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Replace fetchToStore cache by sourcePathToHash cache #14769

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Replace fetchToStore cache by sourcePathToHash cache #14769

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3 changes: 1 addition & 2 deletions src/libexpr/paths.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -23,13 +23,12 @@ SourcePath EvalState::storePath(const StorePath & path)
StorePath
EvalState::mountInput(fetchers::Input & input, const fetchers::Input & originalInput, ref<SourceAccessor> accessor)
{
auto storePath = fetchToStore(fetchSettings, *store, accessor, FetchMode::Copy, input.getName());
auto [storePath, narHash] = fetchToStore2(fetchSettings, *store, accessor, FetchMode::Copy, input.getName());

allowPath(storePath); // FIXME: should just whitelist the entire virtual store

storeFS->mount(CanonPath(store->printStorePath(storePath)), accessor);

auto narHash = store->queryPathInfo(storePath)->narHash;
input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));

if (originalInput.getNarHash() && narHash != *originalInput.getNarHash())
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81 changes: 63 additions & 18 deletions src/libfetchers/fetch-to-store.cc
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Expand Up @@ -5,12 +5,12 @@

namespace nix {

fetchers::Cache::Key makeFetchToStoreCacheKey(
const std::string & name, const std::string & fingerprint, ContentAddressMethod method, const std::string & path)
fetchers::Cache::Key
makeSourcePathToHashCacheKey(std::string_view fingerprint, ContentAddressMethod method, const CanonPath & path)
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@Ericson2314 Ericson2314 Feb 4, 2026

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CanonPath is good

{
return fetchers::Cache::Key{
"fetchToStore",
{{"name", name}, {"fingerprint", fingerprint}, {"method", std::string{method.render()}}, {"path", path}}};
"sourcePathToHash",
{{"fingerprint", std::string(fingerprint)}, {"method", std::string{method.render()}}, {"path", path.abs()}}};
}

StorePath fetchToStore(
Expand All @@ -23,19 +23,39 @@ StorePath fetchToStore(
PathFilter * filter,
RepairFlag repair)
{
// FIXME: add an optimisation for the case where the accessor is
// a `PosixSourceAccessor` pointing to a store path.
Comment on lines -26 to -27
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@roberth roberth Dec 11, 2025

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Should this still be done?

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@edolstra edolstra Feb 4, 2026

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Don't think so, the path fetcher has an optimization that does something like that now.

return fetchToStore2(settings, store, path, mode, name, method, filter, repair).first;
}

std::pair<StorePath, Hash> fetchToStore2(
const fetchers::Settings & settings,
Store & store,
const SourcePath & path,
FetchMode mode,
std::string_view name,
ContentAddressMethod method,
PathFilter * filter,
RepairFlag repair)
{
std::optional<fetchers::Cache::Key> cacheKey;

auto [subpath, fingerprint] = filter ? std::pair<CanonPath, std::optional<std::string>>{path.path, std::nullopt}
: path.accessor->getFingerprint(path.path);

if (fingerprint) {
cacheKey = makeFetchToStoreCacheKey(std::string{name}, *fingerprint, method, subpath.abs());
if (auto res = settings.getCache()->lookupStorePath(*cacheKey, store)) {
debug("store path cache hit for '%s'", path);
return res->storePath;
cacheKey = makeSourcePathToHashCacheKey(*fingerprint, method, subpath);
if (auto res = settings.getCache()->lookup(*cacheKey)) {
auto hash = Hash::parseSRI(fetchers::getStrAttr(*res, "hash"));
auto storePath =
store.makeFixedOutputPathFromCA(name, ContentAddressWithReferences::fromParts(method, hash, {}));
if (mode == FetchMode::DryRun || store.isValidPath(storePath)) {
debug(
"source path '%s' cache hit in '%s' (hash '%s')",
path,
store.printStorePath(storePath),
hash.to_string(HashFormat::SRI, true));
return {storePath, hash};
}
debug("source path '%s' not in store", path);
}
} else {
static auto barf = getEnv("_NIX_TEST_BARF_ON_UNCACHEABLE").value_or("") == "1";
Expand All @@ -53,16 +73,41 @@ StorePath fetchToStore(

auto filter2 = filter ? *filter : defaultPathFilter;

auto storePath = mode == FetchMode::DryRun
? store.computeStorePath(name, path, method, HashAlgorithm::SHA256, {}, filter2).first
: store.addToStore(name, path, method, HashAlgorithm::SHA256, {}, filter2, repair);

debug(mode == FetchMode::DryRun ? "hashed '%s'" : "copied '%s' to '%s'", path, store.printStorePath(storePath));
auto [storePath, hash] =
mode == FetchMode::DryRun
? ({
auto [storePath, hash] =
store.computeStorePath(name, path, method, HashAlgorithm::SHA256, {}, filter2);
debug(
"hashed '%s' to '%s' (hash '%s')",
path,
store.printStorePath(storePath),
hash.to_string(HashFormat::SRI, true));
std::make_pair(storePath, hash);
})
: ({
// FIXME: ideally addToStore() would return the hash
// right away (like computeStorePath()).
auto storePath = store.addToStore(name, path, method, HashAlgorithm::SHA256, {}, filter2, repair);
auto info = store.queryPathInfo(storePath);
assert(info->references.empty());
auto hash = method == ContentAddressMethod::Raw::NixArchive ? info->narHash : ({
if (!info->ca || info->ca->method != method)
throw Error("path '%s' lacks a CA field", store.printStorePath(storePath));
info->ca->hash;
});
debug(
"copied '%s' to '%s' (hash '%s')",
path,
store.printStorePath(storePath),
hash.to_string(HashFormat::SRI, true));
std::make_pair(storePath, hash);
});

if (cacheKey && mode == FetchMode::Copy)
settings.getCache()->upsert(*cacheKey, store, {}, storePath);
if (cacheKey)
settings.getCache()->upsert(*cacheKey, {{"hash", hash.to_string(HashFormat::SRI, true)}});

return storePath;
return {storePath, hash};
}

} // namespace nix
7 changes: 4 additions & 3 deletions src/libfetchers/fetchers.cc
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Expand Up @@ -339,9 +339,10 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(const Settings
// can reuse the existing nar instead of copying the unpacked
// input back into the store on every evaluation.
if (accessor->fingerprint) {
ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive;
auto cacheKey = makeFetchToStoreCacheKey(getName(), *accessor->fingerprint, method, "/");
settings.getCache()->upsert(cacheKey, store, {}, storePath);
settings.getCache()->upsert(
makeSourcePathToHashCacheKey(
*accessor->fingerprint, ContentAddressMethod::Raw::NixArchive, CanonPath::root),
{{"hash", store.queryPathInfo(storePath)->narHash.to_string(HashFormat::SRI, true)}});
Comment on lines +342 to +345
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@roberth roberth Dec 11, 2025

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Suggested change
settings.getCache()->upsert(
makeSourcePathToHashCacheKey(
*accessor->fingerprint, ContentAddressMethod::Raw::NixArchive, CanonPath::root),
{{"hash", store.queryPathInfo(storePath)->narHash.to_string(HashFormat::SRI, true)}});

Tests pass without this code. It could be dead or broken.
Make sure to cover this with the _NIX_TEST_BARF_ON_UNCACHEABLE trick.

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@edolstra edolstra Dec 12, 2025

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_NIX_TEST_BARF_ON_UNCACHEABLE doesn't test that. It fails on inputs that don't have a fingerprint. If you remove the upsert, it disables caching, but there is nothing that tests that.

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@roberth roberth Dec 12, 2025 •
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I see, that is a bit different. Not a bad idea though considering how unreliable the caches have been.

(does need a different name it seems)

}

accessor->setPathDisplay("«" + to_string() + "»");
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14 changes: 12 additions & 2 deletions src/libfetchers/include/nix/fetchers/fetch-to-store.hh
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Expand Up @@ -24,7 +24,17 @@ StorePath fetchToStore(
PathFilter * filter = nullptr,
RepairFlag repair = NoRepair);

fetchers::Cache::Key makeFetchToStoreCacheKey(
const std::string & name, const std::string & fingerprint, ContentAddressMethod method, const std::string & path);
std::pair<StorePath, Hash> fetchToStore2(
const fetchers::Settings & settings,
Store & store,
const SourcePath & path,
FetchMode mode,
std::string_view name = "source",
ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive,
PathFilter * filter = nullptr,
RepairFlag repair = NoRepair);

fetchers::Cache::Key
makeSourcePathToHashCacheKey(std::string_view fingerprint, ContentAddressMethod method, const CanonPath & path);

} // namespace nix
12 changes: 5 additions & 7 deletions src/libfetchers/path.cc
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Expand Up @@ -165,14 +165,12 @@ struct PathInputScheme : InputScheme

// To prevent `fetchToStore()` copying the path again to Nix
// store, pre-create an entry in the fetcher cache.
accessor->fingerprint =
fmt("path:%s", store.queryPathInfo(*storePath)->narHash.to_string(HashFormat::SRI, true));
auto narHash = store.queryPathInfo(*storePath)->narHash.to_string(HashFormat::SRI, true);
accessor->fingerprint = fmt("path:%s", narHash);
settings.getCache()->upsert(
makeFetchToStoreCacheKey(
input.getName(), *accessor->fingerprint, ContentAddressMethod::Raw::NixArchive, "/"),
store,
{},
*storePath);
makeSourcePathToHashCacheKey(
*accessor->fingerprint, ContentAddressMethod::Raw::NixArchive, CanonPath::root),
{{"hash", narHash}});

/* Trust the lastModified value supplied by the user, if
any. It's not a "secure" attribute so we don't care. */
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