     RUN_BATCH,
     ABORT_BATCH,
     SET_RPC_PRIORITY,
-    SHOW_RPC_PRIORITY
+    SHOW_RPC_PRIORITY,
+    SHOW_TRANSACTION_ISOLATION_LEVEL
   }
 
   /**

google-cloud-spanner/src/main/resources/com/google/cloud/spanner/connection/PG_ClientSideStatements.json

@@ -125,6 +125,14 @@
       "method": "statementShowRPCPriority",
       "exampleStatements": ["show spanner.rpc_priority","show variable spanner.rpc_priority"]
     },
+    {
+      "name": "SHOW [VARIABLE] TRANSACTION ISOLATION LEVEL",
+      "executorName": "ClientSideStatementNoParamExecutor",
+      "resultType": "RESULT_SET",
+      "regex": "(?is)\\A\\s*show\\s+(?:variable\\s+)?transaction\\s*isolation\\s*level\\s*\\z",
+      "method": "statementShowTransactionIsolationLevel",
+      "exampleStatements": ["show transaction isolation level","show variable transaction isolation level"]
+    },
     {
       "name": "{START | BEGIN} [TRANSACTION | WORK] [{ (READ ONLY|READ WRITE) | (ISOLATION LEVEL (DEFAULT|SERIALIZABLE)) }]",
       "executorName": "ClientSideStatementPgBeginExecutor",