refactor(3p/nix): Apply clang-tidy's performance-* fixes

This applies the performance fixes listed here:

https://clang.llvm.org/extra/clang-tidy/checks/list.html

third_party/nix/src/libstore/binary-cache-store.cc

@@ -98,7 +98,7 @@ Path BinaryCacheStore::narInfoFileFor(const Path& storePath) {
   return storePathToHash(storePath) + ".narinfo";
 }
 
-void BinaryCacheStore::writeNarInfo(ref<NarInfo> narInfo) {
+void BinaryCacheStore::writeNarInfo(const ref<NarInfo>& narInfo) {
   auto narInfoFile = narInfoFileFor(narInfo->path);
 
   upsertFile(narInfoFile, narInfo->to_string(), "text/x-nix-narinfo");

third_party/nix/src/libstore/binary-cache-store.hh

@@ -65,7 +65,7 @@ class BinaryCacheStore : public Store {
 
   std::string narInfoFileFor(const Path& storePath);
 
-  void writeNarInfo(ref<NarInfo> narInfo);
+  void writeNarInfo(const ref<NarInfo>& narInfo);
 
  public:
   bool isValidPathUncached(const Path& path) override;

third_party/nix/src/libstore/build.cc

@@ -145,7 +145,7 @@ class Goal : public std::enable_shared_from_this<Goal> {
  public:
   virtual void work() = 0;
 
-  void addWaitee(GoalPtr waitee);
+  void addWaitee(const GoalPtr& waitee);
 
   virtual void waiteeDone(GoalPtr waitee, ExitCode result);
 
@@ -280,10 +280,10 @@ class Worker {
                                RepairFlag repair = NoRepair);
 
   /* Remove a dead goal. */
-  void removeGoal(GoalPtr goal);
+  void removeGoal(const GoalPtr& goal);
 
   /* Wake up a goal (i.e., there is something for it to do). */
-  void wakeUp(GoalPtr goal);
+  void wakeUp(const GoalPtr& goal);
 
   /* Return the number of local build and substitution processes
      currently running (but not remote builds via the build
@@ -292,7 +292,7 @@ class Worker {
 
   /* Registers a running child process.  `inBuildSlot' means that
      the process counts towards the jobs limit. */
-  void childStarted(GoalPtr goal, const set<int>& fds, bool inBuildSlot,
+  void childStarted(const GoalPtr& goal, const set<int>& fds, bool inBuildSlot,
                     bool respectTimeouts);
 
   /* Unregisters a running child process.  `wakeSleepers' should be
@@ -303,7 +303,7 @@ class Worker {
 
   /* Put `goal' to sleep until a build slot becomes available (which
      might be right away). */
-  void waitForBuildSlot(GoalPtr goal);
+  void waitForBuildSlot(const GoalPtr& goal);
 
   /* Wait for any goal to finish.  Pretty indiscriminate way to
      wait for some resource that some other goal is holding. */
@@ -332,7 +332,7 @@ class Worker {
 
 //////////////////////////////////////////////////////////////////////
 
-void addToWeakGoals(WeakGoals& goals, GoalPtr p) {
+void addToWeakGoals(WeakGoals& goals, const GoalPtr& p) {
   // FIXME: necessary?
   // FIXME: O(n)
   for (auto& i : goals) {
@@ -343,7 +343,7 @@ void addToWeakGoals(WeakGoals& goals, GoalPtr p) {
   goals.push_back(p);
 }
 
-void Goal::addWaitee(GoalPtr waitee) {
+void Goal::addWaitee(const GoalPtr& waitee) {
   waitees.insert(waitee);
   addToWeakGoals(waitee->waiters, shared_from_this());
 }
@@ -445,7 +445,9 @@ void handleDiffHook(uid_t uid, uid_t gid, Path tryA, Path tryB, Path drvPath,
   auto diffHook = settings.diffHook;
   if (diffHook != "" && settings.runDiffHook) {
     try {
-      RunOptions diffHookOptions(diffHook, {tryA, tryB, drvPath, tmpDir});
+      RunOptions diffHookOptions(
+          diffHook, {std::move(tryA), std::move(tryB), std::move(drvPath),
+                     std::move(tmpDir)});
       diffHookOptions.searchPath = true;
       diffHookOptions.uid = uid;
       diffHookOptions.gid = gid;
@@ -979,7 +981,7 @@ class DerivationGoal : public Goal {
 
   void done(BuildResult::Status status, const string& msg = "");
 
-  PathSet exportReferences(PathSet storePaths);
+  PathSet exportReferences(const PathSet& storePaths);
 };
 
 const Path DerivationGoal::homeDir = "/homeless-shelter";
@@ -1520,7 +1522,7 @@ void DerivationGoal::tryToBuild() {
   started();
 }
 
-void replaceValidPath(const Path& storePath, const Path tmpPath) {
+void replaceValidPath(const Path& storePath, const Path& tmpPath) {
   /* We can't atomically replace storePath (the original) with
      tmpPath (the replacement), so we have to move it out of the
      way first.  We'd better not be interrupted here, because if
@@ -1840,7 +1842,7 @@ int childEntry(void* arg) {
   return 1;
 }
 
-PathSet DerivationGoal::exportReferences(PathSet storePaths) {
+PathSet DerivationGoal::exportReferences(const PathSet& storePaths) {
   PathSet paths;
 
   for (auto storePath : storePaths) {
@@ -4455,7 +4457,7 @@ GoalPtr Worker::makeSubstitutionGoal(const Path& path, RepairFlag repair) {
   return goal;
 }
 
-static void removeGoal(GoalPtr goal, WeakGoalMap& goalMap) {
+static void removeGoal(const GoalPtr& goal, WeakGoalMap& goalMap) {
   /* !!! inefficient */
   for (auto i = goalMap.begin(); i != goalMap.end();) {
     if (i->second.lock() == goal) {
@@ -4469,7 +4471,7 @@ static void removeGoal(GoalPtr goal, WeakGoalMap& goalMap) {
   }
 }
 
-void Worker::removeGoal(GoalPtr goal) {
+void Worker::removeGoal(const GoalPtr& goal) {
   nix::removeGoal(goal, derivationGoals);
   nix::removeGoal(goal, substitutionGoals);
   if (topGoals.find(goal) != topGoals.end()) {
@@ -4492,15 +4494,15 @@ void Worker::removeGoal(GoalPtr goal) {
   waitingForAnyGoal.clear();
 }
 
-void Worker::wakeUp(GoalPtr goal) {
+void Worker::wakeUp(const GoalPtr& goal) {
   goal->trace("woken up");
   addToWeakGoals(awake, goal);
 }
 
 unsigned Worker::getNrLocalBuilds() { return nrLocalBuilds; }
 
-void Worker::childStarted(GoalPtr goal, const set<int>& fds, bool inBuildSlot,
-                          bool respectTimeouts) {
+void Worker::childStarted(const GoalPtr& goal, const set<int>& fds,
+                          bool inBuildSlot, bool respectTimeouts) {
   Child child;
   child.goal = goal;
   child.goal2 = goal.get();
@@ -4542,7 +4544,7 @@ void Worker::childTerminated(Goal* goal, bool wakeSleepers) {
   }
 }
 
-void Worker::waitForBuildSlot(GoalPtr goal) {
+void Worker::waitForBuildSlot(const GoalPtr& goal) {
   DLOG(INFO) << "wait for build slot";
   if (getNrLocalBuilds() < settings.maxBuildJobs) {
     wakeUp(goal); /* we can do it right away */
@@ -4553,12 +4555,12 @@ void Worker::waitForBuildSlot(GoalPtr goal) {
 
 void Worker::waitForAnyGoal(GoalPtr goal) {
   DLOG(INFO) << "wait for any goal";
-  addToWeakGoals(waitingForAnyGoal, goal);
+  addToWeakGoals(waitingForAnyGoal, std::move(goal));
 }
 
 void Worker::waitForAWhile(GoalPtr goal) {
   DLOG(INFO) << "wait for a while";
-  addToWeakGoals(waitingForAWhile, goal);
+  addToWeakGoals(waitingForAWhile, std::move(goal));
 }
 
 void Worker::run(const Goals& _topGoals) {

third_party/nix/src/libstore/crypto.cc

@@ -104,12 +104,12 @@ PublicKeys getDefaultPublicKeys() {
 
   // FIXME: filter duplicates
 
-  for (auto s : settings.trustedPublicKeys.get()) {
+  for (const auto& s : settings.trustedPublicKeys.get()) {
     PublicKey key(s);
     publicKeys.emplace(key.name, key);
   }
 
-  for (auto secretKeyFile : settings.secretKeyFiles.get()) {
+  for (const auto& secretKeyFile : settings.secretKeyFiles.get()) {
     try {
       SecretKey secretKey(readFile(secretKeyFile));
       publicKeys.emplace(secretKey.name, secretKey.toPublicKey());

third_party/nix/src/libstore/derivations.cc

@@ -38,7 +38,7 @@ bool BasicDerivation::isBuiltin() const {
   return string(builder, 0, 8) == "builtin:";
 }
 
-Path writeDerivation(ref<Store> store, const Derivation& drv,
+Path writeDerivation(const ref<Store>& store, const Derivation& drv,
                      const string& name, RepairFlag repair) {
   PathSet references;
   references.insert(drv.inputSrcs.begin(), drv.inputSrcs.end());
@@ -355,7 +355,7 @@ Hash hashDerivationModulo(Store& store, Derivation drv) {
 }
 
 DrvPathWithOutputs parseDrvPathWithOutputs(const string& s) {
-  size_t n = s.find("!");
+  size_t n = s.find('!');
   return n == std::string::npos
              ? DrvPathWithOutputs(s, std::set<string>())
              : DrvPathWithOutputs(

third_party/nix/src/libstore/derivations.hh

@@ -67,7 +67,7 @@ struct Derivation : BasicDerivation {
 class Store;
 
 /* Write a derivation to the Nix store, and return its path. */
-Path writeDerivation(ref<Store> store, const Derivation& drv,
+Path writeDerivation(const ref<Store>& store, const Derivation& drv,
                      const string& name, RepairFlag repair = NoRepair);
 
 /* Read a derivation from a file. */

third_party/nix/src/libstore/download.cc

@@ -130,7 +130,7 @@ struct CurlDownloader : public Downloader {
       }
     }
 
-    void failEx(std::exception_ptr ex) {
+    void failEx(const std::exception_ptr& ex) {
       assert(!done);
       done = true;
       callback.rethrow(ex);
@@ -663,7 +663,7 @@ struct CurlDownloader : public Downloader {
     }
   }
 
-  void enqueueItem(std::shared_ptr<DownloadItem> item) {
+  void enqueueItem(const std::shared_ptr<DownloadItem>& item) {
     if (item->request.data && !hasPrefix(item->request.uri, "http://") &&
         !hasPrefix(item->request.uri, "https://")) {
       throw nix::Error("uploading to '%s' is not supported", item->request.uri);
@@ -858,7 +858,7 @@ void Downloader::download(DownloadRequest&& request, Sink& sink) {
 }
 
 CachedDownloadResult Downloader::downloadCached(
-    ref<Store> store, const CachedDownloadRequest& request) {
+    const ref<Store>& store, const CachedDownloadRequest& request) {
   auto url = resolveUri(request.uri);
 
   auto name = request.name;

third_party/nix/src/libstore/download.hh

@@ -108,7 +108,7 @@ struct Downloader {
      and is more recent than ‘tarball-ttl’ seconds. Otherwise,
      use the recorded ETag to verify if the server has a more
      recent version, and if so, download it to the Nix store. */
-  CachedDownloadResult downloadCached(ref<Store> store,
+  CachedDownloadResult downloadCached(const ref<Store>& store,
                                       const CachedDownloadRequest& request);
 
   enum Error { NotFound, Forbidden, Misc, Transient, Interrupted };

third_party/nix/src/libstore/export-import.cc

@@ -55,7 +55,8 @@ void Store::exportPath(const Path& path, Sink& sink) {
                    << 0;
 }
 
-Paths Store::importPaths(Source& source, std::shared_ptr<FSAccessor> accessor,
+Paths Store::importPaths(Source& source,
+                         const std::shared_ptr<FSAccessor>& accessor,
                          CheckSigsFlag checkSigs) {
   Paths res;
   while (true) {

third_party/nix/src/libstore/globals.cc

@@ -149,17 +149,18 @@ void BaseSetting<SandboxMode>::convertToArg(Args& args,
   args.mkFlag()
       .longName(name)
       .description("Enable sandboxing.")
-      .handler([=](std::vector<std::string> ss) { override(smEnabled); })
+      .handler([=](const std::vector<std::string>& ss) { override(smEnabled); })
       .category(category);
   args.mkFlag()
       .longName("no-" + name)
       .description("Disable sandboxing.")
-      .handler([=](std::vector<std::string> ss) { override(smDisabled); })
+      .handler(
+          [=](const std::vector<std::string>& ss) { override(smDisabled); })
       .category(category);
   args.mkFlag()
       .longName("relaxed-" + name)
       .description("Enable sandboxing, but allow builds to disable it.")
-      .handler([=](std::vector<std::string> ss) { override(smRelaxed); })
+      .handler([=](const std::vector<std::string>& ss) { override(smRelaxed); })
       .category(category);
 }
 

third_party/nix/src/libstore/local-fs-store.cc

@@ -12,7 +12,7 @@ LocalFSStore::LocalFSStore(const Params& params) : Store(params) {}
 struct LocalStoreAccessor : public FSAccessor {
   ref<LocalFSStore> store;
 
-  explicit LocalStoreAccessor(ref<LocalFSStore> store) : store(store) {}
+  explicit LocalStoreAccessor(const ref<LocalFSStore>& store) : store(store) {}
 
   Path toRealPath(const Path& path) {
     Path storePath = store->toStorePath(path);

third_party/nix/src/libstore/machines.cc

@@ -9,12 +9,12 @@
 
 namespace nix {
 
-Machine::Machine(decltype(storeUri) storeUri, decltype(systemTypes) systemTypes,
-                 decltype(sshKey) sshKey, decltype(maxJobs) maxJobs,
-                 decltype(speedFactor) speedFactor,
-                 decltype(supportedFeatures) supportedFeatures,
-                 decltype(mandatoryFeatures) mandatoryFeatures,
-                 decltype(sshPublicHostKey) sshPublicHostKey)
+Machine::Machine(decltype(storeUri)& storeUri,
+                 decltype(systemTypes)& systemTypes, decltype(sshKey)& sshKey,
+                 decltype(maxJobs) maxJobs, decltype(speedFactor) speedFactor,
+                 decltype(supportedFeatures)& supportedFeatures,
+                 decltype(mandatoryFeatures)& mandatoryFeatures,
+                 decltype(sshPublicHostKey)& sshPublicHostKey)
     : storeUri(
           // Backwards compatibility: if the URI is a hostname,
           // prepend ssh://.

third_party/nix/src/libstore/machines.hh

@@ -19,12 +19,12 @@ struct Machine {
 
   bool mandatoryMet(const std::set<string>& features) const;
 
-  Machine(decltype(storeUri) storeUri, decltype(systemTypes) systemTypes,
-          decltype(sshKey) sshKey, decltype(maxJobs) maxJobs,
+  Machine(decltype(storeUri)& storeUri, decltype(systemTypes)& systemTypes,
+          decltype(sshKey)& sshKey, decltype(maxJobs) maxJobs,
           decltype(speedFactor) speedFactor,
-          decltype(supportedFeatures) supportedFeatures,
-          decltype(mandatoryFeatures) mandatoryFeatures,
-          decltype(sshPublicHostKey) sshPublicHostKey);
+          decltype(supportedFeatures)& supportedFeatures,
+          decltype(mandatoryFeatures)& mandatoryFeatures,
+          decltype(sshPublicHostKey)& sshPublicHostKey);
 };
 
 typedef std::vector<Machine> Machines;

third_party/nix/src/libstore/misc.cc

@@ -171,8 +171,9 @@ void Store::queryMissing(const PathSet& targets, PathSet& willBuild_,
     }
   };
 
-  auto checkOutput = [&](const Path& drvPath, ref<Derivation> drv,
-                         const Path& outPath, ref<Sync<DrvState>> drvState_) {
+  auto checkOutput = [&](const Path& drvPath, const ref<Derivation>& drv,
+                         const Path& outPath,
+                         const ref<Sync<DrvState>>& drvState_) {
     if (drvState_->lock()->done) {
       return;
     }

third_party/nix/src/libstore/nar-accessor.cc

@@ -94,7 +94,7 @@ struct NarAccessor : public FSAccessor {
     }
   };
 
-  explicit NarAccessor(ref<const std::string> nar) : nar(nar) {
+  explicit NarAccessor(const ref<const std::string>& nar) : nar(nar) {
     NarIndexer indexer(*this, *nar);
     parseDump(indexer, indexer);
   }
@@ -111,7 +111,7 @@ struct NarAccessor : public FSAccessor {
       if (type == "directory") {
         member.type = FSAccessor::Type::tDirectory;
         for (auto i = v["entries"].begin(); i != v["entries"].end(); ++i) {
-          std::string name = i.key();
+          const std::string& name = i.key();
           recurse(member.children[name], i.value());
         }
       } else if (type == "regular") {
@@ -225,8 +225,8 @@ ref<FSAccessor> makeLazyNarAccessor(const std::string& listing,
   return make_ref<NarAccessor>(listing, getNarBytes);
 }
 
-void listNar(JSONPlaceholder& res, ref<FSAccessor> accessor, const Path& path,
-             bool recurse) {
+void listNar(JSONPlaceholder& res, const ref<FSAccessor>& accessor,
+             const Path& path, bool recurse) {
   auto st = accessor->stat(path);
 
   auto obj = res.object();

third_party/nix/src/libstore/nar-accessor.hh

@@ -23,7 +23,7 @@ class JSONPlaceholder;
 
 /* Write a JSON representation of the contents of a NAR (except file
    contents). */
-void listNar(JSONPlaceholder& res, ref<FSAccessor> accessor, const Path& path,
-             bool recurse);
+void listNar(JSONPlaceholder& res, const ref<FSAccessor>& accessor,
+             const Path& path, bool recurse);
 
 }  // namespace nix

third_party/nix/src/libstore/nar-info.cc

@@ -122,7 +122,7 @@ std::string NarInfo::to_string() const {
     res += "System: " + system + "\n";
   }
 
-  for (auto sig : sigs) {
+  for (const auto& sig : sigs) {
     res += "Sig: " + sig + "\n";
   }
 

third_party/nix/src/libstore/profiles.cc

@@ -35,7 +35,7 @@ static int parseName(const string& profileName, const string& name) {
   return -1;
 }
 
-Generations findGenerations(Path profile, int& curGen) {
+Generations findGenerations(const Path& profile, int& curGen) {
   Generations gens;
 
   Path profileDir = dirOf(profile);
@@ -68,7 +68,8 @@ static void makeName(const Path& profile, unsigned int num, Path& outLink) {
   outLink = prefix + "-link";
 }
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath) {
+Path createGeneration(const ref<LocalFSStore>& store, const Path& profile,
+                      const Path& outPath) {
   /* The new generation number should be higher than old the
      previous ones. */
   int dummy;
@@ -226,7 +227,7 @@ void deleteGenerationsOlderThan(const Path& profile, const string& timeSpec,
   deleteGenerationsOlderThan(profile, oldTime, dryRun);
 }
 
-void switchLink(Path link, Path target) {
+void switchLink(const Path& link, Path target) {
   /* Hacky. */
   if (dirOf(target) == dirOf(link)) {
     target = baseNameOf(target);

third_party/nix/src/libstore/profiles.hh

@@ -19,11 +19,12 @@ typedef list<Generation> Generations;
 
 /* Returns the list of currently present generations for the specified
    profile, sorted by generation number. */
-Generations findGenerations(Path profile, int& curGen);
+Generations findGenerations(const Path& profile, int& curGen);
 
 class LocalFSStore;
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath);
+Path createGeneration(const ref<LocalFSStore>& store, const Path& profile,
+                      const Path& outPath);
 
 void deleteGeneration(const Path& profile, unsigned int gen);
 
@@ -40,7 +41,7 @@ void deleteGenerationsOlderThan(const Path& profile, time_t t, bool dryRun);
 void deleteGenerationsOlderThan(const Path& profile, const string& timeSpec,
                                 bool dryRun);
 
-void switchLink(Path link, Path target);
+void switchLink(const Path& link, Path target);
 
 /* Ensure exclusive access to a profile.  Any command that modifies
    the profile first acquires this lock. */

third_party/nix/src/libstore/remote-fs-accessor.cc

@@ -9,7 +9,8 @@
 
 namespace nix {
 
-RemoteFSAccessor::RemoteFSAccessor(ref<Store> store, const Path& cacheDir)
+RemoteFSAccessor::RemoteFSAccessor(const ref<Store>& store,
+                                   const Path& cacheDir)
     : store(store), cacheDir(cacheDir) {
   if (!cacheDir.empty()) {
     createDirs(cacheDir);
@@ -23,7 +24,7 @@ Path RemoteFSAccessor::makeCacheFile(const Path& storePath,
 }
 
 void RemoteFSAccessor::addToCache(const Path& storePath, const std::string& nar,
-                                  ref<FSAccessor> narAccessor) {
+                                  const ref<FSAccessor>& narAccessor) {
   nars.emplace(storePath, narAccessor);
 
   if (!cacheDir.empty()) {

third_party/nix/src/libstore/remote-fs-accessor.hh

@@ -20,10 +20,10 @@ class RemoteFSAccessor : public FSAccessor {
   Path makeCacheFile(const Path& storePath, const std::string& ext);
 
   void addToCache(const Path& storePath, const std::string& nar,
-                  ref<FSAccessor> narAccessor);
+                  const ref<FSAccessor>& narAccessor);
 
  public:
-  RemoteFSAccessor(ref<Store> store,
+  RemoteFSAccessor(const ref<Store>& store,
                    const /* FIXME: use std::optional */ Path& cacheDir = "");
 
   Stat stat(const Path& path) override;

third_party/nix/src/libstore/store-api.cc

@@ -1,6 +1,7 @@
 #include "store-api.hh"
 
 #include <future>
+#include <utility>
 
 #include <glog/logging.h>
 
@@ -570,7 +571,7 @@ void Store::buildPaths(const PathSet& paths, BuildMode buildMode) {
   }
 }
 
-void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
+void copyStorePath(ref<Store> srcStore, const ref<Store>& dstStore,
                    const Path& storePath, RepairFlag repair,
                    CheckSigsFlag checkSigs) {
   auto srcUri = srcStore->getUri();
@@ -693,7 +694,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore,
       });
 }
 
-void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
+void copyClosure(const ref<Store>& srcStore, const ref<Store>& dstStore,
                  const PathSet& storePaths, RepairFlag repair,
                  CheckSigsFlag checkSigs, SubstituteFlag substitute) {
   PathSet closure;
@@ -824,14 +825,14 @@ void Store::addToStore(const ValidPathInfo& info, Source& narSource,
                        RepairFlag repair, CheckSigsFlag checkSigs,
                        std::shared_ptr<FSAccessor> accessor) {
   addToStore(info, make_ref<std::string>(narSource.drain()), repair, checkSigs,
-             accessor);
+             std::move(accessor));
 }
 
 void Store::addToStore(const ValidPathInfo& info, const ref<std::string>& nar,
                        RepairFlag repair, CheckSigsFlag checkSigs,
                        std::shared_ptr<FSAccessor> accessor) {
   StringSource source(*nar);
-  addToStore(info, source, repair, checkSigs, accessor);
+  addToStore(info, source, repair, checkSigs, std::move(accessor));
 }
 
 }  // namespace nix
@@ -851,7 +852,7 @@ std::pair<std::string, Store::Params> splitUriAndParams(
   Store::Params params;
   auto q = uri.find('?');
   if (q != std::string::npos) {
-    for (auto s : tokenizeString<Strings>(uri.substr(q + 1), "&")) {
+    for (const auto& s : tokenizeString<Strings>(uri.substr(q + 1), "&")) {
       auto e = s.find('=');
       if (e != std::string::npos) {
         auto value = s.substr(e + 1);
@@ -885,7 +886,7 @@ ref<Store> openStore(const std::string& uri_,
   auto params = extraParams;
   params.insert(uriParams.begin(), uriParams.end());
 
-  for (auto fun : *RegisterStoreImplementation::implementations) {
+  for (const auto& fun : *RegisterStoreImplementation::implementations) {
     auto store = fun(uri, params);
     if (store) {
       store->warnUnknownSettings();
@@ -952,11 +953,11 @@ std::list<ref<Store>> getDefaultSubstituters() {
       }
     };
 
-    for (auto uri : settings.substituters.get()) {
+    for (const auto& uri : settings.substituters.get()) {
       addStore(uri);
     }
 
-    for (auto uri : settings.extraSubstituters.get()) {
+    for (const auto& uri : settings.extraSubstituters.get()) {
       addStore(uri);
     }
 

third_party/nix/src/libstore/store-api.hh

@@ -564,7 +564,7 @@ class Store : public std::enable_shared_from_this<Store>, public Config {
      the Nix store. Optionally, the contents of the NARs are
      preloaded into the specified FS accessor to speed up subsequent
      access. */
-  Paths importPaths(Source& source, std::shared_ptr<FSAccessor> accessor,
+  Paths importPaths(Source& source, const std::shared_ptr<FSAccessor>& accessor,
                     CheckSigsFlag checkSigs = CheckSigs);
 
   struct Stats {
@@ -671,7 +671,7 @@ string storePathToHash(const Path& path);
 void checkStoreName(const string& name);
 
 /* Copy a path from one store to another. */
-void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
+void copyStorePath(ref<Store> srcStore, const ref<Store>& dstStore,
                    const Path& storePath, RepairFlag repair = NoRepair,
                    CheckSigsFlag checkSigs = CheckSigs);
 
@@ -686,7 +686,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore,
                SubstituteFlag substitute = NoSubstitute);
 
 /* Copy the closure of the specified paths from one store to another. */
-void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
+void copyClosure(const ref<Store>& srcStore, const ref<Store>& dstStore,
                  const PathSet& storePaths, RepairFlag repair = NoRepair,
                  CheckSigsFlag checkSigs = CheckSigs,
                  SubstituteFlag substitute = NoSubstitute);