libstore: split PathLocks::lockPaths into lock, tryLock

Change-Id: I202cfc077b7e468c74ecd8039175cef34855944c

lix/libstore/build/derivation-goal.cc

@@ -734,7 +734,7 @@ retry:
         }
     }
 
-    if (!outputLocks.lockPaths(lockFiles, "", false)) {
+    if (!outputLocks.tryLockPaths(lockFiles)) {
         if (!actLock)
             actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
                 fmt("waiting for lock on %s", Magenta(showPaths(lockFiles))));

lix/libstore/pathlocks.cc

@@ -113,7 +113,7 @@ PathLocks::PathLocks(const PathSet & paths, const std::string & waitMsg)
 }
 
 
-bool PathLocks::lockPaths(const PathSet & paths,
+bool PathLocks::lockPathsImpl(const PathSet & paths,
     const std::string & waitMsg, bool wait)
 {
     assert(fds.empty());

lix/libstore/pathlocks.hh

@@ -39,13 +39,19 @@ private:
     std::list<FDPair> fds;
     bool deletePaths;
 
+    bool lockPathsImpl(const PathSet & _paths, const std::string & waitMsg, bool wait);
+
 public:
     PathLocks();
-    PathLocks(const PathSet & paths,
-        const std::string & waitMsg = "");
-    bool lockPaths(const PathSet & _paths,
-        const std::string & waitMsg = "",
-        bool wait = true);
+    PathLocks(const PathSet & paths, const std::string & waitMsg = "");
+    void lockPaths(const PathSet & _paths, const std::string & waitMsg = "")
+    {
+        lockPathsImpl(_paths, waitMsg, true);
+    }
+    bool tryLockPaths(const PathSet & _paths)
+    {
+        return lockPathsImpl(_paths, "", false);
+    }
     ~PathLocks();
     void unlock();
     void setDeletion(bool deletePaths);