Filter the Accept-Charset header

app/src/androidTest/java/at/bitfire/icsdroid/MockServer.kt

@@ -21,8 +21,14 @@ object MockServer {
 
     private val queue = mutableListOf<Response>()
 
+    var lastRequestHeaders: Headers? = null
+        private set
+
     val createMockEngine: (CustomCertManager, SSLContext) -> HttpClientEngine = { _, _ ->
-        MockEngine {
+        MockEngine { request ->
+            // record headers from the incoming request
+            lastRequestHeaders = request.headers
+
             if (queue.isNotEmpty()) {
                 val response = lock.withLock { queue.removeAt(0) }
                 respond(response.content, response.status, response.headers)
@@ -34,6 +40,7 @@ object MockServer {
 
     fun clear() {
         queue.clear()
+        lastRequestHeaders = null
     }
 
     private fun enqueue(response: Response) {

app/src/androidTest/java/at/bitfire/icsdroid/TestAppHttpClient.kt

@@ -0,0 +1,57 @@
+package at.bitfire.icsdroid
+
+import android.content.Context
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import io.ktor.client.request.get
+import io.ktor.client.statement.HttpResponse
+import io.ktor.http.HttpHeaders
+import io.ktor.http.HttpStatusCode
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+class TestAppHttpClient {
+
+    private val context = ApplicationProvider.getApplicationContext<Context>()
+    private lateinit var client: AppHttpClient
+
+    @Before
+    fun setUp() {
+        MockServer.clear()
+        client = MockServer.httpClient(context)
+    }
+
+    @After
+    fun tearDown() {
+        MockServer.clear()
+    }
+
+    // Verifies that no Accept-Charset header is sent by default
+    @Test
+    fun request_doesNotContainAcceptCharsetHeader() = runBlocking {
+        // enqueue a simple 200 response
+        MockServer.enqueue(content = "ok", status = HttpStatusCode.OK)
+
+        // perform a GET request to the mock server
+        val uri = MockServer.uri("test")
+        val response: HttpResponse = client.httpClient.get(uri.toString())
+
+        assertEquals(HttpStatusCode.OK, response.status)
+
+        // retrieve the headers recorded by the mock server
+        val headers = MockServer.lastRequestHeaders
+
+        // Ensure headers were recorded
+        assertNotNull(headers)
+
+        // Assert that Accept-Charset header is not present
+        assertFalse(headers!!.contains(HttpHeaders.AcceptCharset))
+    }
+}

app/src/main/java/at/bitfire/icsdroid/AppHttpClient.kt

@@ -17,7 +17,10 @@ import io.ktor.client.engine.HttpClientEngine
 import io.ktor.client.engine.okhttp.OkHttp
 import io.ktor.client.plugins.HttpTimeout
 import io.ktor.client.plugins.UserAgent
+import io.ktor.client.plugins.api.Send
+import io.ktor.client.plugins.api.createClientPlugin
 import io.ktor.client.plugins.cookies.HttpCookies
+import io.ktor.http.HttpHeaders
 import okhttp3.brotli.BrotliInterceptor
 import okhttp3.internal.tls.OkHostnameVerifier
 import javax.net.ssl.SSLContext
@@ -85,6 +88,15 @@ class AppHttpClient @AssistedInject constructor(
         // Enable cookie storage - in memory, will be lost on app restart
         install(HttpCookies)
 
+        // Some servers have issues with the Accept-Charset header. It is actually deprecated/not-recommended by RFC 9110 §12.5.2.
+        // Ktor adds it by default, so we need to manually strip it with a custom plugin.
+        install(createClientPlugin("RemoveAcceptCharsetHeader") {
+            on(Send) { request ->
+                request.headers.remove(HttpHeaders.AcceptCharset)
+                proceed(request)
+            }
+        })
+
         // Disable redirect following, it's handled by CalendarFetcher
         followRedirects = false
     }