feat(loader): add the disk cache option

packages/loader/src/cache.ts

@@ -0,0 +1,22 @@
+import { join } from 'node:path'
+import { SrcSetCacheStorage } from '@srcset/core'
+
+const storages = new Map<string, SrcSetCacheStorage>()
+
+/**
+ * Get the disk cache storage shared between loader runs.
+ * Every run of a compilation gets the same storage, so a variant
+ * generated for one module is a hit for the next one.
+ * @param context - Root context directory of the compiler.
+ * @returns Cache storage.
+ */
+export function getSharedCache(context: string) {
+  let storage = storages.get(context)
+
+  if (!storage) {
+    storage = new SrcSetCacheStorage(join(context, 'node_modules', '.cache', 'srcset'))
+    storages.set(context, storage)
+  }
+
+  return storage
+}

packages/loader/src/loader.spec.ts

@@ -1,7 +1,8 @@
 import {
   describe,
   it,
-  expect
+  expect,
+  vi
 } from 'vitest'
 import sharp from 'sharp'
 import webpack from 'webpack'
@@ -155,6 +156,48 @@ describe('loader', () => {
         expect(assets).toEqual(['image.jpg.webp'])
       })
 
+      it('should reuse the disk cache across builds when enabled', async () => {
+        const dir = await createFixtureProject(defaultEntry)
+        const optimize = vi.fn((contents: Buffer) => contents)
+        const options = {
+          cache: true,
+          optimization: {
+            jpg: optimize
+          }
+        }
+
+        await compile(createCompiler, dir, options)
+
+        const generatedCalls = optimize.mock.calls.length
+
+        expect(generatedCalls).toBeGreaterThan(0)
+
+        const { assets } = await compile(createCompiler, dir, options)
+
+        expect(optimize.mock.calls.length).toBe(generatedCalls)
+        expect(assets.length).toBeGreaterThan(0)
+      })
+
+      it('should regenerate without the cache option', async () => {
+        const dir = await createFixtureProject(defaultEntry)
+        const optimize = vi.fn((contents: Buffer) => contents)
+        const options = {
+          optimization: {
+            jpg: optimize
+          }
+        }
+
+        await compile(createCompiler, dir, options)
+
+        const generatedCalls = optimize.mock.calls.length
+
+        expect(generatedCalls).toBeGreaterThan(0)
+
+        await compile(createCompiler, dir, options)
+
+        expect(optimize.mock.calls.length).toBe(generatedCalls * 2)
+      })
+
       it('should export placeholder data-url when enabled', async () => {
         const dir = await createFixtureProject(defaultEntry)
         const { exports } = await compile(createCompiler, dir, {

packages/loader/src/loader.ts

@@ -14,17 +14,19 @@ import {
   resolvePublicPath
 } from './paths.ts'
 import { getSharedLimit } from './limit.ts'
+import { getSharedCache } from './cache.ts'
 
 async function generateModule(ctx: SrcSetLoaderContext, contents: Buffer) {
-  const options = ctx.getOptions()
   const {
     context = ctx.rootContext,
     emitFile = true,
     name = getDefaultName(ctx.mode),
+    cache = false,
     concurrency,
     outputPath,
-    publicPath
-  } = options
+    publicPath,
+    ...moduleOptions
+  } = ctx.getOptions()
   const source = {
     path: ctx.resourcePath,
     contents
@@ -56,7 +58,10 @@ async function generateModule(ctx: SrcSetLoaderContext, contents: Buffer) {
   return generateSrcSetModule(
     source,
     query,
-    options,
+    {
+      ...moduleOptions,
+      cache: cache ? getSharedCache(ctx.rootContext) : undefined
+    },
     emitImage,
     limit
   )

packages/loader/src/types.ts

@@ -49,7 +49,13 @@ export interface SrcSetLoaderContext {
   async(): (error?: Error | null, content?: string | Buffer) => void
 }
 
-export interface SrcSetLoaderOptions extends SrcSetModuleOptions {
+export interface SrcSetLoaderOptions extends Omit<SrcSetModuleOptions, 'cache'> {
+  /**
+   * Cache generated variants on disk, in `node_modules/.cache/srcset`:
+   * repeated builds skip the generation. Disabled by default - the
+   * persistent cache of the bundler covers it, when it is enabled.
+   */
+  cache?: boolean
   /**
    * Output file name template.
    * Supports `[name]`, `[postfix]`, `[ext]`, `[path]`, `[sourceext]` and