doc: update zlib doc

Just some general improvements to zlib docs and examples

Signed-off-by: James M Snell <[email protected]>

Author: jasnell

doc/api/zlib.md

@@ -4,60 +4,122 @@
 
 > Stability: 2 - Stable
 
-The `zlib` module provides compression functionality implemented using Gzip and
-Deflate/Inflate, as well as Brotli. It can be accessed using:
+The `zlib` module provides compression functionality implemented using Gzip,
+Deflate/Inflate, and Brotli.
+
+To access it:
 
 ```js
 const zlib = require('zlib');
 ```
 
+Compression and decompression are built around the Node.js [Streams API][].
+
 Compressing or decompressing a stream (such as a file) can be accomplished by
-piping the source stream data through a `zlib` stream into a destination stream:
+piping the source stream through a `zlib` `Transform` stream into a destination
+stream:
 
 ```js
-const gzip = zlib.createGzip();
-const fs = require('fs');
-const inp = fs.createReadStream('input.txt');
-const out = fs.createWriteStream('input.txt.gz');
-
-inp.pipe(gzip)
-  .on('error', () => {
-    // handle error
-  })
-  .pipe(out)
-  .on('error', () => {
-    // handle error
+const { createGzip } = require('zlib');
+const { pipeline } = require('stream');
+const {
+  createReadStream,
+  createWriteStream
+} = require('fs');
+
+const gzip = createGzip();
+const source = createReadStream('input.txt');
+const destination = createWriteStream('input.txt.gz');
+
+pipeline(source, gzip, destination, (err) => {
+  if (err) {
+    console.error('An error occurred:', err);
+    process.exitCode = 1;
+  }
+});
+
+// Or, Promisified
+
+const { promisify } = require('util');
+const pipe = promisify(pipeline);
+
+async function do_gzip(input, output) {
+  const gzip = createGzip();
+  const source = createReadStream(input);
+  const destination = createWriteStream(output);
+  await pipe(source, gzip, destination);
+}
+
+do_gzip('input.txt', 'input.txt.gz')
+  .catch((err) => {
+    console.error('An error occurred:', err);
+    process.exitCode = 1;
   });
 ```
 
 It is also possible to compress or decompress data in a single step:
 
 ```js
+const { deflate, unzip } = require('zlib');
+
 const input = '.................................';
-zlib.deflate(input, (err, buffer) => {
-  if (!err) {
-    console.log(buffer.toString('base64'));
-  } else {
-    // handle error
+deflate(input, (err, buffer) => {
+  if (err) {
+    console.error('An error occurred:', err);
+    process.exitCode = 1;
   }
+  console.log(buffer.toString('base64'));
 });
 
 const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
-zlib.unzip(buffer, (err, buffer) => {
-  if (!err) {
-    console.log(buffer.toString());
-  } else {
-    // handle error
+unzip(buffer, (err, buffer) => {
+  if (err) {
+    console.error('An error occurred:', err);
+    process.exitCode = 1;
   }
+  console.log(buffer.toString());
 });
+
+// Or, Promisified
+
+const { promisify } = require('util');
+const do_unzip = promisify(unzip);
+
+do_unzip(buffer)
+  .then((buf) => console.log(buf.toString()))
+  .catch((err) => {
+    console.error('An error occurred:', err);
+    process.exitCode = 1;
+  });
+```
+
+## Threadpool Usage and Performance Considerations
+
+All `zlib` APIs, except those that are explicitly synchronous, use the Node.js
+internal threadpool. This can lead to surprising effects and performance
+limitations in some applications.
+
+Creating and using a large number of zlib objects simultaneously can cause
+significant memory fragmentation.
+
+
+```js
+const zlib = require('zlib');
+
+const payload = Buffer.from('This is some data');
+
+// WARNING: DO NOT DO THIS!
+for (let i = 0; i < 30000; ++i) {
+  zlib.deflate(payload, (err, buffer) => {});
+}
 ```
 
-## Threadpool Usage
+In the preceding example, 30,000 deflate instances are created concurrently. Because
+of how some operating systems handle memory allocation and deallocation,
+this may lead to to significant memory fragmentation.
 
-All zlib APIs, except those that are explicitly synchronous, use libuv's
-threadpool. This can lead to surprising effects in some applications, such as
-subpar performance (which can be mitigated by adjusting the [pool size][])
-and/or unrecoverable and catastrophic memory fragmentation.
+It is strongly recommended that the results of compression
+operations be cached to avoid duplication of effort.
 
 ## Compressing HTTP requests and responses
 
@@ -80,26 +142,35 @@ tradeoffs involved in `zlib` usage.
 const zlib = require('zlib');
 const http = require('http');
 const fs = require('fs');
+const { pipeline } = require('stream');
+
 const request = http.get({ host: 'example.com',
                            path: '/',
                            port: 80,
                            headers: { 'Accept-Encoding': 'br,gzip,deflate' } });
 request.on('response', (response) => {
   const output = fs.createWriteStream('example.com_index.html');
 
+  const onError = (err) => {
+    if (err) {
+      console.error('An error occurred:', err);
+      process.exitCode = 1;
+    }
+  };
+
   switch (response.headers['content-encoding']) {
     case 'br':
-      response.pipe(zlib.createBrotliDecompress()).pipe(output);
+      pipeline(response, zlib.createBrotliDecompress(), output, onError);
       break;
     // Or, just use zlib.createUnzip() to handle both of the following cases:
     case 'gzip':
-      response.pipe(zlib.createGunzip()).pipe(output);
+      pipeline(response, zlib.createGunzip(), output, onError);
       break;
     case 'deflate':
-      response.pipe(zlib.createInflate()).pipe(output);
+      pipeline(response, zlib.createInflate(), outout, onError);
       break;
     default:
-      response.pipe(output);
+      pipeline(response, output, onError);
       break;
   }
 });
@@ -112,6 +183,8 @@ request.on('response', (response) => {
 const zlib = require('zlib');
 const http = require('http');
 const fs = require('fs');
+const { pipeline } = require('stream');
+
 http.createServer((request, response) => {
   const raw = fs.createReadStream('index.html');
   // Store both a compressed and an uncompressed version of the resource.
@@ -121,20 +194,32 @@ http.createServer((request, response) => {
     acceptEncoding = '';
   }
 
+  const onError = (err) => {
+    if (err) {
+      // If an error occurs, there's not much we can do because
+      // the server has already sent the 200 response code and
+      // some amount of data has already been sent to the client.
+      // The best we can do is terminate the response immediately
+      // and log the error.
+      response.end();
+      console.error('An error occurred:', err);
+    }
+  };
+
   // Note: This is not a conformant accept-encoding parser.
   // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
   if (/\bdeflate\b/.test(acceptEncoding)) {
     response.writeHead(200, { 'Content-Encoding': 'deflate' });
-    raw.pipe(zlib.createDeflate()).pipe(response);
+    pipeline(raw, zlib.createDeflate(), response, onError);
   } else if (/\bgzip\b/.test(acceptEncoding)) {
     response.writeHead(200, { 'Content-Encoding': 'gzip' });
-    raw.pipe(zlib.createGzip()).pipe(response);
+    pipeline(raw, zlib.createGzip(), response, onError);
   } else if (/\bbr\b/.test(acceptEncoding)) {
     response.writeHead(200, { 'Content-Encoding': 'br' });
-    raw.pipe(zlib.createBrotliCompress()).pipe(response);
+    pipeline(raw, zlib.createBrotliCompress(), response, onError);
   } else {
     response.writeHead(200, {});
-    raw.pipe(response);
+    pipeline(raw, response, onError);
   }
 }).listen(1337);
 ```
@@ -154,11 +239,11 @@ zlib.unzip(
   // For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.
   { finishFlush: zlib.constants.Z_SYNC_FLUSH },
   (err, buffer) => {
-    if (!err) {
-      console.log(buffer.toString());
-    } else {
-      // handle error
+    if (err) {
+      console.error('An error occurred:', err);
+      process.exitCode = 1;
     }
+    console.log(buffer.toString());
   });
 ```
 
@@ -234,14 +319,28 @@ HTTP response to the client:
 ```js
 const zlib = require('zlib');
 const http = require('http');
+const { pipeline } = require('stream');
 
 http.createServer((request, response) => {
   // For the sake of simplicity, the Accept-Encoding checks are omitted.
   response.writeHead(200, { 'content-encoding': 'gzip' });
   const output = zlib.createGzip();
-  output.pipe(response);
+  let i;
+
+  pipeline(output, response, (err) => {
+    if (err) {
+      // If an error occurs, there's not much we can do because
+      // the server has already sent the 200 response code and
+      // some amount of data has already been sent to the client.
+      // The best we can do is terminate the response immediately
+      // and log the error.
+      clearInterval(i);
+      response.end();
+      console.error('An error occurred:', err);
+    }
+  });
 
-  setInterval(() => {
+  i = setInterval(() => {
     output.write(`The current time is ${Date()}\n`, () => {
       // The data has been passed to zlib, but the compression algorithm may
       // have decided to buffer the data for more efficient compression.
@@ -399,7 +498,7 @@ changes:
 
 <!--type=misc-->
 
-Each zlib-based class takes an `options` object. All options are optional.
+Each zlib-based class takes an `options` object. No options are required.
 
 Some options are only relevant when compressing and are
 ignored by the decompression classes.
@@ -1059,5 +1158,6 @@ Decompress a chunk of data with [`Unzip`][].
 [Memory Usage Tuning]: #zlib_memory_usage_tuning
 [RFC 7932]: https://www.rfc-editor.org/rfc/rfc7932.txt
 [pool size]: cli.html#cli_uv_threadpool_size_size
+[Streams API]: stream.md
 [zlib documentation]: https://zlib.net/manual.html#Constants
 [zlib.createGzip example]: #zlib_zlib