Lazy import either 'anyio' or 'trio' (#639)

* Lazy import either 'anyio' or 'trio'

* Add comments in _synchronization.py

* Add comments in _synchronization.py

httpcore/_synchronization.py

@@ -2,17 +2,44 @@
 from types import TracebackType
 from typing import Optional, Type
 
-import anyio
+import sniffio
 
 from ._exceptions import ExceptionMapping, PoolTimeout, map_exceptions
 
+# Our async synchronization primatives use either 'anyio' or 'trio' depending
+# on if they're running under asyncio or trio.
+#
+# We take care to only lazily import whichever of these two we need.
+
 
 class AsyncLock:
     def __init__(self) -> None:
-        self._lock = anyio.Lock()
+        self._backend = ""
+
+    def setup(self) -> None:
+        """
+        Detect if we're running under 'asyncio' or 'trio' and create
+        a lock with the correct implementation.
+        """
+        self._backend = sniffio.current_async_library()
+        if self._backend == "trio":
+            import trio
+
+            self._trio_lock = trio.Lock()
+        else:
+            import anyio
+
+            self._anyio_lock = anyio.Lock()
 
     async def __aenter__(self) -> "AsyncLock":
-        await self._lock.acquire()
+        if not self._backend:
+            self.setup()
+
+        if self._backend == "trio":
+            await self._trio_lock.acquire()
+        else:
+            await self._anyio_lock.acquire()
+
         return self
 
     async def __aexit__(
@@ -21,32 +48,102 @@ async def __aexit__(
         exc_value: Optional[BaseException] = None,
         traceback: Optional[TracebackType] = None,
     ) -> None:
-        self._lock.release()
+        if self._backend == "trio":
+            self._trio_lock.release()
+        else:
+            self._anyio_lock.release()
 
 
 class AsyncEvent:
     def __init__(self) -> None:
-        self._event = anyio.Event()
+        self._backend = ""
+
+    def setup(self) -> None:
+        """
+        Detect if we're running under 'asyncio' or 'trio' and create
+        a lock with the correct implementation.
+        """
+        self._backend = sniffio.current_async_library()
+        if self._backend == "trio":
+            import trio
+
+            self._trio_event = trio.Event()
+        else:
+            import anyio
+
+            self._anyio_event = anyio.Event()
 
     def set(self) -> None:
-        self._event.set()
+        if not self._backend:
+            self.setup()
+
+        if self._backend == "trio":
+            self._trio_event.set()
+        else:
+            self._anyio_event.set()
 
     async def wait(self, timeout: Optional[float] = None) -> None:
-        exc_map: ExceptionMapping = {TimeoutError: PoolTimeout}
-        with map_exceptions(exc_map):
-            with anyio.fail_after(timeout):
-                await self._event.wait()
+        if not self._backend:
+            self.setup()
+
+        if self._backend == "trio":
+            import trio
+
+            trio_exc_map: ExceptionMapping = {trio.TooSlowError: PoolTimeout}
+            timeout_or_inf = float("inf") if timeout is None else timeout
+            with map_exceptions(trio_exc_map):
+                with trio.fail_after(timeout_or_inf):
+                    await self._trio_event.wait()
+        else:
+            import anyio
+
+            anyio_exc_map: ExceptionMapping = {TimeoutError: PoolTimeout}
+            with map_exceptions(anyio_exc_map):
+                with anyio.fail_after(timeout):
+                    await self._anyio_event.wait()
 
 
 class AsyncSemaphore:
     def __init__(self, bound: int) -> None:
-        self._semaphore = anyio.Semaphore(initial_value=bound, max_value=bound)
+        self._bound = bound
+        self._backend = ""
+
+    def setup(self) -> None:
+        """
+        Detect if we're running under 'asyncio' or 'trio' and create
+        a semaphore with the correct implementation.
+        """
+        self._backend = sniffio.current_async_library()
+        if self._backend == "trio":
+            import trio
+
+            self._trio_semaphore = trio.Semaphore(
+                initial_value=self._bound, max_value=self._bound
+            )
+        else:
+            import anyio
+
+            self._anyio_semaphore = anyio.Semaphore(
+                initial_value=self._bound, max_value=self._bound
+            )
 
     async def acquire(self) -> None:
-        await self._semaphore.acquire()
+        if not self._backend:
+            self.setup()
+
+        if self._backend == "trio":
+            await self._trio_semaphore.acquire()
+        else:
+            await self._anyio_semaphore.acquire()
 
     async def release(self) -> None:
-        self._semaphore.release()
+        if self._backend == "trio":
+            self._trio_semaphore.release()
+        else:
+            self._anyio_semaphore.release()
+
+
+# Our thread-based synchronization primitives...
 
 
 class Lock: