fix: lazily open random access files (#782)

Author: ianbotsf

.changes/0cc34a91-f764-49d8-b24e-b170973e1f9d.json

@@ -0,0 +1,8 @@
+{
+    "id": "0cc34a91-f764-49d8-b24e-b170973e1f9d",
+    "type": "bugfix",
+    "description": "Lazily open random access files to prevent exhausting file handles in highly concurrent scenarios",
+    "issues": [
+        "awslabs/smithy-kotlin#781"
+    ]
+}

runtime/io/jvm/src/aws/smithy/kotlin/runtime/io/RandomAccessFileSource.kt

@@ -13,33 +13,35 @@ import java.io.RandomAccessFile
 import kotlin.jvm.Throws
 
 internal class RandomAccessFileSource(
-    file: File,
+    private val fileObject: File,
     start: Long,
     private val endInclusive: Long,
     private val timeout: Timeout = Timeout.NONE,
 ) : okio.Source {
-    private val file = RandomAccessFile(file, "r")
-
-    init {
-        require(file.exists()) { "cannot create SdkSource, file does not exist: $this" }
-        require(file.isFile) { "cannot create a SdkSource from a directory: $this" }
+    private val file by lazy {
+        require(fileObject.exists()) { "cannot create SdkSource, file does not exist: $this" }
+        require(fileObject.isFile) { "cannot create a SdkSource from a directory: $this" }
         require(start >= 0L) { "start position should be >= 0, found $start" }
-        require(endInclusive >= 0 && endInclusive <= file.length() - 1) {
+        require(endInclusive >= 0 && endInclusive <= fileObject.length() - 1) {
             "endInclusive should be less than or equal to the length of the file, was $endInclusive"
         }
-        if (start > 0) {
-            this.file.seek(start)
+
+        RandomAccessFile(fileObject, "r").also {
+            if (start > 0) {
+                it.seek(start)
+            }
         }
     }
 
     private var position = start
-    private val channel = this.file.channel
 
-    override fun toString(): String = "RandomAccessFileSource($file)"
+    override fun toString(): String = "RandomAccessFileSource($fileObject)"
     override fun timeout(): Timeout = timeout
 
     @Throws(IOException::class)
     override fun read(sink: Buffer, byteCount: Long): Long {
+        val channel = file.channel
+
         check(channel.isOpen) { "channel is closed" }
         if (position > endInclusive) return -1
 
@@ -50,7 +52,6 @@ internal class RandomAccessFileSource(
     }
 
     override fun close() {
-        channel.close()
         file.close()
     }
 }

runtime/io/jvm/test/aws/smithy/kotlin/runtime/io/FileSourceTest.kt

@@ -7,24 +7,31 @@ package aws.smithy.kotlin.runtime.io
 
 import aws.smithy.kotlin.runtime.testing.RandomTempFile
 import io.kotest.matchers.string.shouldContain
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertFailsWith
 
+@OptIn(ExperimentalCoroutinesApi::class)
 class FileSourceTest {
     @Test
-    fun testStartPositionValidation() {
+    fun testStartPositionValidation() = runTest {
         val file = RandomTempFile(1024)
+        val source = file.source(-1)
         assertFailsWith<IllegalArgumentException> {
-            file.source(-1)
+            // Files are opened lazily...IAE won't be thrown until a read is attempted
+            source.readToByteArray()
         }.message.shouldContain("start position should be >= 0, found -1")
     }
 
     @Test
-    fun testEndPositionValidation() {
+    fun testEndPositionValidation() = runTest {
         val file = RandomTempFile(1024)
+        val source = file.source(endInclusive = 1024)
         assertFailsWith<IllegalArgumentException> {
-            file.source(endInclusive = 1024)
+            // Files are opened lazily...IAE won't be thrown until a read is attempted
+            source.readToByteArray()
         }.message.shouldContain("endInclusive should be less than or equal to the length of the file, was 1024")
 
         file.source(1023).close()