Add automatic garbage collection

Nix can now automatically run the garbage collector during builds or
while adding paths to the store. The option "min-free = <bytes>"
specifies that Nix should run the garbage collector whenever free
space in the Nix store drops below <bytes>. It will then delete
garbage until "max-free" bytes are available.

Garbage collection during builds is asynchronous; running builds are
not paused and new builds are not blocked. However, there also is a
synchronous GC run prior to the first build/substitution.

Currently, no old GC roots are deleted (as in "nix-collect-garbage
-d").

src/libstore/local-store.hh

@@ -7,6 +7,8 @@
 #include "sync.hh"
 #include "util.hh"
 
+#include <chrono>
+#include <future>
 #include <string>
 #include <unordered_set>
 
@@ -60,6 +62,21 @@ private:
 
         /* The file to which we write our temporary roots. */
         AutoCloseFD fdTempRoots;
+
+        /* 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;
+
+        /* Whether auto-GC is running. If so, get gcFuture to wait for
+           the GC to finish. */
+        bool gcRunning = false;
+        std::shared_future<void> gcFuture;
+
+        /* How much disk space was available after the previous
+           auto-GC. If the current available disk space is below
+           minFree but not much below availAfterGC, then there is no
+           point in starting a new GC. */
+        uint64_t availAfterGC = std::numeric_limits<uint64_t>::max();
     };
 
     Sync<State, std::recursive_mutex> _state;
@@ -196,6 +213,10 @@ public:
 
     void addSignatures(const Path & storePath, const StringSet & sigs) override;
 
+    /* If free disk space in /nix/store if below minFree, delete
+       garbage until it exceeds maxFree. */
+    void autoGC(bool sync = true);
+
 private:
 
     int getSchema();