Extend CONTRIBUTING.md with error handling information

* Add few informations how we should handle errors inside code.
* Use same function name `PostgresFunctionGuard` for both void and
  non-void PG function calls.

CONTRIBUTING.md

@@ -128,6 +128,10 @@ coding styles.
 ## Error Handling
 
 * Use exceptions **only** when an error is encountered that terminates a query (e.g. parser error, table not found). Exceptions should only be used for **exceptional** situations. For regular errors that do not break the execution flow (e.g. errors you **expect** might occur) use a return value instead.
+* *Exception* should be used in code that is executed inside duckdb engine.
+* Use PostgreSQL *elog* API to report messages back to user. Using *ERROR* is strictly forbiden to use as it can brake execution and lead to
+unexpected memory problems - *ERROR* is only used in duckdb node to early bail out execution in case of query preparation error or execution interruption.
+* Calling PostgreSQL native functions needs to be used with *PostgresFunctionGuard*. *PostgresFunctionGuard* will handle correctly *ERROR* log messages that could be emmited from these functions.
 * Try to add test cases that trigger exceptions. If an exception cannot be easily triggered using a test case then it should probably be an assertion. This is not always true (e.g. out of memory errors are exceptions, but are very hard to trigger).
 * Use `D_ASSERT` to assert. Use **assert** only when failing the assert means a programmer error. Assert should never be triggered by user input. Avoid code like `D_ASSERT(a > b + 3);` without comments or context.
 * Assert liberally, but make it clear with comments next to the assert what went wrong when the assert is triggered.

include/pgduckdb/pgduckdb_utils.hpp

@@ -46,8 +46,8 @@ PostgresFunctionGuard(FuncType postgres_function, FuncArgs... args) {
 }
 
 template <typename FuncType, typename... FuncArgs>
-bool
-PostgresVoidFunctionGuard(FuncType postgres_function, FuncArgs... args) {
+std::optional<bool>
+PostgresFunctionGuard(FuncType postgres_function, FuncArgs... args) {
 	bool error = false;
 	// clang-format off
 	PG_TRY();

src/pgduckdb_detoast.cpp

@@ -124,8 +124,8 @@ ToastFetchDatum(struct varlena *attr) {
 	}
 
 	bool error_fetch_toast = false;
-	if (PostgresVoidFunctionGuard(table_relation_fetch_toast_slice, toast_rel, toast_pointer.va_valueid, attrsize, 0,
-	                              attrsize, result)) {
+	if (PostgresFunctionGuard(table_relation_fetch_toast_slice, toast_rel, toast_pointer.va_valueid, attrsize, 0,
+	                          attrsize, result).value()) {
 		error_fetch_toast = true;
 	}
 

src/scan/heap_reader.cpp

@@ -100,7 +100,7 @@ HeapReader::ReadPageTuples(duckdb::DataChunk &output) {
 
 			m_buffer = opt_buffer.value();
 
-			if (PostgresVoidFunctionGuard(LockBuffer, m_buffer, BUFFER_LOCK_SHARE)) {
+			if (PostgresFunctionGuard(LockBuffer, m_buffer, BUFFER_LOCK_SHARE).value()) {
 				DuckdbProcessLock::GetLock().unlock();
 				throw duckdb::InternalException("(PGDuckdDB/ReadPageTuples) LockBuffer failed");
 			}