threadshare/runtime: Add a dummy context when blocking the current thread

This allows downstreams to function as if a normal context thread is
operating upstream apart from not being able to spawn new tasks.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/issues/96

gst-plugin-threadshare/src/runtime/executor.rs

@@ -77,28 +77,86 @@ tokio::task_local! {
     static CURRENT_TASK_ID: TaskId;
 }
 
+/// Blocks on `future` in one way or another if possible.
+///
+/// IO & time related `Future`s must be handled within their own [`Context`].
+/// Wait for the result using a [`JoinHandle`] or a `channel`.
+///
+/// If there's currently an active `Context` with a task, then the future is only queued up as a
+/// pending sub task for that task.
+///
+/// Otherwise the current thread is blocking and the passed in future is executed.
+///
+/// Note that you must not pass any futures here that wait for the currently active task in one way
+/// or another as this would deadlock!
+pub fn block_on_or_add_sub_task<Fut: Future + Send + 'static>(future: Fut) -> Option<Fut::Output> {
+    if let Some((cur_context, cur_task_id)) = Context::current_task() {
+        gst_debug!(
+            RUNTIME_CAT,
+            "Adding subtask to task {:?} on context {}",
+            cur_task_id,
+            cur_context.name()
+        );
+        let _ = Context::add_sub_task(async move {
+            future.await;
+            Ok(())
+        });
+        return None;
+    }
+
+    // Not running in a Context thread so we can block
+    Some(block_on(future))
+}
+
 /// Blocks on `future`.
 ///
 /// IO & time related `Future`s must be handled within their own [`Context`].
 /// Wait for the result using a [`JoinHandle`] or a `channel`.
 ///
-/// This function must NOT be called within a [`Context`] thread.
+/// The current thread is blocking and the passed in future is executed.
-/// The reason is this would prevent any task operating on the
-/// [`Context`] from making progress.
 ///
 /// # Panics
 ///
 /// This function panics if called within a [`Context`] thread.
-///
-/// [`Context`]: struct.Context.html
-/// [`JoinHandle`]: enum.JoinHandle.html
 pub fn block_on<Fut: Future>(future: Fut) -> Fut::Output {
-    if Context::is_context_thread() {
+    assert!(!Context::is_context_thread());
-        panic!("Attempt to `block_on` within a `Context` thread");
-    }
 
     // Not running in a Context thread so we can block
-    futures::executor::block_on(future)
+    gst_debug!(RUNTIME_CAT, "Blocking on new dummy context");
+
+    let context = Context(Arc::new(ContextInner {
+        real: None,
+        task_queues: Mutex::new((0, HashMap::new())),
+    }));
+
+    CURRENT_THREAD_CONTEXT.with(move |cur_ctx| {
+        *cur_ctx.borrow_mut() = Some(context.downgrade());
+
+        let res = futures::executor::block_on(async move {
+            let res = CURRENT_TASK_ID
+                .scope(TaskId(0), async move {
+                    let task_id = CURRENT_TASK_ID.try_with(|task_id| *task_id).ok();
+                    assert_eq!(task_id, Some(TaskId(0)));
+
+                    let res = future.await;
+
+                    while Context::current_has_sub_tasks() {
+                        if Context::drain_sub_tasks().await.is_err() {
+                            break;
+                        }
+                    }
+
+                    res
+                })
+                .await;
+
+            res
+        });
+
+        *cur_ctx.borrow_mut() = None;
+
+        res
+    })
 }
 
 struct ContextThread {
@@ -114,7 +172,15 @@ impl ContextThread {
         });
 
         let context = context_receiver.recv().expect("Context thread init failed");
-        *context.0.shutdown.join.lock().unwrap() = Some(join);
+        *context
+            .0
+            .real
+            .as_ref()
+            .unwrap()
+            .shutdown
+            .join
+            .lock()
+            .unwrap() = Some(join);
 
         context
     }
@@ -139,9 +205,11 @@ impl ContextThread {
         };
 
         let context = Context(Arc::new(ContextInner {
+            real: Some(ContextRealInner {
                 name: self.name.clone(),
                 handle: Mutex::new(runtime.handle().clone()),
                 shutdown,
+            }),
             task_queues: Mutex::new((0, HashMap::new())),
         }));
 
@@ -279,19 +347,27 @@ impl<T> fmt::Debug for JoinHandle<T> {
 }
 
 #[derive(Debug)]
-struct ContextInner {
+struct ContextRealInner {
     name: String,
     handle: Mutex<tokio::runtime::Handle>,
     // Only used for dropping
     shutdown: ContextShutdown,
+}
+
+#[derive(Debug)]
+struct ContextInner {
+    // Otherwise a dummy context
+    real: Option<ContextRealInner>,
     task_queues: Mutex<(u64, HashMap<u64, SubTaskQueue>)>,
 }
 
 impl Drop for ContextInner {
     fn drop(&mut self) {
+        if let Some(ref real) = self.real {
             let mut contexts = CONTEXTS.lock().unwrap();
-        gst_debug!(RUNTIME_CAT, "Finalizing context '{}'", self.name);
+            gst_debug!(RUNTIME_CAT, "Finalizing context '{}'", real.name);
-        contexts.remove(&self.name);
+            contexts.remove(&real.name);
+        }
     }
 }
 
@@ -329,11 +405,17 @@ impl Eq for Context {}
 
 impl Context {
     pub fn acquire(context_name: &str, wait: u32) -> Result<Self, io::Error> {
+        assert_ne!(context_name, "DUMMY");
+
         let mut contexts = CONTEXTS.lock().unwrap();
 
         if let Some(inner_weak) = contexts.get(context_name) {
             if let Some(inner_strong) = inner_weak.upgrade() {
-                gst_debug!(RUNTIME_CAT, "Joining Context '{}'", inner_strong.name);
+                gst_debug!(
+                    RUNTIME_CAT,
+                    "Joining Context '{}'",
+                    inner_strong.real.as_ref().unwrap().name
+                );
                 return Ok(Context(inner_strong));
             }
         }
@@ -341,7 +423,11 @@ impl Context {
         let context = ContextThread::start(context_name, wait);
         contexts.insert(context_name.into(), Arc::downgrade(&context.0));
 
-        gst_debug!(RUNTIME_CAT, "New Context '{}'", context.0.name);
+        gst_debug!(
+            RUNTIME_CAT,
+            "New Context '{}'",
+            context.0.real.as_ref().unwrap().name
+        );
         Ok(context)
     }
 
@@ -350,7 +436,10 @@ impl Context {
     }
 
     pub fn name(&self) -> &str {
-        self.0.name.as_str()
+        match self.0.real {
+            Some(ref real) => real.name.as_str(),
+            None => "DUMMY",
+        }
     }
 
     /// Returns `true` if a `Context` is running on current thread.
@@ -376,7 +465,7 @@ impl Context {
                 .as_ref()
                 .and_then(|ctx_weak| ctx_weak.upgrade())
                 .and_then(|ctx| {
-                    let task_id = ctx.enter(|| CURRENT_TASK_ID.try_with(|task_id| *task_id).ok());
+                    let task_id = CURRENT_TASK_ID.try_with(|task_id| *task_id).ok();
 
                     task_id.map(move |task_id| (ctx, task_id))
                 })
@@ -387,7 +476,12 @@ impl Context {
     where
         F: FnOnce() -> R,
     {
-        self.0.handle.lock().unwrap().enter(f)
+        let real = match self.0.real {
+            Some(ref real) => real,
+            None => panic!("Can't enter on dummy context"),
+        };
+
+        real.handle.lock().unwrap().enter(f)
     }
 
     pub fn spawn<Fut>(&self, future: Fut) -> JoinHandle<Fut::Output>
@@ -395,6 +489,11 @@ impl Context {
         Fut: Future + Send + 'static,
         Fut::Output: Send + 'static,
     {
+        let real = match self.0.real {
+            Some(ref real) => real,
+            None => panic!("Can't spawn new tasks on dummy context"),
+        };
+
         let mut task_queues = self.0.task_queues.lock().unwrap();
         let id = task_queues.0;
         task_queues.0 += 1;
@@ -405,16 +504,18 @@ impl Context {
             RUNTIME_CAT,
             "Spawning new task {:?} on context {}",
             id,
-            self.0.name
+            real.name
         );
 
-        let join_handle = self.0.handle.lock().unwrap().spawn(async move {
+        let join_handle = real.handle.lock().unwrap().spawn(async move {
             let ctx = Context::current().unwrap();
+            let real = ctx.0.real.as_ref().unwrap();
+
             gst_trace!(
                 RUNTIME_CAT,
                 "Running task {:?} on context {}",
                 id,
-                ctx.0.name
+                real.name
             );
             let res = CURRENT_TASK_ID.scope(id, future).await;
 
@@ -428,14 +529,14 @@ impl Context {
                             RUNTIME_CAT,
                             "Task {:?} on context {} has {} pending sub tasks",
                             id,
-                            ctx.0.name,
+                            real.name,
                             l
                         );
                     }
                 }
             }
 
-            gst_trace!(RUNTIME_CAT, "Task {:?} on context {} done", id, ctx.0.name);
+            gst_trace!(RUNTIME_CAT, "Task {:?} on context {} done", id, real.name);
 
             res
         });
@@ -479,12 +580,20 @@ impl Context {
         let mut task_queues = ctx.0.task_queues.lock().unwrap();
         match task_queues.1.get_mut(&task_id.0) {
             Some(task_queue) => {
+                if let Some(ref real) = ctx.0.real {
                     gst_trace!(
                         RUNTIME_CAT,
                         "Adding subtask to {:?} on context {}",
                         task_id,
-                    ctx.0.name
+                        real.name
                     );
+                } else {
+                    gst_trace!(
+                        RUNTIME_CAT,
+                        "Adding subtask to {:?} on dummy context",
+                        task_id,
+                    );
+                }
                 task_queue.0.push_back(sub_task.boxed());
                 Ok(())
             }
@@ -519,7 +628,12 @@ impl Context {
             }
         };
 
-        let name = self.0.name.clone();
+        let name = self
+            .0
+            .real
+            .as_ref()
+            .map(|r| r.name.clone())
+            .unwrap_or_else(|| String::from("DUMMY"));
         async move {
             if !task_queue.0.is_empty() {
                 gst_log!(

gst-plugin-threadshare/src/runtime/pad.rs

@@ -83,7 +83,7 @@ use std::marker::PhantomData;
 use std::sync;
 use std::sync::{Arc, Weak};
 
-use super::executor::Context;
+use super::executor::{block_on_or_add_sub_task, Context};
 use super::task::Task;
 use super::RUNTIME_CAT;
 
@@ -986,7 +986,8 @@ impl<'a> PadSinkRef<'a> {
             // Note: we don't use `crate::runtime::executor::block_on` here
             // because `Context::is_context_thread()` is checked in the `if`
             // statement above.
-            futures::executor::block_on(fut.map(|res| res.map(|_| gst::FlowSuccess::Ok)))
+            block_on_or_add_sub_task(fut.map(|res| res.map(|_| gst::FlowSuccess::Ok)))
+                .unwrap_or(Ok(gst::FlowSuccess::Ok))
         } else {
             Ok(gst::FlowSuccess::Ok)
         }

gst-plugin-threadshare/src/runtime/task.rs

@@ -27,6 +27,7 @@ use gst::{gst_debug, gst_log, gst_trace, gst_warning};
 use std::fmt;
 use std::sync::{Arc, Mutex};
 
+use super::executor::block_on;
 use super::{Context, JoinHandle, RUNTIME_CAT};
 
 #[derive(Clone, Debug, Eq, PartialEq)]
@@ -209,7 +210,7 @@ impl Task {
                         cur_task_id,
                         cur_context.name()
                     );
-                    let _ = futures::executor::block_on(prepare_handle);
+                    let _ = block_on(prepare_handle);
                 }
             } else {
                 gst_debug!(
@@ -217,7 +218,7 @@ impl Task {
                     "Synchronously waiting for task {:?}",
                     prepare_handle
                 );
-                let _ = futures::executor::block_on(prepare_handle);
+                let _ = block_on(prepare_handle);
             }
         }
 
@@ -422,7 +423,7 @@ impl Task {
                         cur_task_id,
                         cur_context.name()
                     );
-                    let _ = futures::executor::block_on(loop_handle);
+                    let _ = block_on(loop_handle);
                 }
             } else {
                 gst_debug!(
@@ -430,7 +431,7 @@ impl Task {
                     "Synchronously waiting for task {:?}",
                     loop_handle
                 );
-                let _ = futures::executor::block_on(loop_handle);
+                let _ = block_on(loop_handle);
             }
         }