Support thread pools (#283)

Create a safe `ThreadPool` type wrapping `ZSTD_threadPool`. Allow
binding a `&ThreadPool` to a `CCtx`, which requires that the lifetime of
the `&ThreadPool` outlive the `CCtx`.

zstd-safe/src/lib.rs

@@ -804,6 +804,38 @@ impl<'a> CCtx<'a> {
         // Safety: Just FFI
         unsafe { zstd_sys::ZSTD_CStreamOutSize() }
     }
+
+    /// Use a shared thread pool for this context.
+    ///
+    /// Thread pool must outlive the context.
+    #[cfg(all(feature = "experimental", feature = "zstdmt"))]
+    #[cfg_attr(
+        feature = "doc-cfg",
+        doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+    )]
+    pub fn ref_thread_pool<'b>(&mut self, pool: &'b ThreadPool) -> SafeResult
+    where
+        'b: 'a,
+    {
+        parse_code(unsafe {
+            zstd_sys::ZSTD_CCtx_refThreadPool(self.0.as_ptr(), pool.0.as_ptr())
+        })
+    }
+
+    /// Return to using a private thread pool for this context.
+    #[cfg(all(feature = "experimental", feature = "zstdmt"))]
+    #[cfg_attr(
+        feature = "doc-cfg",
+        doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+    )]
+    pub fn disable_thread_pool(&mut self) -> SafeResult {
+        parse_code(unsafe {
+            zstd_sys::ZSTD_CCtx_refThreadPool(
+                self.0.as_ptr(),
+                core::ptr::null_mut(),
+            )
+        })
+    }
 }
 
 impl<'a> Drop for CCtx<'a> {
@@ -1401,6 +1433,64 @@ impl<'a> Drop for DDict<'a> {
 unsafe impl<'a> Send for DDict<'a> {}
 unsafe impl<'a> Sync for DDict<'a> {}
 
+/// A shared thread pool for one or more compression contexts
+#[cfg(all(feature = "experimental", feature = "zstdmt"))]
+#[cfg_attr(
+    feature = "doc-cfg",
+    doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+)]
+pub struct ThreadPool(NonNull<zstd_sys::ZSTD_threadPool>);
+
+#[cfg(all(feature = "experimental", feature = "zstdmt"))]
+#[cfg_attr(
+    feature = "doc-cfg",
+    doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+)]
+impl ThreadPool {
+    /// Create a thread pool with the specified number of threads.
+    ///
+    /// # Panics
+    ///
+    /// If creating the thread pool failed.
+    pub fn new(num_threads: usize) -> Self {
+        Self::try_new(num_threads)
+            .expect("zstd returned null pointer when creating thread pool")
+    }
+
+    /// Create a thread pool with the specified number of threads.
+    pub fn try_new(num_threads: usize) -> Option<Self> {
+        Some(Self(NonNull::new(unsafe {
+            zstd_sys::ZSTD_createThreadPool(num_threads)
+        })?))
+    }
+}
+
+#[cfg(all(feature = "experimental", feature = "zstdmt"))]
+#[cfg_attr(
+    feature = "doc-cfg",
+    doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+)]
+impl Drop for ThreadPool {
+    fn drop(&mut self) {
+        unsafe {
+            zstd_sys::ZSTD_freeThreadPool(self.0.as_ptr());
+        }
+    }
+}
+
+#[cfg(all(feature = "experimental", feature = "zstdmt"))]
+#[cfg_attr(
+    feature = "doc-cfg",
+    doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+)]
+unsafe impl Send for ThreadPool {}
+#[cfg(all(feature = "experimental", feature = "zstdmt"))]
+#[cfg_attr(
+    feature = "doc-cfg",
+    doc(cfg(all(feature = "experimental", feature = "zstdmt")))
+)]
+unsafe impl Sync for ThreadPool {}
+
 /// Wraps the `ZSTD_decompress_usingDDict()` function.
 pub fn decompress_using_ddict(
     dctx: &mut DCtx<'_>,