Migrated concurrency module from Java to Kotiln (commons-app#6110)

* Rename .java to .kt

* Migrated concurrency module to Kotlin

Author: Saifuddin53

app/src/main/java/fr/free/nrw/commons/concurrency/BackgroundPoolExceptionHandler.java

@@ -0,0 +1,21 @@
+package fr.free.nrw.commons.concurrency
+
+import fr.free.nrw.commons.BuildConfig
+
+
+class BackgroundPoolExceptionHandler : ExceptionHandler {
+    /**
+     * If an exception occurs on a background thread, this handler will crash for debug builds
+     * but fail silently for release builds.
+     * @param t
+     */
+    override fun onException(t: Throwable) {
+        // Crash for debug build
+        if (BuildConfig.DEBUG) {
+            val thread = Thread {
+                throw RuntimeException(t)
+            }
+            thread.start()
+        }
+    }
+}

app/src/main/java/fr/free/nrw/commons/concurrency/BackgroundPoolExceptionHandler.kt

@@ -0,0 +1,40 @@
+package fr.free.nrw.commons.concurrency
+
+import java.util.concurrent.CancellationException
+import java.util.concurrent.ExecutionException
+import java.util.concurrent.Future
+import java.util.concurrent.ScheduledThreadPoolExecutor
+import java.util.concurrent.ThreadFactory
+
+
+class ExceptionAwareThreadPoolExecutor(
+    corePoolSize: Int,
+    threadFactory: ThreadFactory,
+    private val exceptionHandler: ExceptionHandler?
+) : ScheduledThreadPoolExecutor(corePoolSize, threadFactory) {
+
+    override fun afterExecute(r: Runnable, t: Throwable?) {
+        super.afterExecute(r, t)
+        var throwable = t
+
+        if (throwable == null && r is Future<*>) {
+            try {
+                if (r.isDone) {
+                    r.get()
+                }
+            } catch (e: CancellationException) {
+                // ignore
+            } catch (e: InterruptedException) {
+                // ignore
+            } catch (e: ExecutionException) {
+                throwable = e.cause ?: e
+            } catch (e: Exception) {
+                throwable = e
+            }
+        }
+
+        throwable?.let {
+            exceptionHandler?.onException(it)
+        }
+    }
+}

app/src/main/java/fr/free/nrw/commons/concurrency/ExceptionAwareThreadPoolExecutor.java

@@ -0,0 +1,7 @@
+package fr.free.nrw.commons.concurrency
+
+interface ExceptionHandler {
+
+    fun onException(t: Throwable)
+
+}

app/src/main/java/fr/free/nrw/commons/concurrency/ExceptionAwareThreadPoolExecutor.kt

@@ -0,0 +1,122 @@
+package fr.free.nrw.commons.concurrency
+
+import java.util.concurrent.Callable
+import java.util.concurrent.Executor
+import java.util.concurrent.ScheduledFuture
+import java.util.concurrent.ScheduledThreadPoolExecutor
+import java.util.concurrent.ThreadFactory
+import java.util.concurrent.TimeUnit
+
+
+/**
+ * This class is a thread pool which provides some additional features:
+ * - it sets the thread priority to a value lower than foreground priority by default, or you can
+ * supply your own priority
+ * - it gives you a way to handle exceptions thrown in the thread pool
+ */
+class ThreadPoolService private constructor(builder: Builder) : Executor {
+    private val backgroundPool: ScheduledThreadPoolExecutor = ExceptionAwareThreadPoolExecutor(
+        builder.poolSize,
+        object : ThreadFactory {
+            private var count = 0
+            override fun newThread(r: Runnable): Thread {
+                count++
+                val t = Thread(r, "${builder.name}-$count")
+                // If the priority is specified out of range, we set the thread priority to
+                // Thread.MIN_PRIORITY
+                // It's done to prevent IllegalArgumentException and to prevent setting of
+                // improper high priority for a less priority task
+                t.priority =
+                    if (
+                        builder.priority > Thread.MAX_PRIORITY
+                        ||
+                        builder.priority < Thread.MIN_PRIORITY
+                    ) {
+                    Thread.MIN_PRIORITY
+                } else {
+                    builder.priority
+                }
+                return t
+            }
+        },
+        builder.exceptionHandler
+    )
+
+    fun <V> schedule(callable: Callable<V>, time: Long, timeUnit: TimeUnit): ScheduledFuture<V> {
+        return backgroundPool.schedule(callable, time, timeUnit)
+    }
+
+    fun schedule(runnable: Runnable): ScheduledFuture<*> {
+        return schedule(runnable, 0, TimeUnit.SECONDS)
+    }
+
+    fun schedule(runnable: Runnable, time: Long, timeUnit: TimeUnit): ScheduledFuture<*> {
+        return backgroundPool.schedule(runnable, time, timeUnit)
+    }
+
+    fun scheduleAtFixedRate(
+        task: Runnable,
+        initialDelay: Long,
+        period: Long,
+        timeUnit: TimeUnit
+    ): ScheduledFuture<*> {
+        return backgroundPool.scheduleWithFixedDelay(task, initialDelay, period, timeUnit)
+    }
+
+    fun executor(): ScheduledThreadPoolExecutor {
+        return backgroundPool
+    }
+
+    fun shutdown() {
+        backgroundPool.shutdown()
+    }
+
+    override fun execute(command: Runnable) {
+        backgroundPool.execute(command)
+    }
+
+    /**
+     * Builder class for [ThreadPoolService]
+     */
+    class Builder(val name: String) {
+        var poolSize: Int = 1
+        var priority: Int = Thread.MIN_PRIORITY
+        var exceptionHandler: ExceptionHandler? = null
+
+        /**
+         * @param poolSize the number of threads to keep in the pool
+         * @throws IllegalArgumentException if size of pool <= 0
+         */
+        fun setPoolSize(poolSize: Int): Builder {
+            if (poolSize <= 0) {
+                throw IllegalArgumentException("Pool size must be greater than 0")
+            }
+            this.poolSize = poolSize
+            return this
+        }
+
+        /**
+         * @param priority Priority of the threads in the service. You can supply a constant from
+         *                 [java.lang.Thread] or
+         *                 specify your own priority in the range 1(MIN_PRIORITY)
+         *                 to 10(MAX_PRIORITY)
+         *                 By default, the priority is set to [java.lang.Thread.MIN_PRIORITY]
+         */
+        fun setPriority(priority: Int): Builder {
+            this.priority = priority
+            return this
+        }
+
+        /**
+         * @param handler The handler to use to handle exceptions in the service
+         */
+        fun setExceptionHandler(handler: ExceptionHandler): Builder {
+            exceptionHandler = handler
+            return this
+        }
+
+        fun build(): ThreadPoolService {
+            return ThreadPoolService(this)
+        }
+    }
+}