mercurial/cffi/mpatchbuild.py

tags-fnode-cache: skip building a changectx in getfnode

Building a changectx object is costly, doing it just to retrieve the revision
number is suboptimal. Directly fetching the revision number from the changelog
provide a sizeable speedup to `hg debugupdatecache`.

### data-env-vars.name           = mercurial-2018-08-01-zstd-sparse-revlog
  # benchmark.name               = debug-update-cache
  # benchmark.variants.pre-state = warm
before: 0.213229 seconds
after:  0.165577 seconds  (-22.35%)
  # data-env-vars.name           = mercurial-filtered-2019-11-22-zstd-sparse-revlog
before: 1.200383 seconds
after:  1.071618 seconds  (-10.73%)
  # data-env-vars.name           = mozilla-central-2018-08-01-zstd-sparse-revlog
before: 1.465735 seconds
after:  0.923128 seconds  (-37.02%)
  # data-env-vars.name           = mozilla-try-2019-02-18-zstd-sparse-revlog
before: 6.511771 seconds
after:  4.507316 seconds  (-30.78%)
  # data-env-vars.name           = netbeans-2018-08-01-zstd-sparse-revlog
before: 1.023007 seconds
after:  0.645026 seconds  (-36.95%)
  # data-env-vars.name           = pypy-2018-08-01-zstd-sparse-revlog
before: 0.381141 seconds
after:  0.268654 seconds  (-29.51%)

import cffi
import os

ffi = cffi.FFI()
mpatch_c = os.path.join(
    os.path.join(os.path.dirname(__file__), '..', 'mpatch.c')
)
with open(mpatch_c) as f:
    ffi.set_source(
        "mercurial.cffi._mpatch", f.read(), include_dirs=["mercurial"]
    )
ffi.cdef(
    """

struct mpatch_frag {
       int start, end, len;
       const char *data;
};

struct mpatch_flist {
       struct mpatch_frag *base, *head, *tail;
};

extern "Python" struct mpatch_flist* cffi_get_next_item(void*, ssize_t);

int mpatch_decode(const char *bin, ssize_t len, struct mpatch_flist** res);
ssize_t mpatch_calcsize(size_t len, struct mpatch_flist *l);
void mpatch_lfree(struct mpatch_flist *a);
static int mpatch_apply(char *buf, const char *orig, size_t len,
                        struct mpatch_flist *l);
struct mpatch_flist *mpatch_fold(void *bins,
                       struct mpatch_flist* (*get_next_item)(void*, ssize_t),
                       ssize_t start, ssize_t end);
"""
)

if __name__ == '__main__':
    ffi.compile()