Static file caching for the Website service (#6)

This PR contains the work done to add cache-control headers to static file responses by configuring the `FileMiddleware` middleware to tag served static files with *Cache-Control* directives, tuned per media type.

To provide further details about the work done:

* Added a cache control that sets per-type policies: text assets (CSS/JS) get public, max-age, must-revalidate; images get public, max-age; everything else gets a default public, max-age.
* Files stay validated via `ETag/Last-Modified` header.
* Made the max-age values configurable, with defaults, via new cache constants.

Reviewed-on: rock-n-code/loud-amsterdam#6
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/App/App+build.swift

@@ -12,6 +12,20 @@ import WebsiteCore
 func application(
     reader: ConfigReader
 ) async -> some ApplicationProtocol {
+    let cacheControl = cacheControl(
+        textMaxAge: reader.int(
+            forKey: .Cache.maxAgeText,
+            default: .Cache.maxAgeText
+        ),
+        imageMaxAge: reader.int(
+            forKey: .Cache.maxAgeImage,
+            default: .Cache.maxAgeImage
+        ),
+        defaultMaxAge: reader.int(
+            forKey: .Cache.maxAgeDefault,
+            default: .Cache.maxAgeDefault
+        )
+    )
     let logLevel = reader.string(
         forKey: .Log.level,
         as: Logger.Level.self,
@@ -29,6 +43,7 @@ func application(
     return Application(
         router: router(
             staticFilesPath: staticFilesPath,
+            cacheControl: cacheControl,
             logLevel: logLevel
         ),
         configuration: ApplicationConfiguration(
@@ -46,6 +61,29 @@ func application(
 // Request context used by application
 private typealias AppRequestContext = BasicRequestContext
 
+/// Builds the cache-control policy applied to the served static files.
+///
+/// Static files are public and validated by `FileMiddleware` through their `ETag` and
+/// `Last-Modified` headers, so each media type is given a `max-age` after which the browser
+/// revalidates. Text-based assets (CSS, JavaScript) are additionally marked `must-revalidate`
+/// since they change between deployments while keeping their filenames.
+/// - Parameters:
+///   - textMaxAge: the max-age, in seconds, applied to text-based static files (CSS, JavaScript, plain text).
+///   - imageMaxAge: the max-age, in seconds, applied to image static files (ICO, PNG, SVG).
+///   - defaultMaxAge: the max-age, in seconds, applied to all other static files (e.g. the web manifest).
+/// - Returns: the configured cache-control policy.
+private func cacheControl(
+    textMaxAge: Int,
+    imageMaxAge: Int,
+    defaultMaxAge: Int
+) -> CacheControl {
+    .init([
+        (.text, [.public, .maxAge(textMaxAge), .mustRevalidate]),
+        (.image, [.public, .maxAge(imageMaxAge)]),
+        (.init(type: .any), [.public, .maxAge(defaultMaxAge)]),
+    ])
+}
+
 /// Builds the application's logger.
 /// - Parameters:
 ///   - serverName: the label applied to the logger.
@@ -65,14 +103,17 @@ private func logger(
 /// Builds the application's router.
 ///
 /// Registers the request-logging middleware, the not-found middleware that serves the error
-/// page, and the static file middleware that serves the contents of `staticFilesPath`, then
-/// adds the `RootController` routes that render the landing page.
+/// page, and the static file middleware that serves the contents of `staticFilesPath` (tagging
+/// responses with the given `cacheControl` directives), then adds the `RootController` routes
+/// that render the landing page.
 /// - Parameters:
 ///   - staticFilesPath: the folder, relative to the working directory, the static files are served from.
+///   - cacheControl: the cache-control directives applied to the served static files.
 ///   - logLevel: the level the request-logging middleware logs at.
 /// - Returns: the configured router.
 private func router(
     staticFilesPath: String,
+    cacheControl: CacheControl,
     logLevel: Logger.Level
 ) -> Router<AppRequestContext> {
     let router = Router(context: AppRequestContext.self)
@@ -80,7 +121,10 @@ private func router(
     router.addMiddleware {
         LogRequestsMiddleware(logLevel)
         NotFoundMiddleware()
-        FileMiddleware(staticFilesPath)
+        FileMiddleware(
+            staticFilesPath,
+            cacheControl: cacheControl
+        )
     }
 
     router.addRoutes(RootController<AppRequestContext>().routes)