Warn if PostgreSQL appears stalled

src/hydra-queue-runner/counter.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <atomic>
+#include <functional>
 
 typedef std::atomic<unsigned long> counter;
 
@@ -8,5 +9,9 @@ struct MaintainCount
 {
     counter & c;
     MaintainCount(counter & c) : c(c) { c++; }
+    MaintainCount(counter & c, std::function<void(unsigned long)> warn) : c(c)
+    {
+        warn(++c);
+    }
     ~MaintainCount() { auto prev = c--; assert(prev); }
 };