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nix nar {ls,cat}: Optimize

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The whole NarAccessor -> listing -> lazy NarAccessor is very weird. Source
can now be seek-ed over when supported, so we can support it pretty easily.
Alternatively we could also make it single-pass very easily with a custom
FileSystemObjectSink. It will get removed in a follow-up commit anyway.
This commit is contained in:
Sergei Zimmerman 2025-12-08 02:48:35 +03:00
parent 22f993fab6
commit b9b6defca6
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4 changed files with 41 additions and 18 deletions
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43
src/libutil/nar-accessor.cc
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@ -111,6 +111,7 @@ struct NarAccessor : public SourceAccessor
path,
NarMember{.stat = {.type = Type::tRegular, .fileSize = 0, .isExecutable = false, .narOffset = 0}});
NarMemberConstructor nmc{nm, pos};
nmc.skipContents = true; /* Don't care about contents. */
func(nmc);
}
@ -141,6 +142,13 @@ struct NarAccessor : public SourceAccessor
parseDump(indexer, indexer);
}
NarAccessor(Source & source, GetNarBytes getNarBytes)
: getNarBytes(std::move(getNarBytes))
{
NarIndexer indexer(*this, source);
parseDump(indexer, indexer);
}
NarAccessor(const nlohmann::json & listing, GetNarBytes getNarBytes)
: getNarBytes(getNarBytes)
{
@ -249,24 +257,35 @@ ref<SourceAccessor> makeLazyNarAccessor(const nlohmann::json & listing, GetNarBy
return make_ref<NarAccessor>(listing, getNarBytes);
}
ref<SourceAccessor> makeLazyNarAccessor(Source & source, GetNarBytes getNarBytes)
{
return make_ref<NarAccessor>(source, getNarBytes);
}
GetNarBytes seekableGetNarBytes(const Path & path)
{
return [path](uint64_t offset, uint64_t length) {
AutoCloseFD fd = toDescriptor(open(
path.c_str(),
O_RDONLY
#ifndef _WIN32
| O_CLOEXEC
AutoCloseFD fd = toDescriptor(open(
path.c_str(),
O_RDONLY
#ifdef O_CLOEXEC
| O_CLOEXEC
#endif
));
if (!fd)
throw SysError("opening NAR cache file '%s'", path);
));
if (!fd)
throw SysError("opening NAR cache file '%s'", path);
if (lseek(fromDescriptorReadOnly(fd.get()), offset, SEEK_SET) != (off_t) offset)
throw SysError("seeking in '%s'", path);
return [inner = seekableGetNarBytes(fd.get()), fd = make_ref<AutoCloseFD>(std::move(fd))](
uint64_t offset, uint64_t length) { return inner(offset, length); };
}
GetNarBytes seekableGetNarBytes(Descriptor fd)
{
return [fd](uint64_t offset, uint64_t length) {
if (::lseek(fromDescriptorReadOnly(fd), offset, SEEK_SET) == -1)
throw SysError("seeking in file");
std::string buf(length, 0);
readFull(fd.get(), buf.data(), length);
readFull(fd, buf.data(), length);
return buf;
};