zlib: fix memory leak for unused zlib instances

An oversight in an earlier commit led to a memory leak
in the untypical situation that zlib instances are created
but never used, because zlib handles no longer started
out their life as weak handles.

The bug was introduced in bd201102862a194f3f5ce669e0a3c8143eafc900.

Refs: https://github.com/nodejs/node/pull/20455

PR-URL: https://github.com/nodejs/node/pull/21607
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

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}`);