feat: support drop cascade (#841)

* feat: support drop cascade

* test: start adding tests

* fix: drop table should not remove foreign keys by default

* test: add integration test

* chore: address review comments

src/main/java/com/google/cloud/spanner/pgadapter/error/PGExceptionFactory.java

@@ -27,6 +27,10 @@
 public class PGExceptionFactory {
   private static final Pattern RELATION_NOT_FOUND_PATTERN =
       Pattern.compile("relation .+ does not exist");
+  private static final Pattern CANNOT_DROP_TABLE_WITH_INDICES_PATTERN =
+      Pattern.compile("Cannot drop table test with indices");
+  private static final Pattern ONLY_RESTRICT_BEHAVIOR =
+      Pattern.compile("Only <RESTRICT> behavior is supported by <DROP> statement\\.");
 
   private PGExceptionFactory() {}
 
@@ -86,6 +90,20 @@ public static PGException toPGException(SpannerException spannerException) {
       return PGException.newBuilder(extractMessage(spannerException))
           .setSQLState(SQLState.UndefinedTable)
           .build();
+    } else if (spannerException.getErrorCode() == ErrorCode.FAILED_PRECONDITION
+        && CANNOT_DROP_TABLE_WITH_INDICES_PATTERN.matcher(spannerException.getMessage()).find()) {
+      return PGException.newBuilder(extractMessage(spannerException))
+          .setSQLState(SQLState.FeatureNotSupported)
+          .setHints(
+              "Execute 'set spanner.support_drop_cascade=true' to enable dropping tables with indices")
+          .build();
+    } else if (spannerException.getErrorCode() == ErrorCode.INVALID_ARGUMENT
+        && ONLY_RESTRICT_BEHAVIOR.matcher(spannerException.getMessage()).find()) {
+      return PGException.newBuilder(extractMessage(spannerException))
+          .setSQLState(SQLState.FeatureNotSupported)
+          .setHints(
+              "Execute 'set spanner.support_drop_cascade=true' to enable 'drop {table|schema} cascade' statements.")
+          .build();
     }
     return newPGException(extractMessage(spannerException));
   }

src/main/java/com/google/cloud/spanner/pgadapter/session/SessionState.java

@@ -399,6 +399,14 @@ public boolean isForceAutocommit() {
     return getBoolSetting("spanner", "force_autocommit", false);
   }
 
+  /**
+   * Returns whether DROP ... CASCADE statements are supported. This also includes implicit
+   * cascading operations, such as dropping indexes of tables that are being dropped.
+   */
+  public boolean isSupportDropCascade() {
+    return getBoolSetting("spanner", "support_drop_cascade", false);
+  }
+
   /**
    * Returns whether statements with an OFFSET clause that uses a parameter should be automatically
    * appended with a LIMIT clause. The LIMIT clause will use the literal Long.MAX_VALUE for unbound

src/main/java/com/google/cloud/spanner/pgadapter/statements/BackendConnection.java

@@ -715,7 +715,7 @@ void execute() {
 
   private static final ImmutableMap<String, LocalStatement> EMPTY_LOCAL_STATEMENTS =
       ImmutableMap.of();
-  private static final StatementResult NO_RESULT = new NoResult();
+  static final StatementResult NO_RESULT = new NoResult();
   private static final StatementResult ROLLBACK_RESULT = new NoResult("ROLLBACK");
   private static final Statement ROLLBACK = Statement.of("ROLLBACK");