[PATCH v3 7/7] Add merger for tuple streams (Lua part)

[Vladimir Davydov]

On Wed, Apr 10, 2019 at 06:21:25PM +0300, Alexander Turenko wrote:
> A merger is a special kind of a source, which is created from a key_def
> object and a set of sources. It performs a kind of the merge sort:
> chooses a source with a minimal / maximal tuple on each step, consumes
> a tuple from this source and repeats. The API to create a merger is the
> following:
> 
> ```lua
> local ctx = merger.context.new(key_def.new(<...>))

Discussed verbally, agreed that a performance gain from using
merger.context is somewhat dubious. It's compelling to drop it
altogether and pass key_def to merger.new directly, creating
a format every time (we can reuse formats if necessary in future).
Alexander will think about it.

Other than that the API and the patch looks good to me.

See a few minor comments below.

> local sources = {<...>}
> local merger_inst = merger.new(ctx, sources, {
>     -- Ascending (false) or descending (true) order.
>     -- Default is ascending.
>     reverse = <boolean> or <nil>,
> })
> ```

> diff --git a/src/box/lua/merger.c b/src/box/lua/merger.c
> new file mode 100644
> index 000000000..ebe60bc8d
> --- /dev/null
> +++ b/src/box/lua/merger.c
> @@ -0,0 +1,1184 @@

> +static uint32_t merger_source_type_id = 0;
> +static uint32_t merger_context_type_id = 0;
> +static uint32_t ibuf_type_id = 0;

We typically use upper case for naming variables storing Lua type ids,
e.g. CTID_STRUCT_TUPLE_REF.

> +static int
> +lbox_merger_source_gc(struct lua_State *L)
> +{
> +	struct merger_source *source;
> +	if ((source = luaT_check_merger_source(L, 1)) == NULL)
> +		return 0;

Is it actually possible?

> +	merger_source_unref(source);
> +	return 0;
> +}

> +static struct merger_source **
> +luaT_merger_new_parse_sources(struct lua_State *L, int idx,
> +			      uint32_t *sources_count_ptr)
> +{
> +	/* Allocate sources array. */
> +	uint32_t sources_count = lua_objlen(L, idx);
> +	const ssize_t sources_size =
> +		sources_count * sizeof(struct merger_source *);
> +	struct merger_source **sources =
> +		(struct merger_source **) malloc(sources_size);

Pointless type conversion.

> +	if (sources == NULL) {
> +		diag_set(OutOfMemory, sources_size, "malloc", "sources");
> +		return NULL;
> +	}
> +
> +	/* Save all sources. */
> +	for (uint32_t i = 0; i < sources_count; ++i) {
> +		lua_pushinteger(L, i + 1);
> +		lua_gettable(L, idx);
> +
> +		/* Extract a source from a Lua stack. */
> +		struct merger_source *source = luaT_check_merger_source(L, -1);
> +		if (source == NULL) {
> +			free(sources);
> +			diag_set(IllegalParams,
> +				 "Unknown source type at index %d", i + 1);
> +			return NULL;
> +		}
> +		sources[i] = source;
> +	}
> +	lua_pop(L, sources_count);
> +
> +	*sources_count_ptr = sources_count;

source_count

> +struct merger_source_buffer {
> +	struct merger_source base;
> +	/*
> +	 * A reference to a Lua iterator to fetch a next chunk of
> +	 * tuples.
> +	 */
> +	struct luaL_iterator *fetch_it;
> +	/*
> +	 * A reference a buffer with a current chunk of tuples.

A reference to a buffer storing the current chunk of tuples.

> +	 * It is needed to prevent LuaJIT from collecting the
> +	 * buffer while the source consider it as the current
> +	 * one.
> +	 */
> +	int ref;
> +	/*
> +	 * A buffer with a current chunk of tuples.
> +	 */
> +	struct ibuf *buf;
> +	/*
> +	 * A merger stops before end of a buffer when it is not
> +	 * the last merger in the chain.
> +	 */
> +	size_t remaining_tuples_cnt;

Please rename to remaining_tuple_count.

> +static int
> +luaL_merger_source_buffer_fetch(struct merger_source_buffer *source)
> +{
> +	struct lua_State *L = fiber()->storage.lua.stack;
> +	int nresult = luaL_iterator_next(L, source->fetch_it);
> +
> +	/* Handle a Lua error in a gen function. */
> +	if (nresult == -1)
> +		return -1;
> +
> +	/* No more data: do nothing. */
> +	if (nresult == 0)
> +		return 0;
> +
> +	/* Handle incorrect results count. */
> +	if (nresult != 2) {
> +		diag_set(IllegalParams, "Expected <state>, <buffer>, got %d "
> +			 "return values", nresult);
> +		return -1;
> +	}
> +
> +	/* Set a new buffer as the current chunk. */
> +	if (source->ref > 0)
> +		luaL_unref(L, LUA_REGISTRYINDEX, source->ref);
> +	lua_pushvalue(L, -nresult + 1); /* Popped by luaL_ref(). */
> +	source->ref = luaL_ref(L, LUA_REGISTRYINDEX);
> +	source->buf = luaT_check_ibuf(L, -1);
> +	assert(source->buf != NULL);

 | tarantool> m = merger.new(c, {merger.new_buffer_source(box.space._space:pairs())})
 | ---
 | ...
 | 
 | tarantool> m:select()
 | luaL_merger_source_buffer_fetch: Assertion `source->buf != NULL' failed.

Please fail gracefully in this case.

> +	lua_pop(L, nresult);
> +
> +	/* Update remaining_tuples_cnt and skip the header. */
> +	if (decode_header(source->buf, &source->remaining_tuples_cnt) != 0) {
> +		diag_set(IllegalParams, "Invalid merger source %p",
> +			 &source->base);
> +		return -1;
> +	}
> +	return 1;
> +}
> +
> +/* Virtual methods */
> +
> +static void
> +luaL_merger_source_buffer_delete(struct merger_source *base)
> +{
> +	struct merger_source_buffer *source = container_of(base,
> +		struct merger_source_buffer, base);
> +
> +	assert(source->fetch_it != NULL);
> +	luaL_iterator_delete(source->fetch_it);
> +	if (source->ref > 0)
> +		luaL_unref(tarantool_L, LUA_REGISTRYINDEX, source->ref);
> +
> +	free(source);
> +}
> +
> +static int
> +luaL_merger_source_buffer_next(struct merger_source *base,
> +			       box_tuple_format_t *format,
> +			       struct tuple **out)
> +{
> +	struct merger_source_buffer *source = container_of(base,
> +		struct merger_source_buffer, base);
> +
> +	/*
> +	 * Handle the case when all data were processed: ask a
> +	 * next chunk until a non-empty chunk is received or a
> +	 * chunks iterator ends.
> +	 */
> +	while (source->remaining_tuples_cnt == 0) {
> +		int rc = luaL_merger_source_buffer_fetch(source);
> +		if (rc < 0)
> +			return -1;
> +		if (rc == 0) {
> +			*out = NULL;
> +			return 0;
> +		}
> +	}
> +	if (ibuf_used(source->buf) == 0) {
> +		diag_set(IllegalParams, "Unexpected msgpack buffer end");
> +		return -1;
> +	}
> +	const char *tuple_beg = source->buf->rpos;
> +	const char *tuple_end = tuple_beg;
> +	/*
> +	 * mp_next() is faster then mp_check(), but can	read bytes
> +	 * outside of the buffer and so can cause segmentation
> +	 * faults or an incorrect result.
> +	 *
> +	 * We check buffer boundaries after the mp_next() call and
> +	 * throw an error when the boundaries are violated, but it
> +	 * does not save us from possible segmentation faults.
> +	 *
> +	 * It is in a user responsibility to provide valid
> +	 * msgpack.

Ugh, I'd check the buffer with mp_check anyway. Would probably provide
an option to skip the check if required ('unchecked'). Not sure if it's
really necessary though.

> +	 */
> +	mp_next(&tuple_end);
> +	--source->remaining_tuples_cnt;
> +	if (tuple_end > source->buf->wpos) {
> +		diag_set(IllegalParams, "Unexpected msgpack buffer end");
> +		return -1;
> +	}
> +	source->buf->rpos = (char *) tuple_end;
> +	if (format == NULL)
> +		format = box_tuple_format_default();

I'd pass tuple_format_runtime explicitly to the ->next callback.
Special-casing NULL is kinda ugly.

> +	struct tuple *tuple = box_tuple_new(format, tuple_beg, tuple_end);
> +	if (tuple == NULL)
> +		return -1;
> +
> +	box_tuple_ref(tuple);
> +	*out = tuple;
> +	return 0;
> +}

> +static int
> +encode_result_buffer(struct lua_State *L, struct merger_source *source,
> +		     struct ibuf *obuf, uint32_t limit)

Better rename 'obuf' to 'out' or 'buf' or something - we have struct
obuf so 'struct ibuf *obuf' looks misleading.

> +{
> +	uint32_t result_len = 0;
> +	uint32_t result_len_offset = 4;
> +
> +	/*
> +	 * Reserve maximum size for the array around resulting
> +	 * tuples to set it later.
> +	 */
> +	encode_header(obuf, UINT32_MAX);
> +
> +	/* Fetch, merge and copy tuples to the buffer. */
> +	struct tuple *tuple;
> +	int rc = 0;
> +	while (result_len < limit && (rc =
> +	       source->vtab->next(source, NULL, &tuple)) == 0 &&
> +	       tuple != NULL) {
> +		uint32_t bsize = tuple->bsize;
> +		ibuf_reserve(obuf, bsize);
> +		memcpy(obuf->wpos, tuple_data(tuple), bsize);
> +		obuf->wpos += bsize;
> +		result_len_offset += bsize;
> +		++result_len;
> +
> +		/* The received tuple is not more needed. */
> +		box_tuple_unref(tuple);
> +	}
> +
> +	if (rc != 0)
> +		return luaT_error(L);
> +
> +	/* Write the real array size. */
> +	mp_store_u32(obuf->wpos - result_len_offset, result_len);
> +
> +	return 0;
> +}

> +static int
> +lbox_merger_source_select(struct lua_State *L)
> +{
> +	struct merger_source *source;
> +	int top = lua_gettop(L);
> +	bool ok = (top == 1 || top == 2) &&
> +		/* Merger source. */
> +		(source = luaT_check_merger_source(L, 1)) != NULL &&
> +		/* Opts. */
> +		(lua_isnoneornil(L, 2) == 1 || lua_istable(L, 2) == 1);
> +	if (!ok)
> +		return lbox_merger_source_select_usage(L, NULL);
> +
> +	uint32_t limit = 0xFFFFFFFF;

Hmm, UINT32_MAX?