Exclusive Cache Lock

okhttp/src/main/kotlin/okhttp3/internal/cache/CacheLock.kt

@@ -0,0 +1,44 @@
+package okhttp3.internal.cache
+
+import android.annotation.SuppressLint
+import java.io.Closeable
+import java.nio.channels.FileChannel
+import java.nio.file.StandardOpenOption
+import java.util.Collections
+import okhttp3.internal.platform.Platform
+import okio.FileSystem
+import okio.Path
+
+internal object CacheLock {
+  private val openCaches = Collections.synchronizedMap(mutableMapOf<Path, Exception>())
+
+  fun openLock(fileSystem: FileSystem, directory: Path): Closeable {
+    return if (fileSystem == FileSystem.SYSTEM && !Platform.isAndroid) {
+      fileSystemLock(directory)
+    } else {
+      inMemoryLock(directory)
+    }
+  }
+
+  @SuppressLint("NewApi")
+  fun inMemoryLock(directory: Path): Closeable {
+    // TODO solution for Android N?
+    val existing = openCaches.putIfAbsent(directory, Exception("CacheLock($directory)"))
+    if (existing != null) {
+      throw IllegalStateException("Cache already open at '$directory'", existing)
+    }
+    return okio.Closeable {
+      openCaches.remove(directory)
+    }
+  }
+
+  @SuppressLint("NewApi")
+  fun fileSystemLock(directory: Path): Closeable {
+    val lockFile = directory / "lock"
+    val channel = FileChannel.open(lockFile.toNioPath(), StandardOpenOption.APPEND)
+
+    return okio.Closeable {
+      channel.close()
+    }
+  }
+}

okhttp/src/main/kotlin/okhttp3/internal/cache/DiskLruCache.kt

@@ -20,6 +20,7 @@ import java.io.EOFException
 import java.io.Flushable
 import java.io.IOException
 import okhttp3.internal.assertThreadHoldsLock
+import okhttp3.internal.cache.CacheLock.openLock
 import okhttp3.internal.cache.DiskLruCache.Editor
 import okhttp3.internal.closeQuietly
 import okhttp3.internal.concurrent.Task
@@ -95,6 +96,8 @@ class DiskLruCache(
   /** Used for asynchronous journal rebuilds. */
   taskRunner: TaskRunner,
 ) : Closeable, Flushable {
+  lateinit var cacheLock: Closeable
+
   internal val fileSystem: FileSystem =
     object : ForwardingFileSystem(fileSystem) {
       override fun sink(
@@ -242,6 +245,8 @@ class DiskLruCache(
 
     civilizedFileSystem = fileSystem.isCivilized(journalFileBackup)
 
+    cacheLock = openLock(fileSystem, directory)
+
     // Prefer to pick up where we left off.
     if (fileSystem.exists(journalFile)) {
       try {
@@ -705,6 +710,8 @@ class DiskLruCache(
       return
     }
 
+    cacheLock.close()
+
     // Copying for concurrent iteration.
     for (entry in lruEntries.values.toTypedArray()) {
       if (entry.currentEditor != null) {