nix/ls: fall back to downloading the nar if the .ls is bad

fixes #959

Change-Id: I1715f6fcc938dac7287d9cf4a85e93c9d66768a0

lix/nix/ls.cc

@@ -137,13 +137,21 @@ struct CmdLsStore : StoreCommand, MixLs
                 );
                 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");
                     });
+                    path = restPath;
                 }
             }
-        } catch (NoSuchBinaryCacheFile &) { }
+        } catch (NoSuchBinaryCacheFile &) {
+        } catch (Error & e) {
+            warn(
+                "nar listing for %s on %s is bad (falling back to full nar download): %s",
+                path,
+                store->getUri(),
+                e.what()
+            );
+        }
 
         list(accessor);
     }

tests/functional/nar-access.sh

@@ -82,11 +82,21 @@ diff -u \
     <(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
+# Confirm that we are reading from ".ls" file by moving the nar
+mv $cacheDir/nar $cacheDir/nar.gone
 diff -u \
     <(nix store ls --json -R $storePath/foo/bar --store "file://$cacheDir" | jq -S) \
     <(echo '{"narOffset": 368,"type":"regular","size":0}' | jq -S)
+mv $cacheDir/nar.gone $cacheDir/nar
+
+# confirm that we read the nar if the listing is missing offsets
+narls=$(echo "$cacheDir/"*.ls)
+cp "$narls" "$narls.old"
+jq 'walk(if type == "object" then del(.narOffset) else . end)' < "$narls.old" >"$narls"
+diff -u \
+    <(nix store ls --json $storePath/foo/bar --store "file://$cacheDir" | jq -S) \
+    <(echo '{"narOffset": 368,"type":"regular","size":0}' | jq -S)
+mv "$narls.old" "$narls"
 
 if canWriteNonUtf8Inodes; then
     # Confirm that there's no more than one `.ls` in the `$cacheDir` because non-UTF8 inodes cannot have `.ls` generated for them.