zlib: fix decompression of empty data streams

add4b0a made the assumption that compressed data
would never lead to an empty decompressed stream.

Fix that by explicitly checking the number of read bytes.

Fixes: nodejs#17041
Refs: nodejs#13322

lib/zlib.js

@@ -95,6 +95,8 @@ function zlibBufferOnEnd() {
   var err;
   if (this.nread >= kMaxLength) {
     err = new errors.RangeError('ERR_BUFFER_TOO_LARGE');
+  } else if (this.nread === 0) {
+    buf = Buffer.alloc(0);
   } else {
     var bufs = this.buffers;
     buf = (bufs.length === 1 ? bufs[0] : Buffer.concat(bufs, this.nread));
@@ -485,6 +487,9 @@ function processChunkSync(self, chunk, flushFlag) {
 
   _close(self);
 
+  if (nread === 0)
+    return Buffer.alloc(0);
+
   return (buffers.length === 1 ? buffers[0] : Buffer.concat(buffers, nread));
 }
 

test/parallel/test-zlib-empty-buffer.js

@@ -0,0 +1,26 @@
+'use strict';
+const common = require('../common');
+const zlib = require('zlib');
+const { inspect, promisify } = require('util');
+const assert = require('assert');
+const emptyBuffer = new Buffer(0);
+
+common.crashOnUnhandledRejection();
+
+(async function() {
+  for (const [ compress, decompress, method ] of [
+    [ zlib.deflateRawSync, zlib.inflateRawSync, 'raw sync' ],
+    [ zlib.deflateSync, zlib.inflateSync, 'deflate sync' ],
+    [ zlib.gzipSync, zlib.gunzipSync, 'gzip sync' ],
+    [ promisify(zlib.deflateRaw), promisify(zlib.inflateRaw), 'raw' ],
+    [ promisify(zlib.deflate), promisify(zlib.inflate), 'deflate' ],
+    [ promisify(zlib.gzip), promisify(zlib.gunzip), 'gzip' ]
+  ]) {
+    const compressed = await compress(emptyBuffer);
+    const decompressed = await decompress(compressed);
+    assert.deepStrictEqual(
+      emptyBuffer, decompressed,
+      `Expected ${inspect(compressed)} to match ${inspect(decompressed)} ` +
+      `to match for ${method}`);
+  }
+})();