daemon: Sacrifice builders on OOM.

* nix/libstore/build.cc (DerivationGoal::runChild):
Maximise our OOM score adjustment.

Change-Id: I418c763b499ca16e1ffe3c6033319112b9744f51

nix/libstore/build.cc

@@ -2154,6 +2154,12 @@ void DerivationGoal::runChild()
            determinism. */
         int cur = personality(0xffffffff);
         if (cur != -1) personality(cur | ADDR_NO_RANDOMIZE);
+
+	/* Ask the kernel to eagerly kill us & our children if it runs out of
+	   memory, regardless of blame, to preserve ‘real’ user data & state. */
+	try {
+	    writeFile("/proc/self/oom_score_adj", "1000"); // 100%
+	} catch (...) { ignoreException(); }
 #endif
 
         /* Fill in the environment. */