Return correct default from PgDatabaseMetaData.getDefaultTransactionI…

…solation (#2992)

Fixes: #2991

This commit updates `PgDatabaseMetaData.getDefaultTransactionIsolation`
to query the database for the default transaction isolation level,
instead of assuming that the default isolation level is READ COMMITTED.

pgjdbc/src/main/java/org/postgresql/jdbc/PgDatabaseMetaData.java

@@ -38,6 +38,7 @@
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.StringTokenizer;
 
@@ -53,6 +54,7 @@ public PgDatabaseMetaData(PgConnection conn) {
 
   private int nameDataLength = 0; // length for name datatype
   private int indexMaxKeys = 0; // maximum number of keys in an index.
+  private int defaultTransactionIsolation = 0;
 
   protected int getMaxIndexKeys() throws SQLException {
     if (indexMaxKeys == 0) {
@@ -957,7 +959,40 @@ public int getMaxUserNameLength() throws SQLException {
   }
 
   public int getDefaultTransactionIsolation() throws SQLException {
-    return Connection.TRANSACTION_READ_COMMITTED;
+    if (defaultTransactionIsolation == 0) {
+      String sql;
+      sql = "SELECT setting FROM pg_catalog.pg_settings WHERE name='default_transaction_isolation'";
+
+      try (Statement stmt = connection.createStatement()) {
+        try (ResultSet rs = stmt.executeQuery(sql)) {
+          String level = null;
+          if (rs.next()) {
+            level = rs.getString(1);
+          }
+          if (level == null) {
+            throw new PSQLException(
+                GT.tr(
+                    "Unable to determine a value for DefaultTransactionIsolation due to missing "
+                    + "system catalog data."),
+                PSQLState.UNEXPECTED_ERROR);
+          }
+
+          level = level.toUpperCase(Locale.US);
+          if (level.equals("READ COMMITTED")) {
+            defaultTransactionIsolation = Connection.TRANSACTION_READ_COMMITTED;
+          } else if (level.equals("READ UNCOMMITTED")) {
+            defaultTransactionIsolation = Connection.TRANSACTION_READ_UNCOMMITTED;
+          } else if (level.equals("REPEATABLE READ")) {
+            defaultTransactionIsolation = Connection.TRANSACTION_REPEATABLE_READ;
+          } else if (level.equals("SERIALIZABLE")) {
+            defaultTransactionIsolation = Connection.TRANSACTION_SERIALIZABLE;
+          } else {
+            defaultTransactionIsolation = Connection.TRANSACTION_READ_COMMITTED; // Best guess.
+          }
+        }
+      }
+    }
+    return defaultTransactionIsolation;
   }
 
   public boolean supportsTransactions() throws SQLException {

pgjdbc/src/test/java/org/postgresql/test/jdbc2/DatabaseMetaDataPropertiesTest.java

@@ -147,6 +147,41 @@ public void testIdentifiers() throws SQLException {
 
   }
 
+  @Test
+  public void testDefaultTransactionIsolation() throws SQLException {
+    DatabaseMetaData dbmd = con.getMetaData();
+    assertNotNull(dbmd);
+
+    int transactionIsolation = dbmd.getDefaultTransactionIsolation();
+    assertEquals(Connection.TRANSACTION_READ_COMMITTED, transactionIsolation);
+
+    String [] isolationLevels = {"\"read committed\"","\"read uncommitted\"", "\"repeatable read\"","serializable"};
+    try {
+      for (int i = 0; i < isolationLevels.length; i++) {
+        con.createStatement().execute("alter database test set default_transaction_isolation to " + isolationLevels[i]);
+        try (Connection con1 = TestUtil.openDB()) {
+          transactionIsolation = con1.getMetaData().getDefaultTransactionIsolation();
+          switch (i) {
+            case 0:
+              assertEquals(Connection.TRANSACTION_READ_COMMITTED, transactionIsolation);
+              break;
+            case 1:
+              assertEquals(Connection.TRANSACTION_READ_UNCOMMITTED, transactionIsolation);
+              break;
+            case 2:
+              assertEquals(Connection.TRANSACTION_REPEATABLE_READ, transactionIsolation);
+              break;
+            case 3:
+              assertEquals(Connection.TRANSACTION_SERIALIZABLE, transactionIsolation);
+              break;
+          }
+        }
+      }
+    } finally {
+      con.createStatement().execute("alter database test set default_transaction_isolation to DEFAULT");
+    }
+  }
+
   @Test
   public void testTables() throws SQLException {
     DatabaseMetaData dbmd = con.getMetaData();