lix/src/libstore/nar-accessor.hh
Eelco Dolstra 2df9cbeb47 Provide random access to cached NARs
E.g.

  $ time nix cat-store --store https://cache.nixos.org?local-nar-cache=/tmp/nars \
    /nix/store/b0w2hafndl09h64fhb86kw6bmhbmnpm1-blender-2.79/share/icons/hicolor/scalable/apps/blender.svg > /dev/null
  real    0m4.139s

  $ time nix cat-store --store https://cache.nixos.org?local-nar-cache=/tmp/nars \
    /nix/store/b0w2hafndl09h64fhb86kw6bmhbmnpm1-blender-2.79/share/icons/hicolor/scalable/apps/blender.svg > /dev/null
  real    0m0.024s

(Before, the second call took ~0.220s.)

This will use a NAR listing in
/tmp/nars/b0w2hafndl09h64fhb86kw6bmhbmnpm1.ls containing all metadata,
including the offsets of regular files inside the NAR. Thus, we don't
need to read the entire NAR. (We do read the entire listing, but
that's generally pretty small. We could use a SQLite DB by borrowing
some more code from nixos-channel-scripts/file-cache.hh.)

This is primarily useful when Hydra is serving files from an S3 binary
cache, in particular when you have giant NARs. E.g. we had some 12 GiB
NARs, so accessing individuals files was pretty slow.
2017-12-07 01:07:07 +01:00

29 lines
811 B
C++

#pragma once
#include "fs-accessor.hh"
namespace nix {
/* Return an object that provides access to the contents of a NAR
file. */
ref<FSAccessor> makeNarAccessor(ref<const std::string> nar);
/* Create a NAR accessor from a NAR listing (in the format produced by
listNar()). The callback getNarBytes(offset, length) is used by the
readFile() method of the accessor to get the contents of files
inside the NAR. */
typedef std::function<std::string(uint64_t, uint64_t)> GetNarBytes;
ref<FSAccessor> makeLazyNarAccessor(
const std::string & listing,
GetNarBytes getNarBytes);
class JSONPlaceholder;
/* Write a JSON representation of the contents of a NAR (except file
contents). */
void listNar(JSONPlaceholder & res, ref<FSAccessor> accessor,
const Path & path, bool recurse);
}