BinaryCacheStore: Implement getBuildLog()

We assume that build logs are stored under log/<drv>, e.g.

  /nix/store/q7ab198v13p0f8x8wgnd75dva7d5mip6-friday-devil-0.1.1.1.drv

maps to

  https://cache.nixos.org/log/q7ab198v13p0f8x8wgnd75dva7d5mip6-friday-devil-0.1.1.1.drv

src/libstore/binary-cache-store.cc

@@ -382,4 +382,28 @@ ref<FSAccessor> BinaryCacheStore::getFSAccessor()
     return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()));
 }
 
+std::shared_ptr<std::string> BinaryCacheStore::getBuildLog(const Path & path)
+{
+    Path drvPath;
+
+    if (isDerivation(path))
+        drvPath = path;
+    else {
+        try {
+            auto info = queryPathInfo(path);
+            // FIXME: add a "Log" field to .narinfo
+            if (info->deriver == "") return nullptr;
+            drvPath = info->deriver;
+        } catch (InvalidPath &) {
+            return nullptr;
+        }
+    }
+
+    auto logPath = "log/" + baseNameOf(drvPath);
+
+    debug("fetching build log from binary cache ‘%s/%s’", getUri(), logPath);
+
+    return getFile(logPath);
+}
+
 }

src/libstore/binary-cache-store.hh

@@ -122,6 +122,8 @@ public:
     void addSignatures(const Path & storePath, const StringSet & sigs) override
     { notImpl(); }
 
+    std::shared_ptr<std::string> getBuildLog(const Path & path) override;
+
 };
 
 }

src/libstore/local-fs-store.cc

@@ -95,7 +95,11 @@ std::shared_ptr<std::string> LocalFSStore::getBuildLog(const Path & path_)
     assertStorePath(path);
 
     if (!isDerivation(path)) {
+        try {
             path = queryPathInfo(path)->deriver;
+        } catch (InvalidPath &) {
+            return nullptr;
+        }
         if (path == "") return nullptr;
     }