Fix deadlock between auto-GC and addTempRoot()

Previously addTempRoot() acquired the LocalStore state lock and waited
for the garbage collector to reply. If the garbage collector is in the
same process (as it the case with auto-GC), this would deadlock as
soon as the garbage collector thread needs the LocalStore state lock.

So now addTempRoot() uses separate Syncs for the state that it
needs. As long at the auto-GC thread doesn't call addTempRoot() (which
it shouldn't), it shouldn't deadlock.

Fixes #3224.

src/libstore/local-store.hh

@@ -59,12 +59,6 @@ private:
         struct Stmts;
         std::unique_ptr<Stmts> stmts;
 
-        /* The global GC lock  */
-        AutoCloseFD fdGCLock;
-
-        /* Connection to the garbage collector. */
-        AutoCloseFD fdRootsSocket;
-
         /* The last time we checked whether to do an auto-GC, or an
            auto-GC finished. */
         std::chrono::time_point<std::chrono::steady_clock> lastGCCheck;
@@ -160,6 +154,12 @@ private:
     /* The file to which we write our temporary roots. */
     Sync<AutoCloseFD> _fdTempRoots;
 
+    /* The global GC lock. */
+    Sync<AutoCloseFD> _fdGCLock;
+
+    /* Connection to the garbage collector. */
+    Sync<AutoCloseFD> _fdRootsSocket;
+
 public:
 
     void addIndirectRoot(const Path & path) override;