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fetchToStore() cache: Use content hashes instead of store paths

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We can always compute the store path from the content hash, but not
vice versa. Storing the content hash allows `hashPath()` to be
replaced by `fetchToStore(...FetchMode::DryRun...)`, which gets us
caching in lazy-trees mode.
This commit is contained in:
Eelco Dolstra 2025-06-13 22:19:53 +02:00 • committed by Graham Christensen
parent b0aa12df25
commit dc5e620032
6 changed files with 72 additions and 58 deletions
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35
src/libfetchers/path.cc
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@ -144,37 +144,22 @@ struct PathInputScheme : InputScheme
storePath = store->addToStoreFromDump(*src, "source");
}
// To avoid copying the path again to the /nix/store, we need to add a cache entry.
ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive;
auto fp = getFingerprint(store, input);
if (fp) {
auto cacheKey = makeFetchToStoreCacheKey(input.getName(), *fp, method, "/");
fetchers::getCache()->upsert(cacheKey, *store, {}, *storePath);
}
auto accessor = makeStorePathAccessor(store, *storePath);
// To prevent `fetchToStore()` copying the path again to Nix
// store, pre-create an entry in the fetcher cache.
auto info = store->queryPathInfo(*storePath);
accessor->fingerprint = fmt("path:%s", store->queryPathInfo(*storePath)->narHash.to_string(HashFormat::SRI, true));
fetchers::getCache()->upsert(
makeSourcePathToHashCacheKey(*accessor->fingerprint, ContentAddressMethod::Raw::NixArchive, "/"),
{{"hash", info->narHash.to_string(HashFormat::SRI, true)}});
/* Trust the lastModified value supplied by the user, if
any. It's not a "secure" attribute so we don't care. */
if (!input.getLastModified())
input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
return {makeStorePathAccessor(store, *storePath), std::move(input)};
}
std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
{
if (isRelative(input))
return std::nullopt;
/* If this path is in the Nix store, use the hash of the
store object and the subpath. */
auto path = getAbsPath(input);
try {
auto [storePath, subPath] = store->toStorePath(path.string());
auto info = store->queryPathInfo(storePath);
return fmt("path:%s:%s", info->narHash.to_string(HashFormat::Base16, false), subPath);
} catch (Error &) {
return std::nullopt;
}
return {accessor, std::move(input)};
}
};