doc,test: update the v8.startupSnapshot doc and test the example

The API is now available to user-land run-time snapshots. So update
the example. This also makes the intention of the examples a bit
clearer and test it in our test suite.

doc/api/v8.md

@@ -921,15 +921,12 @@ added:
 > Stability: 1 - Experimental
 
 The `v8.startupSnapshot` interface can be used to add serialization and
-deserialization hooks for custom startup snapshots. Currently the startup
-snapshots can only be built into the Node.js binary from source.
+deserialization hooks for custom startup snapshots.
 
 ```console
-$ cd /path/to/node
-$ ./configure --node-snapshot-main=entry.js
-$ make node
-# This binary contains the result of the execution of entry.js
-$ out/Release/node
+$ node --snapshot-blob snapshot.blob --build-snapshot entry.js
+# This launches a process with the snapshot
+$ node --snapshot-blob snapshot.blob
 ```
 
 In the example above, `entry.js` can use methods from the `v8.startupSnapshot`
@@ -946,43 +943,63 @@ const zlib = require('node:zlib');
 const path = require('node:path');
 const assert = require('node:assert');
 
-const {
-  isBuildingSnapshot,
-  addSerializeCallback,
-  addDeserializeCallback,
-  setDeserializeMainFunction,
-} = require('node:v8').startupSnapshot;
+const v8 = require('node:v8');
 
-const filePath = path.resolve(__dirname, '../x1024.txt');
-const storage = {};
+class BookShelf {
+  storage = new Map();
 
-assert(isBuildingSnapshot());
+  // Reading a series of files from directory and store them into storage.
+  constructor(directory, books) {
+    for (const book of books) {
+      this.storage.set(book, fs.readFileSync(path.join(directory, book)));
+    }
+  }
 
-addSerializeCallback(({ filePath }) => {
-  storage[filePath] = zlib.gzipSync(fs.readFileSync(filePath));
-}, { filePath });
+  static compressAll(shelf) {
+    for (const [ book, content ] of shelf.storage) {
+      shelf.storage.set(book, zlib.gzipSync(content));
+    }
+  }
 
-addDeserializeCallback(({ filePath }) => {
-  storage[filePath] = zlib.gunzipSync(storage[filePath]);
-}, { filePath });
+  static decompressAll(shelf) {
+    for (const [ book, content ] of shelf.storage) {
+      shelf.storage.set(book, zlib.gunzipSync(content));
+    }
+  }
+}
 
-setDeserializeMainFunction(({ filePath }) => {
-  console.log(storage[filePath].toString());
-}, { filePath });
+// __dirname here is where the snapshot script is placed
+// during snapshot building time.
+const shelf = new BookShelf(__dirname, [
+  'book1.en_US.txt',
+  'book1.es_ES.txt',
+  'book2.zh_CN.txt',
+]);
+
+assert(v8.startupSnapshot.isBuildingSnapshot());
+// On snapshot serialization, compress the books to reduce size.
+v8.startupSnapshot.addSerializeCallback(BookShelf.compressAll, shelf);
+// On snapshot deserialization, decompress the books.
+v8.startupSnapshot.addDeserializeCallback(BookShelf.decompressAll, shelf);
+v8.startupSnapshot.setDeserializeMainFunction((shelf) => {
+  // process.env and process.argv are refreshed during snapshot
+  // deserialization.
+  const lang = process.env.BOOK_LANG || 'en_US';
+  const book = process.argv[1];
+  const name = `${book}.${lang}.txt`;
+  console.log(shelf.storage.get(name));
+}, shelf);
 ```
 
-The resulted binary will simply print the data deserialized from the snapshot
-during start up:
+The resulted binary will get print the data deserialized from the snapshot
+during start up, using the refreshed `process.env` and `process.argv` of
+the launched process:
 
 ```console
-$ out/Release/node
-# Prints content of ./test/fixtures/x1024.txt
+$ BOOK_LANG=es_ES node --snapshot-blob snapshot.blob book1
+# Prints content of book1.es_ES.txt deserialized from the snapshot.
 ```
 
-Currently the API is only available to a Node.js instance launched from the
-default snapshot, that is, the application deserialized from a user-land
-snapshot cannot use these APIs again.
-
 ### `v8.startupSnapshot.addSerializeCallback(callback[, data])`
 
 <!-- YAML

test/fixtures/snapshot/v8-startup-snapshot-api.js

@@ -1,32 +1,54 @@
 'use strict';
 
-const fs = require('fs');
-const zlib = require('zlib');
-const path = require('path');
-const assert = require('assert');
-
-const {
-  isBuildingSnapshot,
-  addSerializeCallback,
-  addDeserializeCallback,
-  setDeserializeMainFunction
-} = require('v8').startupSnapshot;
-
-const filePath = path.resolve(__dirname, '../x1024.txt');
-const storage = {};
-
-assert(isBuildingSnapshot());
-
-addSerializeCallback(({ filePath }) => {
-  console.error('serializing', filePath);
-  storage[filePath] = zlib.gzipSync(fs.readFileSync(filePath));
-}, { filePath });
-
-addDeserializeCallback(({ filePath }) => {
-  console.error('deserializing', filePath);
-  storage[filePath] = zlib.gunzipSync(storage[filePath]);
-}, { filePath });
-
-setDeserializeMainFunction(({ filePath }) => {
-  console.log(storage[filePath].toString());
-}, { filePath });
+const fs = require('node:fs');
+const zlib = require('node:zlib');
+const path = require('node:path');
+const assert = require('node:assert');
+
+const v8 = require('node:v8');
+
+class BookShelf {
+  storage = new Map();
+
+  // Reading a series of files from directory and store them into storage.
+  constructor(directory, books) {
+    for (const book of books) {
+      this.storage.set(book, fs.readFileSync(path.join(directory, book)));
+    };
+  }
+
+  static compressAll(shelf) {
+    for (const [ book, content ] of shelf.storage) {
+      shelf.storage.set(book, zlib.gzipSync(content));
+    }
+  }
+
+  static decompressAll(shelf) {
+    for (const [ book, content ] of shelf.storage) {
+      shelf.storage.set(book, zlib.gunzipSync(content));
+    }
+  }
+}
+
+// __dirname here is where the snapshot script is placed
+// during snapshot building time.
+const shelf = new BookShelf(__dirname, [
+  'book1.en_US.txt',
+  'book1.es_ES.txt',
+  'book2.zh_CN.txt',
+]);
+
+assert(v8.startupSnapshot.isBuildingSnapshot());
+// On snapshot serialization, compress the books to reduce size.
+v8.startupSnapshot.addSerializeCallback(BookShelf.compressAll, shelf);
+// On snapshot deserialization, decompress the books.
+v8.startupSnapshot.addDeserializeCallback(BookShelf.decompressAll, shelf);
+v8.startupSnapshot.setDeserializeMainFunction((shelf) => {
+  // process.env and process.argv are refreshed during snapshot
+  // deserialization.
+  const lang = process.env.BOOK_LANG || 'en_US';
+  const book = process.argv[1];
+  const name = `${book}.${lang}.txt`;
+  console.error('Reading', name);
+  console.log(shelf.storage.get(name).toString());
+}, shelf);

test/parallel/test-snapshot-api.js

@@ -1,6 +1,6 @@
 'use strict';
 
-// This tests snapshot JS API
+// This tests snapshot JS API using the example in the docs.
 
 require('../common');
 const assert = require('assert');
@@ -20,11 +20,20 @@ tmpdir.refresh();
 const blobPath = path.join(tmpdir.path, 'snapshot.blob');
 const entry = fixtures.path('snapshot', 'v8-startup-snapshot-api.js');
 {
+  for (const book of [
+    'book1.en_US.txt',
+    'book1.es_ES.txt',
+    'book2.zh_CN.txt',
+  ]) {
+    const content = `This is ${book}`;
+    fs.writeFileSync(path.join(tmpdir.path, book), content, 'utf8');
+  }
+  fs.copyFileSync(entry, path.join(tmpdir.path, 'entry.js'));
   const child = spawnSync(process.execPath, [
     '--snapshot-blob',
     blobPath,
     '--build-snapshot',
-    entry,
+    'entry.js',
   ], {
     cwd: tmpdir.path
   });
@@ -41,15 +50,18 @@ const entry = fixtures.path('snapshot', 'v8-startup-snapshot-api.js');
   const child = spawnSync(process.execPath, [
     '--snapshot-blob',
     blobPath,
+    'book1',
   ], {
     cwd: tmpdir.path,
     env: {
       ...process.env,
+      BOOK_LANG: 'en_US',
     }
   });
 
   const stdout = child.stdout.toString().trim();
-  const file = fs.readFileSync(fixtures.path('x1024.txt'), 'utf8');
-  assert.strictEqual(stdout, file);
+  const stderr = child.stderr.toString().trim();
+  assert.strictEqual(stderr, 'Reading book1.en_US.txt');
+  assert.strictEqual(stdout, 'This is book1.en_US.txt');
   assert.strictEqual(child.status, 0);
 }