fetchers::downloadTarball(): Return a cacheable accessor

downloadTarball() is used by `-I foo=<url>` etc. fetchToStore() needs
the accessor to have a fingerprint to enable caching.

Fixes NixOS#11271.

src/libcmd/common-eval-args.cc

@@ -171,7 +171,9 @@ SourcePath lookupFileArg(EvalState & state, std::string_view s, const Path * bas
 {
     if (EvalSettings::isPseudoUrl(s)) {
         auto accessor = fetchers::downloadTarball(
-            EvalSettings::resolvePseudoUrl(s)).accessor;
+            state.store,
+            state.fetchSettings,
+            EvalSettings::resolvePseudoUrl(s));
         auto storePath = fetchToStore(*state.store, SourcePath(accessor), FetchMode::Copy);
         return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }

src/libexpr/eval.cc

@@ -3088,7 +3088,9 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
     if (EvalSettings::isPseudoUrl(value)) {
         try {
             auto accessor = fetchers::downloadTarball(
-                EvalSettings::resolvePseudoUrl(value)).accessor;
+                store,
+                fetchSettings,
+                EvalSettings::resolvePseudoUrl(value));
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
             return finish(store->toRealPath(storePath));
         } catch (Error & e) {

src/libexpr/primops/fetchTree.cc

@@ -507,7 +507,11 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     //       https://github.com/NixOS/nix/issues/4313
     auto storePath =
         unpack
-        ? fetchToStore(*state.store, fetchers::downloadTarball(*url).accessor, FetchMode::Copy, name)
+        ? fetchToStore(
+            *state.store,
+            fetchers::downloadTarball(state.store, state.fetchSettings, *url),
+            FetchMode::Copy,
+            name)
         : fetchers::downloadFile(state.store, *url, name).storePath;
 
     if (expectedHash) {

src/libfetchers/tarball.cc

@@ -102,7 +102,7 @@ DownloadFileResult downloadFile(
     };
 }
 
-DownloadTarballResult downloadTarball(
+static DownloadTarballResult downloadTarball_(
     const std::string & url,
     const Headers & headers)
 {
@@ -202,6 +202,22 @@ DownloadTarballResult downloadTarball(
     return attrsToResult(infoAttrs);
 }
 
+ref<SourceAccessor> downloadTarball(
+    ref<Store> store,
+    const Settings & settings,
+    const std::string & url)
+{
+    /* Go through Input::getAccessor() to ensure that the resulting
+       accessor has a fingerprint. */
+    fetchers::Attrs attrs;
+    attrs.insert_or_assign("type", "tarball");
+    attrs.insert_or_assign("url", url);
+
+    auto input = Input::fromAttrs(settings, std::move(attrs));
+
+    return input.getAccessor(store).first;
+}
+
 // An input scheme corresponding to a curl-downloadable resource.
 struct CurlInputScheme : InputScheme
 {
@@ -353,7 +369,7 @@ struct TarballInputScheme : CurlInputScheme
     {
         auto input(_input);
 
-        auto result = downloadTarball(getStrAttr(input.attrs, "url"), {});
+        auto result = downloadTarball_(getStrAttr(input.attrs, "url"), {});
 
         result.accessor->setPathDisplay("«" + input.to_string() + "»");
 

src/libfetchers/tarball.hh

@@ -14,6 +14,8 @@ struct SourceAccessor;
 
 namespace nix::fetchers {
 
+struct Settings;
+
 struct DownloadFileResult
 {
     StorePath storePath;
@@ -40,8 +42,9 @@ struct DownloadTarballResult
  * Download and import a tarball into the Git cache. The result is the
  * Git tree hash of the root directory.
  */
-DownloadTarballResult downloadTarball(
-    const std::string & url,
-    const Headers & headers = {});
+ref<SourceAccessor> downloadTarball(
+    ref<Store> store,
+    const Settings & settings,
+    const std::string & url);
 
 }