Fix readUtf8Line with multiple buffers

ktor-io/common/src/io/ktor/utils/io/ByteReadChannelOperations.kt

@@ -357,32 +357,42 @@ public suspend fun ByteReadChannel.discard(max: Long = Long.MAX_VALUE): Long {
     return max - remaining
 }
 
-@OptIn(InternalAPI::class)
+@OptIn(InternalAPI::class, InternalIoApi::class)
 public suspend fun ByteReadChannel.readUTF8LineTo(out: Appendable, max: Int): Boolean {
     if (isClosedForRead) return false
     if (readBuffer.exhausted()) awaitContent()
     if (isClosedForRead) return false
 
+    val buffer = Buffer()
     var cr = readBuffer.indexOf('\r'.code.toByte())
     var lf = readBuffer.indexOf('\n'.code.toByte())
     var searchEnd = readBuffer.remaining
-    while (cr < 0 && lf < 0 && readBuffer.remaining < max && awaitContent()) {
+    while (cr < 0 && lf < 0 && buffer.remaining + readBuffer.remaining <= max && !isClosedForRead) {
+        readBuffer.transferTo(buffer)
+        awaitContent()
+
         cr = readBuffer.indexOf('\r'.code.toByte(), searchEnd)
         lf = readBuffer.indexOf('\n'.code.toByte(), searchEnd)
-        searchEnd = readBuffer.remaining
+        searchEnd = buffer.remaining
     }
 
     if (cr < 0 && lf < 0) {
-        val count = minOf(max.toLong(), readBuffer.remaining)
-        out.append(readBuffer.readString(count))
+        if (max > buffer.remaining && !readBuffer.exhausted()) {
+            readBuffer.readTo(buffer, max - buffer.remaining)
+        }
+        out.append(buffer.readString())
     } else {
         val eol = when {
             cr >= 0 && lf >= 0 -> minOf(cr, lf)
             cr >= 0 -> cr
             else -> lf
         }
 
-        out.append(readBuffer.readString(eol))
+        val remaining = eol - buffer.remaining
+        if (remaining > 0) {
+            readBuffer.readTo(buffer, remaining)
+        }
+        out.append(buffer.readString())
         readBuffer.readByte()
         if (eol == cr && lf == cr + 1) readBuffer.readByte()
     }

ktor-io/common/test/ReadUtf8LineTest.kt

@@ -0,0 +1,27 @@
+import io.ktor.test.dispatcher.*
+import io.ktor.utils.io.*
+import kotlin.test.*
+
+/*
+ * Copyright 2014-2024 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+class ReadUtf8LineTest {
+
+    @Test
+    fun testReadUtf8LineWithLongLineWithLimit() = testSuspend {
+        val lineSize = 1024
+        val line = "A".repeat(lineSize)
+
+        val channel = writer {
+            repeat(10) {
+                channel.writeStringUtf8(line)
+            }
+        }.channel
+
+        val builder = StringBuilder()
+        channel.readUTF8LineTo(builder, 8 * 1024)
+        assertEquals(8 * 1024, builder.length)
+        assertEquals("A".repeat(8 * 1024), builder.toString())
+    }
+}