KTOR-8236: support for 'Vary' header (#4777)

* KTOR-8236: support for 'Vary' header

* KTOR-8236: resolve lint issues

* KTOR-8236: add tests for 'Vary' header

* KTOR-8236: remove check for null expectedVary header branch

Author: tre3p

ktor-http/common/src/io/ktor/http/content/CompressedContent.kt

@@ -46,6 +46,11 @@ private class CompressedReadChannelResponse(
         Headers.build {
             appendFiltered(original.headers) { name, _ -> !name.equals(HttpHeaders.ContentLength, true) }
             append(HttpHeaders.ContentEncoding, encoder.name)
+            append(
+                HttpHeaders.Vary,
+                original.headers[HttpHeaders.Vary]?.plus(", ${HttpHeaders.AcceptEncoding}")
+                    ?: HttpHeaders.AcceptEncoding
+            )
         }
     }
 
@@ -67,6 +72,11 @@ private class CompressedWriteChannelResponse(
         Headers.build {
             appendFiltered(original.headers) { name, _ -> !name.equals(HttpHeaders.ContentLength, true) }
             append(HttpHeaders.ContentEncoding, encoder.name)
+            append(
+                HttpHeaders.Vary,
+                original.headers[HttpHeaders.Vary]?.plus(", ${HttpHeaders.AcceptEncoding}")
+                    ?: HttpHeaders.AcceptEncoding
+            )
         }
     }
 

ktor-server/ktor-server-core/jvm/src/io/ktor/server/http/content/PreCompressed.kt

@@ -135,6 +135,10 @@ internal suspend fun ApplicationCall.respondStaticFile(
     suppressCompression()
     val compressedFile = File("${requestedFile.absolutePath}.${bestCompressionFit.extension}")
     if (cacheControlValues.isNotEmpty()) response.header(HttpHeaders.CacheControl, cacheControlValues)
+    response.header(
+        HttpHeaders.Vary,
+        response.headers[HttpHeaders.Vary]?.plus(", ${HttpHeaders.AcceptEncoding}") ?: HttpHeaders.AcceptEncoding
+    )
     modify(requestedFile, this)
     val localFileContent = LocalFileContent(compressedFile, contentType(requestedFile))
     respond(PreCompressedResponse(localFileContent, bestCompressionFit.encoding))
@@ -195,6 +199,10 @@ internal suspend fun ApplicationCall.respondStaticResource(
         suppressCompression()
         val cacheControlValues = cacheControl(bestCompressionFit.url).joinToString(", ")
         if (cacheControlValues.isNotEmpty()) response.header(HttpHeaders.CacheControl, cacheControlValues)
+        response.header(
+            HttpHeaders.Vary,
+            response.headers[HttpHeaders.Vary]?.plus(", ${HttpHeaders.AcceptEncoding}") ?: HttpHeaders.AcceptEncoding
+        )
         modifier(bestCompressionFit.url, this)
         respond(PreCompressedResponse(bestCompressionFit.content, bestCompressionFit.compression.encoding))
         return

ktor-server/ktor-server-tests/jvm/test/io/ktor/server/plugins/CompressionTest.kt

@@ -38,6 +38,19 @@ class CompressionTest {
     private val textToCompress = "text to be compressed\n".repeat(100)
     private val textToCompressAsBytes = textToCompress.encodeToByteArray()
 
+    @Test
+    fun testVaryHeaderPresent() = testApplication {
+        install(Compression)
+
+        routing {
+            get("/") {
+                call.respondText(textToCompress)
+            }
+        }
+
+        handleAndAssert("/", "gzip", "gzip", textToCompress, HttpHeaders.AcceptEncoding)
+    }
+
     @Test
     fun testCompressionNotSpecified() = testApplication {
         install(Compression)
@@ -837,7 +850,8 @@ class CompressionTest {
         url: String,
         acceptHeader: String?,
         expectedEncoding: String?,
-        expectedContent: String
+        expectedContent: String,
+        expectedVary: String? = null,
     ): HttpResponse {
         val response = client.get(url) {
             if (acceptHeader != null) {
@@ -872,6 +886,10 @@ class CompressionTest {
             assertNotNull(response.headers[HttpHeaders.ContentLength])
         }
 
+        expectedVary?.let {
+            assertEquals(response.headers[HttpHeaders.Vary], expectedVary)
+        }
+
         return response
     }
 

ktor-server/ktor-server-tests/jvm/test/io/ktor/server/plugins/StaticContentTest.kt

@@ -31,7 +31,26 @@ class StaticContentTest {
             .map { File(it, "io/ktor/server") }
             .first(File::exists)
 
-    private operator fun File.get(relativePath: String) = File(this, relativePath)
+    @Test
+    fun testVaryHeaderWithPreCompressedStaticResources() = testApplication {
+        routing {
+            staticResources("static", "public") {
+                preCompressed(CompressedFileType.BROTLI)
+            }
+        }
+
+        client.get("static/nested/file-nested.txt").let { response ->
+            assertNull(response.headers[HttpHeaders.Vary])
+        }
+
+        client.get("static/nested/file-nested.txt") {
+            headers {
+                append(HttpHeaders.AcceptEncoding, "br")
+            }
+        }.let { response ->
+            assertEquals(HttpHeaders.AcceptEncoding, response.headers[HttpHeaders.Vary])
+        }
+    }
 
     @Test
     fun testStaticContentBuilder() = testApplication {