reftable/blocksource: use mmap to read tables

reftable/blocksource: use mmap to read tables

The blocksource interface provides an interface to read blocks from a
reftable table. This interface is implemented using read(3P) calls on
the underlying file descriptor. While this works alright, this pattern
is very inefficient when repeatedly querying the reftable stack for one
or more refs. This inefficiency can mostly be attributed to the fact
that we often need to re-read the same blocks over and over again, and
every single time we need to call read(3P) again.

A natural fit in this context is to use mmap(3P) instead of read(3P),
which has a bunch of benefits:

  - We do not need to come up with a caching strategy for some of the
    blocks as this will be handled by the kernel already.

  - We can avoid the overhead of having to call into the read(3P)
    syscall repeatedly.

  - We do not need to allocate returned blocks repeatedly, but can
    instead hand out pointers into the mmapped region directly.

Using mmap comes with a significant drawback on Windows though, because
mmapped files cannot be deleted and neither is it possible to rename
files onto an mmapped file. But for one, the reftable library gracefully
handles the case where auto-compaction cannot delete a still-open stack
already and ignores any such errors. Also, `reftable_stack_clean()` will
prune stale tables which are not referenced by "tables.list" anymore so
that those files can eventually be pruned. And second, we never rewrite
already-written stacks, so it does not matter that we cannot rename a
file over an mmaped file, either.

Another unfortunate property of mmap is that it is not supported by all
systems. But given that the size of reftables should typically be rather
limited (megabytes at most in the vast majority of repositories), we can
use the fallback implementation provided by `git_mmap()` which reads the
whole file into memory instead. This is the same strategy that the
"packed" backend uses.

While this change doesn't significantly improve performance in the case
where we're seeking through stacks once (like e.g. git-for-each-ref(1)
would). But it does speed up usecases where there is lots of random
access to refs, e.g. when writing. The following benchmark demonstrates
these savings with git-update-ref(1) creating N refs in an otherwise
empty repository:

  Benchmark 1: update-ref: create many refs (refcount = 1, revision = HEAD~)
    Time (mean ± σ):       5.1 ms ±   0.2 ms    [User: 2.5 ms, System: 2.5 ms]
    Range (min … max):     4.8 ms …   7.1 ms    111 runs

  Benchmark 2: update-ref: create many refs (refcount = 100, revision = HEAD~)
    Time (mean ± σ):      14.8 ms ±   0.5 ms    [User: 7.1 ms, System: 7.5 ms]
    Range (min … max):    14.1 ms …  18.7 ms    84 runs

  Benchmark 3: update-ref: create many refs (refcount = 10000, revision = HEAD~)
    Time (mean ± σ):     926.4 ms ±   5.6 ms    [User: 448.5 ms, System: 477.7 ms]
    Range (min … max):   920.0 ms … 936.1 ms    10 runs

  Benchmark 4: update-ref: create many refs (refcount = 1, revision = HEAD)
    Time (mean ± σ):       5.0 ms ±   0.2 ms    [User: 2.4 ms, System: 2.5 ms]
    Range (min … max):     4.7 ms …   5.4 ms    111 runs

  Benchmark 5: update-ref: create many refs (refcount = 100, revision = HEAD)
    Time (mean ± σ):      10.5 ms ±   0.2 ms    [User: 5.0 ms, System: 5.3 ms]
    Range (min … max):    10.0 ms …  10.9 ms    93 runs

  Benchmark 6: update-ref: create many refs (refcount = 10000, revision = HEAD)
    Time (mean ± σ):     529.6 ms ±   9.1 ms    [User: 268.0 ms, System: 261.4 ms]
    Range (min … max):   522.4 ms … 547.1 ms    10 runs

  Summary
    update-ref: create many refs (refcount = 1, revision = HEAD) ran
      1.01 ± 0.06 times faster than update-ref: create many refs (refcount = 1, revision = HEAD~)
      2.08 ± 0.07 times faster than update-ref: create many refs (refcount = 100, revision = HEAD)
      2.95 ± 0.14 times faster than update-ref: create many refs (refcount = 100, revision = HEAD~)
    105.33 ± 3.76 times faster than update-ref: create many refs (refcount = 10000, revision = HEAD)
    184.24 ± 5.89 times faster than update-ref: create many refs (refcount = 10000, revision = HEAD~)

Theoretically, we could also replicate the strategy of the "packed"
backend where small tables are read into memory instead of using mmap.
Benchmarks did not confirm that this has a performance benefit though.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>