protobuf_unpack_palloc.patch

Unpack deparse input with palloc instead of malloc, and skip the free pass.

protobuf-c's default allocator does one malloc per message and, worse,
protobuf_c_message_free_unpacked walks every field of every message's
descriptor to find pointers to free. pg_query's Node descriptor has 271
fields and every value in a parse tree is wrapped in a Node, so profiling
puts ~80% of pg_query_deparse_protobuf inside protobuf-c: 45% of samples
in free_unpacked alone, 34% in unpack.

pg_query_protobuf_to_nodes has exactly one caller, pg_query_deparse_protobuf,
which always runs it inside a pg_query memory context -- the same context
_readRawStmt already pallocs the Node tree into. palloc is an arena here:
allocation is a bump, and MemoryContextDelete frees everything at once. So
the unpacked protobuf structs can live in the context too, the free pass
becomes unnecessary, and the 271-field descriptor walk disappears with it.

Byte-level behaviour is unchanged: re-packing an arena-unpacked message
reproduces the input exactly, and the deparse test suite pins output SQL.

diff --git a/src/pg_query_readfuncs_protobuf.c b/src/pg_query_readfuncs_protobuf.c
index d0a7e21..2993096 100644
--- a/src/pg_query_readfuncs_protobuf.c
+++ b/src/pg_query_readfuncs_protobuf.c
@@ -152,13 +152,27 @@ static Node * _readNode(PgQuery__Node *msg)
 	}
 }
 
+static void *unpack_palloc(void *allocator_data, size_t size)
+{
+	return palloc(size);
+}
+
+static void unpack_free_noop(void *allocator_data, void *pointer)
+{
+	/* freed wholesale when the caller's memory context is deleted */
+}
+
+static ProtobufCAllocator unpack_allocator = {
+	unpack_palloc, unpack_free_noop, NULL
+};
+
 List * pg_query_protobuf_to_nodes(PgQueryProtobuf protobuf)
 {
 	PgQuery__ParseResult *result = NULL;
 	List * list = NULL;
 	size_t i = 0;
 
-	result = pg_query__parse_result__unpack(NULL, protobuf.len, (const uint8_t *) protobuf.data);
+	result = pg_query__parse_result__unpack(&unpack_allocator, protobuf.len, (const uint8_t *) protobuf.data);
 
 	// TODO: Handle this by returning an error instead
 	Assert(result != NULL);
@@ -171,7 +185,5 @@ List * pg_query_protobuf_to_nodes(PgQueryProtobuf protobuf)
     for (i = 1; i < result->n_stmts; i++)
 		list = lappend(list, _readRawStmt(result->stmts[i]));
 
-	pg_query__parse_result__free_unpacked(result, NULL);
-
 	return list;
 }