handle etags in WebMapTileServer

modules/events-server/src/main/kotlin/dev/adamko/kafkatorio/processor/tileserver/WebMapTileServer.kt

@@ -1,10 +1,16 @@
 package dev.adamko.kafkatorio.processor.tileserver
 
 import dev.adamko.kafkatorio.processor.config.ApplicationProperties
+import java.security.MessageDigest
 import kotlin.io.path.absolutePathString
+import org.http4k.core.Filter
 import org.http4k.core.HttpHandler
+import org.http4k.core.MemoryBody
+import org.http4k.core.Method.GET
+import org.http4k.core.Request
+import org.http4k.core.Response
+import org.http4k.core.Status
 import org.http4k.core.then
-import org.http4k.filter.CachingFilters
 import org.http4k.routing.ResourceLoader
 import org.http4k.routing.RoutingHttpHandler
 import org.http4k.routing.bind
@@ -12,15 +18,114 @@ import org.http4k.routing.routes
 import org.http4k.routing.static
 
 internal class WebMapTileServer(
-  appProps: ApplicationProperties = ApplicationProperties()
+  private val appProps: ApplicationProperties = ApplicationProperties()
 ) {
 
+  private val tilesLoader = ResourceLoader.Directory(appProps.webmapTileDir.absolutePathString())
+
+  private val filters = listOf(
+    ETagChecker()(),
+  ).reduce(Filter::then)
+
   private val routes: RoutingHttpHandler = routes(
-    "/tiles" bind CachingFilters.Response.NoCache()
-      .then(static(ResourceLoader.Directory(appProps.webmapTileDir.absolutePathString())))
+    "/tiles" bind GET to
+        static(tilesLoader).withFilter(filters)
   )
 
   fun build(): HttpHandler {
     return routes
   }
+
+
+  inner class ETagChecker
+//    : CoroutineScope
+  {
+
+//    private val tagState: MutableStateFlow<MutableMap<String, String>> =
+//      MutableStateFlow(mutableMapOf())
+//
+//    override val coroutineContext: CoroutineContext =
+//      CoroutineName("ETagChecker") +
+//          Dispatchers.Unconfined +
+//          KafkatorioTopology.rootJob
+
+//    init {
+//      launch {
+//        appProps.webmapTileDir
+//          .fileEventsFlow()
+//          .filterIsInstance<KWatchEvent.PathEvent>()
+//          .filter { it.path.isRegularFile() }
+////          .filter { it.path.extension == "png" }
+//          .runningFold(mutableMapOf<String, String>()) { tags, event ->
+////            println("[WebMapTileServer] handling file update ${event.path}")
+//
+//            val relativePath = event.path
+//              .relativeTo(appProps.webmapTileDir)
+//              .invariantSeparatorsPathString
+//
+//            when (event) {
+//              is KWatchEvent.Deleted  -> tags.remove(relativePath)
+//              is KWatchEvent.Created,
+//              is KWatchEvent.Modified -> {
+//                val md5 = event.path.md5()
+//                if (md5 != null) {
+//                  tags[relativePath] = md5
+//                } else {
+//                  println("null md5 for ${event.path}")
+//                }
+//              }
+//            }
+//
+//            tags
+//          }.cancellable()
+//          .collect(tagState)
+//      }
+//    }
+
+
+    private val Request.etagKey: String get() = uri.path.removePrefix("/tiles/")
+
+
+    operator fun invoke() = Filter { next ->
+      { request ->
+        val requestedETag = request.header("If-None-Match")
+
+        var response = next(request)
+        val responseBytes = response.body.stream.readAllBytes()
+        response.body.stream.close()
+        val currentETag = responseBytes.md5()
+
+        response = response.body(MemoryBody(responseBytes))
+
+        when {
+          currentETag == null          -> {
+//            println("[${request.etagKey}] currentETag is null (requested: $requestedETag), executing request")
+            response
+              .header("Cache-Control", "no-cache")
+          }
+          requestedETag == currentETag -> {
+            println("[${request.etagKey}] currentETag == requested, returning 302 $requestedETag")
+            Response(Status.NOT_MODIFIED)
+              .header("Cache-Control", "no-cache")
+          }
+          else                         -> {
+            println("[${request.etagKey}] currentETag != requested, executing request and adding currentETag:$currentETag (requested:$requestedETag)")
+            response
+              .header("ETag", currentETag)
+              .header("Cache-Control", "public, max-age=1, stale-while-revalidate=60, stale-if-error=300")
+          }
+        }
+      }
+    }
+
+    private fun ByteArray.md5(): String? {
+      return if (isEmpty()) {
+        null
+      } else {
+        MessageDigest.getInstance("MD5")
+          .digest(this)
+          .joinToString("") { "%02x".format(it) }
+      }
+    }
+  }
 }