Fix readAvailableTo indexes; Add test

Author: e5l

core/commonMain/src/kotlinx/io/Input.kt

@@ -151,11 +151,13 @@ public abstract class Input : Closeable {
      * If no bytes are available in the input, [fill] method will be called directly on
      * the [destination] buffer without an extra copy.
      * Otherwise, available bytes are copied to the destination.
+     *
+     * @return number of bytes written in the [destination].
      */
     public fun readAvailableTo(
         destination: Buffer,
         startIndex: Int = 0,
-        endIndex: Int = destination.size - startIndex
+        endIndex: Int = destination.size
     ): Int {
         checkBufferAndIndexes(destination, startIndex, endIndex)
 

core/commonTest/src/kotlinx/io/InputOutputTest.common.kt

@@ -1,3 +1,4 @@
+@file:Suppress("FORBIDDEN_IDENTITY_EQUALS")
 package kotlinx.io
 
 import kotlinx.io.buffer.*
@@ -138,6 +139,26 @@ class InputOutputTest {
         assertArrayEquals(content.sliceArray(0 until size), output.toByteArray())
     }
 
+    @Test
+    fun testReadAvailableToRange() {
+        var executed = false
+        val input: Input = object : Input() {
+            override fun fill(buffer: Buffer, startIndex: Int, endIndex: Int): Int {
+                assertEquals(1024, endIndex)
+                executed = true
+                return endIndex - startIndex
+            }
+
+            override fun closeSource() {
+            }
+
+        }
+        val buffer = bufferOf(ByteArray(1024))
+        val end = input.readAvailableTo(buffer, 1)
+        assertTrue(executed)
+        assertEquals(1023, end)
+    }
+
     private fun checkException(block: () -> Unit) {
         var fail = false
         try {