Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libstore/pathlocks.hh

@@ -0,0 +1,57 @@
+#pragma once
+///@file
+
+#include "file-descriptor.hh"
+
+namespace nix {
+
+/**
+ * Open (possibly create) a lock file and return the file descriptor.
+ * -1 is returned if create is false and the lock could not be opened
+ * because it doesn't exist.  Any other error throws an exception.
+ */
+AutoCloseFD openLockFile(const Path & path, bool create);
+
+/**
+ * Delete an open lock file.
+ */
+void deleteLockFile(const Path & path, Descriptor desc);
+
+enum LockType { ltRead, ltWrite, ltNone };
+
+bool lockFile(Descriptor desc, LockType lockType, bool wait);
+
+class PathLocks
+{
+private:
+    typedef std::pair<Descriptor, Path> FDPair;
+    std::list<FDPair> fds;
+    bool deletePaths;
+
+public:
+    PathLocks();
+    PathLocks(const PathSet & paths,
+        const std::string & waitMsg = "");
+    bool lockPaths(const PathSet & _paths,
+        const std::string & waitMsg = "",
+        bool wait = true);
+    ~PathLocks();
+    void unlock();
+    void setDeletion(bool deletePaths);
+};
+
+struct FdLock
+{
+    Descriptor desc;
+    bool acquired = false;
+
+    FdLock(Descriptor desc, LockType lockType, bool wait, std::string_view waitMsg);
+
+    ~FdLock()
+    {
+        if (acquired)
+            lockFile(desc, ltNone, false);
+    }
+};
+
+}