On 11/14/2017 8:12 PM, Junio C Hamano wrote:
Ben Peart <peartben@xxxxxxxxx> writes:
I have no thoughts or plans for changes in the future of IEOT (which I
plan to rename ITOC). At this point in time, I can't even imagine
what else we'd want as the index only contains cache entries, ...
Yeah, but the thing is that this is open source, and the imagination
of the originator of the initial idea does not limit how the code
and data structure evolves.
Back when I added the index extensions to the system, I didn't have
perfect foresight, and I didn't have specific plans to add anything
beyond "TREE" to optimize "write-tree" and "diff-index --cached".
In hindsight, I got one thing right and one thing wrong.
Even though I didn't have any plan to add a mandatory extension, I
made the code to ignore optional ones and error out on mandatory
ones if an index extension section is not understood. It turns out
that later (in fact much later---the "TREE" extension dates back to
April 2006, while "link" came in June 2014) we could add the
split-index mode without having to worry about older versions of Git
doing random wrong things when they see this new extension, thanks
to that design decision. That went well.
On the other hand, I didn't think things through to realize that
some operations may want to peek only the extensions without ever
using the main table, that some other operations may want to read
some extensions first before reading the main table, or more
importantly, that these limitations would start mattering once Git
becomes popular enough and starts housing a project tree with very
many paths in the main table.
I really wish somebody had brought it up as a potential issue during
the review---I would have defined the extended index format to have
the simple extension at the end that points back to the tail end of
the main table back then, and we wouldn't be having this back and
forth now. But I was just too happy and excited that I have found a
room to squeeze extension sections into the index file format
without breaking existing implementations of Git (which checked the
trailer checksum matches to the result of hashing the whole thing,
and read the recorded number of entries from the main table, without
even noticing that there is a gap in between), and that excitement
blinded me.
I understand the risk but the list of offsets into the cache entries
is pretty simple as well. I prefer the simplicity of a single TOC
extension that gives us random access to the entire index rather than
having to piece one together using multiple extensions. That model
has its own set of risks and tradeoffs.
I thought that you are not using the "where does the series of
extensions begin" information in the first place, no? That piece of
information is useful independent of the usefulness of "index into
the main table to list entries where the prefix-compression is
reset". So if anything, I'd prefer the simplicity of introducing
that "where does the series of extensions begin" that does not say
anything else, and build on other things like ITOC as mere users of
the mechanism.
OK. I'll call this new extension "EOIE" ("end of index entries"). Other
than the standard header/footer, it will only contain a 32 bit offset to
the beginning of the extension entries. I'll always write out that
extension unless you would prefer it be behind a setting (if so, what do
you want it called and what should the default be)? I won't add support
in update-index for this extension.
Since the goal was to find a way to load the IEOT extension before the
cache entries, I'll also refactor the extension reading loop into a
function that takes a function pointer and add a
preread_index_extension() function that can be passed in when that loop
is run before the cache entries are loaded. When the loop is run again
after the cache entries are loaded, it will pass in the existing
read_index_extension() function. Extensions can then choose which
function they want to be loaded in.
The code to read/write/use the IEOT to parallelize the cache entry
loading will stay behind a config setting that defaults to false (at
least for now). I'll stick with "core.fastindex" until someone can
(please) propose a better name.
