KTOR-3757 Allow specifying custom CacheControl directives

ktor-http/api/ktor-http.api

@@ -7,7 +7,7 @@ public final class io/ktor/http/BadContentTypeFormatException : java/lang/Except
 }
 
 public abstract class io/ktor/http/CacheControl {
-	public synthetic fun <init> (Lio/ktor/http/CacheControl$Visibility;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public fun <init> (Lio/ktor/http/CacheControl$Visibility;)V
 	public final fun getVisibility ()Lio/ktor/http/CacheControl$Visibility;
 }
 

ktor-http/common/src/io/ktor/http/CacheControl.kt

@@ -9,7 +9,7 @@ package io.ktor.http
  *
  * @param visibility specifies an optional visibility such as private or public
  */
-public sealed class CacheControl(public val visibility: Visibility?) {
+public abstract class CacheControl(public val visibility: Visibility?) {
 
     /**
      * Controls caching by proxies

ktor-server/ktor-server-plugins/ktor-server-caching-headers/jvmAndNix/src/io/ktor/server/plugins/cachingheaders/CacheControlMerge.kt

@@ -12,8 +12,8 @@ import io.ktor.http.*
  * Currently, visibility is pinned to the expiration directive,
  * then to the first no-cache,
  * then to the no-store directive.
- * The RFC doesn't state, where a visibility modifier should be so we can place it at any place
- * so there is nothing behind the rule beyond, therefore could be changed.
+ * The RFC doesn't state where a visibility modifier should be, so we can place it at any place
+ * so there is nothing behind the rule beyond, therefore, could be changed.
  *
  * Only one visibility specifier is kept.
  *
@@ -26,7 +26,7 @@ import io.ktor.http.*
  *
  * Revalidation directives are collected as well.
  * Currently, revalidation directives are tied to max age by-design.
- * This is not fair according to RFC so will be changed in the future.
+ * This is not fair, according to RFC, so it will be changed in the future.
  */
 internal fun List<CacheControl>.mergeCacheControlDirectives(): List<CacheControl> {
     if (size < 2) return this
@@ -50,6 +50,12 @@ internal fun List<CacheControl>.mergeCacheControlDirectives(): List<CacheControl
         noCacheDirective?.let { add(CacheControl.NoCache(null)) }
         noStoreDirective?.let { add(CacheControl.NoStore(null)) }
 
+        this@mergeCacheControlDirectives
+            .filter {
+                it !is CacheControl.NoCache && it !is CacheControl.NoStore && it !is CacheControl.MaxAge
+            }
+            .let { addAll(it) }
+
         if (maxAgeDirectives.isNotEmpty()) {
             add(
                 CacheControl.MaxAge(

ktor-server/ktor-server-tests/jvmAndNix/test/io/ktor/tests/server/plugins/CachingHeadersTest.kt

@@ -62,6 +62,26 @@ class CachingHeadersTest {
         }
     )
 
+    object Immutable: CacheControl(null) {
+        override fun toString(): String = "immutable"
+    }
+
+    @Test
+    fun testCustomCacheControl(): Unit = test(
+        configure = {
+            install(CachingHeaders) {
+                options { _, _ -> CachingOptions(CacheControl.NoStore(CacheControl.Visibility.Private)) }
+                options { _, _ -> CachingOptions(Immutable) }
+            }
+        },
+        test = { response ->
+            assertEquals(
+                "no-cache, private, no-store, immutable",
+                response.headers[HttpHeaders.CacheControl]
+            )
+        }
+    )
+
     @Test
     fun testSetInCall() = testApplication {
         install(CachingHeaders)