* A simple hack to fix NIX-18: the garbage collector cannot run when

the disk is full (because to delete something from the Nix store, we
  need a Berkeley DB transaction, which takes up disk space).  Under
  normal operation, we make sure that there exists a file
  /nix/var/nix/db/reserved of 1 MB.  When running the garbage
  collector, we delete that file before we open the Berkeley DB
  environment.

src/libstore/store.cc

@@ -76,10 +76,24 @@ static void upgradeStore07();
 static void upgradeStore09();
 
 
-void openDB()
+void openDB(bool reserveSpace)
 {
     if (readOnlyMode) return;
 
+    try {
+        Path reservedPath = nixDBPath + "/reserved";
+        off_t reservedSize = 1024 * 1024;
+        if (reserveSpace) {
+            struct stat st;
+            if (stat(reservedPath.c_str(), &st) == -1 ||
+                st.st_size != reservedSize)
+                writeFile(reservedPath, string(1024 * 1024, 'X'));
+        }
+        else
+            deletePath(reservedPath);
+    } catch (SysError & e) { /* don't care about errors */
+    }
+
     try {
         nixDB.open(nixDBPath);
     } catch (DbNoPermission & e) {

src/libstore/store.hh

@@ -37,8 +37,14 @@ struct Substitute
 typedef list<Substitute> Substitutes;
 
 
-/* Open the database environment. */
+/* Open the database environment.  If `reserveSpace' is true, make
-void openDB();
+   sure that a big empty file exists in /nix/var/nix/db/reserved.  If
+   `reserveSpace' is false, delete this file if it exists.  The idea
+   is that on normal operation, the file exists; but when we run the
+   garbage collector, it is deleted.  This is to ensure that the
+   garbage collector has a small amount of disk space available, which
+   is required to open the Berkeley DB environment. */
+void openDB(bool reserveSpace = true);
 
 /* Create the required database tables. */
 void initDB();

src/nix-store/main.cc

@@ -686,7 +686,7 @@ void run(Strings args)
     if (!op) throw UsageError("no operation specified");
 
     if (op != opDump && op != opRestore) /* !!! hack */
-        openDB();
+        openDB(op != opGC);
 
     op(opFlags, opArgs);
 }