Fix from_thread.run(_sync?) not setting sniffio on asyncio

This also fixes a deadlock when using start_blocking_portal("asyncio")
on a non-asyncio async backend known to sniffio.

Fixes agronholm#523.

docs/versionhistory.rst

@@ -44,6 +44,15 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
   the event loop to be closed
 - Fixed ``current_effective_deadline()`` not returning ``-inf`` on asyncio when the
   currently active cancel scope has been cancelled (PR by Ganden Schaffner)
+- Fixed ``from_thread.run`` and ``from_thread.run_sync`` not setting sniffio on asyncio.
+  As a result:
+
+  - Fixed ``from_thread.run_sync`` failing when used to call sniffio-dependent functions
+    on asyncio
+  - Fixed ``from_thread.run`` failing when used to call sniffio-dependent functions on
+    asyncio from a thread running trio or curio
+  - Fixed deadlock when using ``from_thread.start_blocking_portal(backend="asyncio")``
+    in a thread running trio or curio (PR by Ganden Schaffner)
 
 **3.6.1**
 

src/anyio/_backends/_asyncio.py

@@ -2035,8 +2035,10 @@ def run_async_from_thread(
         token: object,
     ) -> T_Retval:
         loop = cast(AbstractEventLoop, token)
-        f: concurrent.futures.Future[T_Retval] = asyncio.run_coroutine_threadsafe(
-            func(*args), loop
+        context = copy_context()
+        context.run(sniffio.current_async_library_cvar.set, "asyncio")
+        f: concurrent.futures.Future[T_Retval] = context.run(
+            asyncio.run_coroutine_threadsafe, func(*args), loop
         )
         return f.result()
 
@@ -2047,6 +2049,7 @@ def run_sync_from_thread(
         @wraps(func)
         def wrapper() -> None:
             try:
+                sniffio.current_async_library_cvar.set("asyncio")
                 f.set_result(func(*args))
             except BaseException as exc:
                 f.set_exception(exc)