Respect output.hashSalt in RealContentHashPlugin

Fix #16788

Update RealContentHashPlugin to initialize hash instances with
the value of `output.hashSalt`, if provided.

lib/optimize/RealContentHashPlugin.js

@@ -363,6 +363,9 @@ ${referencingAssets
 						let newHash = hooks.updateHash.call(assetsContent, oldHash);
 						if (!newHash) {
 							const hash = createHash(this._hashFunction);
+							if (compilation.outputOptions.hashSalt) {
+								hash.update(compilation.outputOptions.hashSalt);
+							}
 							for (const content of assetsContent) {
 								hash.update(content);
 							}

test/configCases/contenthash/salt/index.js

@@ -0,0 +1,5 @@
+import img from "./img.jpg";
+
+it("should compile", () => {
+	expect(typeof img).toBe("string");
+});

test/configCases/contenthash/salt/test.config.js

@@ -0,0 +1,24 @@
+const findOutputFiles = require("../../../helpers/findOutputFiles");
+
+const allAssets = new Set();
+const allBundles = new Set();
+
+module.exports = {
+	findBundle: function(i, options) {
+		const bundle = findOutputFiles(options, new RegExp(`^bundle${i}`))[0];
+		allBundles.add(/\.([^.]+)\./.exec(bundle)[1]);
+
+		const assets = findOutputFiles(options, /^img/);
+		for (const asset of assets) {
+			allAssets.add(asset);
+		}
+
+		return `./${bundle}`;
+	},
+	afterExecute: () => {
+		// Since there are exactly 2 unique values of output.hashSalt,
+		// there should be exactly 2 unique output hashes for each file.
+		expect(allBundles.size).toBe(2);
+		expect(allAssets.size).toBe(2);
+	}
+};

test/configCases/contenthash/salt/webpack.config.js

@@ -0,0 +1,48 @@
+/** @type {import("../../../../").Configuration[]} */
+module.exports = [
+	{
+		output: {
+			filename: "bundle0.[contenthash].js",
+			assetModuleFilename: "[name].[contenthash][ext]",
+			hashSalt: "1"
+		},
+		module: {
+			rules: [
+				{
+					test: /\.jpg$/,
+					type: "asset/resource"
+				}
+			]
+		}
+	},
+	{
+		output: {
+			filename: "bundle1.[contenthash].js",
+			assetModuleFilename: "[name].[contenthash][ext]",
+			hashSalt: "1"
+		},
+		module: {
+			rules: [
+				{
+					test: /\.jpg$/,
+					type: "asset/resource"
+				}
+			]
+		}
+	},
+	{
+		output: {
+			filename: "bundle2.[contenthash].js",
+			assetModuleFilename: "[name].[contenthash][ext]",
+			hashSalt: "2"
+		},
+		module: {
+			rules: [
+				{
+					test: /\.jpg$/,
+					type: "asset/resource"
+				}
+			]
+		}
+	}
+];