Catch PG errors when handling 'catch' section (#690)

Fixes #469

Author: Y--

Diff for: include/pgduckdb/pgduckdb_utils.hpp

@@ -35,11 +35,6 @@ struct PgExceptionGuard {
 	}
 
 	~PgExceptionGuard() noexcept {
-		RestoreStacks();
-	}
-
-	void
-	RestoreStacks() const noexcept {
 		PG_exception_stack = _save_exception_stack;
 		error_context_stack = _save_context_stack;
 	}
@@ -83,20 +78,33 @@ typename std::invoke_result<Func, FuncArgs...>::type
 __PostgresFunctionGuard__(const char *func_name, FuncArgs... args) {
 	std::lock_guard<std::recursive_mutex> lock(pgduckdb::GlobalProcessLock::GetLock());
 	MemoryContext ctx = CurrentMemoryContext;
-	ErrorData *edata = nullptr;
-	{ // Scope for PG_END_TRY
+
+	{ // PG_TRY
 		PgExceptionGuard g;
 		sigjmp_buf _local_sigjmp_buf;
 		if (sigsetjmp(_local_sigjmp_buf, 0) == 0) {
 			PG_exception_stack = &_local_sigjmp_buf;
 			return func(std::forward<FuncArgs>(args)...);
-		} else {
-			g.RestoreStacks();
-			CurrentMemoryContext = ctx;
+		}
+	}
+
+	CurrentMemoryContext = ctx;
+
+	ErrorData *edata = nullptr;
+	{ // PG_CATCH
+		// Extract the error message (edata) within a PG_TRY block.
+		PgExceptionGuard g;
+		sigjmp_buf _local_sigjmp_buf;
+		if (sigsetjmp(_local_sigjmp_buf, 0) == 0) {
+			PG_exception_stack = &_local_sigjmp_buf;
+
 			edata = CopyErrorData();
 			FlushErrorState();
+		} else {
+			// This is a pretty bad situation - we failed to extract the error message.
+			throw duckdb::Exception(duckdb::ExceptionType::EXECUTOR, "Failed to extract Postgres error message");
 		}
-	} // PG_END_TRY();
+	} // PG_END_TRY
 
 	auto message = duckdb::StringUtil::Format("(PGDuckDB/%s) %s", func_name, pg::GetErrorDataMessage(edata));
 	throw duckdb::Exception(duckdb::ExceptionType::EXECUTOR, message);