fix: allow for recursive block-on calls

Fixes #798,#795,#760

Author: dignifiedquire

Diff for: Cargo.toml

@@ -75,7 +75,7 @@ futures-timer = { version = "3.0.2", optional = true }
 surf = { version = "1.0.3", optional = true }
 
 [target.'cfg(not(target_os = "unknown"))'.dependencies]
-smol = { version = "0.1.10", optional = true }
+smol = { version = "0.1.11", optional = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 futures-timer = { version = "3.0.2", optional = true, features = ["wasm-bindgen"] }
@@ -103,3 +103,4 @@ required-features = ["unstable"]
 [[example]]
 name = "surf-web"
 required-features = ["surf"]
+

Diff for: src/task/builder.rs

@@ -1,3 +1,4 @@
+use std::cell::Cell;
 use std::future::Future;
 use std::pin::Pin;
 use std::sync::Arc;
@@ -150,8 +151,31 @@ impl Builder {
             parent_task_id: TaskLocalsWrapper::get_current(|t| t.id().0).unwrap_or(0),
         });
 
+        thread_local! {
+            /// Tracks the number of nested block_on calls.
+            static NUM_NESTED_BLOCKING: Cell<usize> = Cell::new(0);
+        }
+
         // Run the future as a task.
-        unsafe { TaskLocalsWrapper::set_current(&wrapped.tag, || smol::run(wrapped)) }
+        NUM_NESTED_BLOCKING.with(|num_nested_blocking| {
+            let count = num_nested_blocking.get();
+            let should_run = count == 0;
+            // increase the count
+            num_nested_blocking.replace(count + 1);
+
+            unsafe {
+                TaskLocalsWrapper::set_current(&wrapped.tag, || {
+                    let res = if should_run {
+                        // The first call should use run.
+                        smol::run(wrapped)
+                    } else {
+                        smol::block_on(wrapped)
+                    };
+                    num_nested_blocking.replace(num_nested_blocking.get() - 1);
+                    res
+                })
+            }
+        })
     }
 }
 

Diff for: tests/block_on.rs

@@ -1,18 +1,63 @@
 #![cfg(not(target_os = "unknown"))]
 
-use async_std::task;
+use async_std::{future::ready, task::block_on};
 
 #[test]
 fn smoke() {
-    let res = task::block_on(async { 1 + 2 });
+    let res = block_on(async { 1 + 2 });
     assert_eq!(res, 3);
 }
 
 #[test]
 #[should_panic = "boom"]
 fn panic() {
-    task::block_on(async {
+    block_on(async {
         // This panic should get propagated into the parent thread.
         panic!("boom");
     });
 }
+
+#[cfg(feature = "unstable")]
+#[test]
+fn nested_block_on_local() {
+    use async_std::task::spawn_local;
+
+    let x = block_on(async {
+        let a = block_on(async { block_on(async { ready(3).await }) });
+        let b = spawn_local(async { block_on(async { ready(2).await }) }).await;
+        let c = block_on(async { block_on(async { ready(1).await }) });
+        a + b + c
+    });
+
+    assert_eq!(x, 3 + 2 + 1);
+
+    let y = block_on(async {
+        let a = block_on(async { block_on(async { ready(3).await }) });
+        let b = spawn_local(async { block_on(async { ready(2).await }) }).await;
+        let c = block_on(async { block_on(async { ready(1).await }) });
+        a + b + c
+    });
+
+    assert_eq!(y, 3 + 2 + 1);
+}
+
+#[test]
+fn nested_block_on() {
+    let x = block_on(async {
+        let a = block_on(async { block_on(async { ready(3).await }) });
+        let b = block_on(async { block_on(async { ready(2).await }) });
+        let c = block_on(async { block_on(async { ready(1).await }) });
+        a + b + c
+    });
+
+    assert_eq!(x, 3 + 2 + 1);
+
+    let y = block_on(async {
+        let a = block_on(async { block_on(async { ready(3).await }) });
+        let b = block_on(async { block_on(async { ready(2).await }) });
+        let c = block_on(async { block_on(async { ready(1).await }) });
+        a + b + c
+    });
+
+    assert_eq!(y, 3 + 2 + 1);
+}