feat(nix store ls): support reading nar listings from binary cache

Remote binary caches support `write-nar-listing` options where they create a `HASH.ls` file for quick indexing without having to download the nar.
This commit makes experimental `nix store ls` attempt to read these files instead of downloading the full nar.

The difference is very obvious with large packages like stellarium:

nix store ls --store "https://cache.nixos.org" /nix/store/ijpvwgs9zamqaax5dy2cd0kxgz7lr7an-stellarium-25.1 -R

Change-Id: I6a37e0788b3a91c319331a8de69c51daf3efa955

doc/manual/change-authors.yml

@@ -236,6 +236,11 @@ vigress8:
   forgejo: vigress8
   github: vigress8
 
+vlinkz:
+  display_name: Victor Fuentes
+  forgejo: vlinkz
+  github: vlinkz
+
 winter:
   forgejo: winter
   github: winterqt

doc/manual/rl-next/nix-store-ls-read-nar-listings.md

@@ -0,0 +1,12 @@
+---
+synopsis: Allow `nix store ls` to read nar listings from binary cache stores.
+issues: []
+cls: [3225]
+category: Improvements
+credits: [vlinkz]
+---
+
+The `nix store ls` command now supports reading `.ls` nar listings from binary cache stores.
+If a listing is detected for the store path being queried, the nar is no longer downloaded.
+These nar listings are available in binary cache stores where the `write-nar-listing` option is
+enabled, such as cache.nixos.org.

lix/nix/ls.cc

@@ -1,4 +1,5 @@
 #include "lix/libcmd/command.hh"
+#include "lix/libstore/binary-cache-store.hh"
 #include "lix/libstore/store-api.hh"
 #include "lix/libstore/fs-accessor.hh"
 #include "lix/libstore/nar-accessor.hh"
@@ -123,7 +124,24 @@ struct CmdLsStore : StoreCommand, MixLs
 
     void run(ref<Store> store) override
     {
-        list(store->getFSAccessor());
+        auto accessor = store->getFSAccessor();
+
+        try {
+            auto binaryCacheStore = store.try_cast_shared<BinaryCacheStore>();
+            if (binaryCacheStore) {
+                const auto [storePath, restPath] = store->toStorePath(path);
+                auto file = binaryCacheStore->getFile(fmt("%s.ls", storePath.hashPart()))->drain();
+                JSON j = json::parse(std::move(file), "a nar content listing");
+                if (j["version"] == 1) {
+                    path = restPath;
+                    accessor = makeLazyNarAccessor(j["root"].dump(), [](uint64_t, uint64_t) -> std::string {
+                        throw Error("attempted to read NAR content during listing");
+                    });
+                }
+            }
+        } catch (NoSuchBinaryCacheFile &) { }
+
+        list(accessor);
     }
 };
 

tests/functional/nar-access.sh

@@ -59,3 +59,23 @@ if nix-store --dump $storePath >/dev/full ; then
     echo "dumping to /dev/full should fail"
     exit -1
 fi
+
+
+# Test reading from remote nar listings if available
+
+nix copy --to "file://$cacheDir?write-nar-listing=true" $storePath
+
+export _NIX_FORCE_HTTP=1
+
+diff -u \
+    <(nix store ls --json $storePath --store "file://$cacheDir" | jq -S) \
+    <(echo '{"type":"directory","entries":{"foo":{},"foo-x":{},"qux":{},"zyx":{}}}' | jq -S)
+diff -u \
+    <(nix store ls --json -R $storePath/foo/bar --store "file://$cacheDir" | jq -S) \
+    <(echo '{"narOffset": 368,"type":"regular","size":0}' | jq -S)
+
+# Confirm that we are reading from ".ls" file by deleting the nar
+rm -rf $cacheDir/nar
+diff -u \
+    <(nix store ls --json -R $storePath/foo/bar --store "file://$cacheDir" | jq -S) \
+    <(echo '{"narOffset": 368,"type":"regular","size":0}' | jq -S)