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+/* SPDX-License-Identifier: LGPL-3.0-or-later */
+/*
+ * threadpool.h
+ *
+ * Copyright (C) 2021 David Oberhollenzer <goliath@infraroot.at>
+ */
+#ifndef THREADPOOL_H
+#define THREADPOOL_H
+
+#include "sqfs/predef.h"
+
+typedef int (*thread_pool_worker_t)(void *user, void *work_item);
+
+/**
+ * @struct thread_pool_t
+ *
+ * @brief A thread pool with a ticket number based work item ordering.
+ *
+ * While the order in which items are non-deterministic, the thread pool
+ * implementation internally uses a ticket system to ensure the completed
+ * items are deqeueued in the same order that they were enqueued.
+ */
+typedef struct thread_pool_t {
+ /**
+ * @brief Shutdown and destroy a thread pool.
+ *
+ * @param pool A pointer to a pool returned by thread_pool_create
+ */
+ void (*destroy)(struct thread_pool_t *pool);
+
+ /**
+ * @brief Get the actual number of worker threads available.
+ *
+ * @return A number greater or equal to 1.
+ */
+ size_t (*get_worker_count)(struct thread_pool_t *pool);
+
+ /**
+ * @brief Change the user data pointer for a thread pool worker
+ * by index.
+ *
+ * @param idx A zero-based index into the worker list.
+ * @param ptr A user pointer that this specific worker thread should
+ * pass to the worker callback.
+ */
+ void (*set_worker_ptr)(struct thread_pool_t *pool, size_t idx,
+ void *ptr);
+
+ /**
+ * @brief Submit a work item to a thread pool.
+ *
+ * This function will fail on allocation failure or if the internal
+ * error state is set was set by one of the workers.
+ *
+ * @param ptr A pointer to a work object to enqueue.
+ *
+ * @return Zero on success.
+ */
+ int (*submit)(struct thread_pool_t *pool, void *ptr);
+
+ /**
+ * @brief Wait for a work item to be completed.
+ *
+ * This function dequeues a single completed work item. It may block
+ * until one of the worker threads signals completion of an additional
+ * item.
+ *
+ * This function guarantees to return the items in the same order as
+ * they were submitted, so the function can actually block longer than
+ * necessary, because it has to wait until the next item in sequence
+ * is finished.
+ *
+ * @return A pointer to a new work item or NULL if there are none
+ * in the pipeline.
+ */
+ void *(*dequeue)(struct thread_pool_t *pool);
+
+ /**
+ * @brief Get the internal worker return status value.
+ *
+ * If the worker functions returns a non-zero exit status in one of the
+ * worker threads, the thread pool stors the value internally and shuts
+ * down. This function can be used to retrieve the value.
+ *
+ * @return A non-zero value returned by the worker callback or zero if
+ * everything is A-OK.
+ */
+ int (*get_status)(struct thread_pool_t *pool);
+} thread_pool_t;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Create a thread pool instance.
+ *
+ * @param num_jobs The number of worker threads to launch.
+ * @param worker A function to call from the worker threads to process
+ * the work items.
+ *
+ * @return A pointer to a thread pool on success, NULL on failure.
+ */
+SQFS_INTERNAL thread_pool_t *thread_pool_create(size_t num_jobs,
+ thread_pool_worker_t worker);
+
+/**
+ * @brief Create a serial mockup thread pool implementation.
+ *
+ * This returns a @ref thread_pool_t implementation that, instead of running a
+ * thread pool actually does the work in-situ when dequeueing.
+ *
+ * @param worker A function to call from the worker threads to process
+ * the work items.
+ *
+ * @return A pointer to a thread pool on success, NULL on failure.
+ */
+SQFS_INTERNAL
+thread_pool_t *thread_pool_create_serial(thread_pool_worker_t worker);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* THREADPOOL_H */