zlib: fix memory leak for unused zlib instances

src/node_zlib.cc

@@ -90,6 +90,7 @@ class ZCtx : public AsyncWrap, public ThreadPoolWork {
         refs_(0),
         gzip_id_bytes_read_(0),
         write_result_(nullptr) {
+    MakeWeak();
   }
 
 

test/parallel/test-zlib-unused-weak.js

@@ -0,0 +1,18 @@
+'use strict';
+// Flags: --expose-gc
+require('../common');
+const assert = require('assert');
+const zlib = require('zlib');
+
+// Tests that native zlib handles start out their life as weak handles.
+
+const before = process.memoryUsage().external;
+for (let i = 0; i < 100; ++i)
+  zlib.createGzip();
+const afterCreation = process.memoryUsage().external;
+global.gc();
+const afterGC = process.memoryUsage().external;
+
+assert((afterGC - before) / (afterCreation - before) <= 0.05,
+       `Expected after-GC delta ${afterGC - before} to be less than 5 %` +
+       ` of before-GC delta ${afterCreation - before}`);