Re: [PATCH v2 12/17] chunk-format: create read chunk API

"Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:

Writing it here won't help future developers very much. I think that belongs to API/function doc for init_chunkfile().

It's a bit of mystery, having seen how the table-of-contents is laid out by reading the writing side of the code, how toc_offset and toc_length are discovered by the caller. IIRC, the size to cover everything from the beginning of the file to the end of the table-of-contents was recorded as the length of a non-existent chunk with id 0, but we need to be able to somehow find it to use that as a way to get to the (end of) table-of-contents from the beginning of the file. I guess we'll learn enough when we get to read the code that calls this function.

The chunks have to be recorded in toc in the order they appear, and there must be enough room to store the hashfile trailer checksum after the last chunk. OK.

Shouldn't we be validating the size component associated with this "id=0" fake chunk that appears at the end as well? IIRC, the size field should be pointing at the byte immediately after the TOC entry for the last true chunk, immediately before this zero chunk id?

OK, the assumption here is that there will be at most one chunk that has the chunk_id we seek to find (or putting it differently, second and subsequent chunks with the same ID are ignored). We may want to write it down somewhere.

It is curious why pair_chunk() exists in the first place. With something like this:

        int pair_chunk_fn(const unsigned char *chunk_start,
                          size_t chunk_size,
                          void *data)
        {
                const unsigned char **p = data;
                *p = chunk_start;
                return 0;
        }

instead of

	const unsigned char *location;

	pair_chunk(cf, chunk_id, &location);

we can write

	const unsigned char *location;

	read_chunk(cf, chunk_id, pair_chunk_fn, &location);

no? That would allow us to reorganize the in-core TOC more easily if it turns out to be necessary in the future.

Is the assumption throughout the chunked file API that reading is actually not done as a "seek+read+process", but as a normal memory access over mmapped region? The reason I ask is because the answer affects the choice of the right type for the offset. The function signature of read_table_of_contents() uses size_t to represent the length of the entire mmapped region that holds the data from the file, and that is better than off_t, especially if size_t were smaller than off_t (i.e. we may not be able to mmap a huge size that filesystem can handle and let us access with seek+read).

But the assumption that the whole mfile can be mmapped in as a whole is only in read_table_of_contents(), and users of read_chunk() API can be oblivious, I think---IOW, we could "page in" the chunk's data in read_chunk() while the callback function works on it in-core, and then discard it, if we wanted to change the implementation [*].

	Side note: for that to work, the API must say that the
	callback function MUST NOT assume that the memory region
	starting at chunk_start it is given will stay in memory
	after it returns.  Otherwise, we cannot "page in" and "page
	out".

I am not advocating that we should not assume the entire file can be mapped in. I would however advocate to be explicit in documenting what the users of API can and cannot do (e.g. if we want the "read" callbacks to take advantage of the fact that mfile will stay mapped until the chunkfile is discarded, we should say so, so that they will not make unnecessary copies out of the mmapped region).

Did I miss an update to free_chunkfile() to release resources used to read this file? For some reason, unlike the writing side, the reading side of this API feels a bit incomplete to me.

Thanks.