fix tracing and add tests

llama_stack/distribution/library_client.py

@@ -376,19 +376,18 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
 
         body = self._convert_body(path, options.method, body)
 
-        await start_trace(options.url, {"__location__": "library_client"})
-
         async def gen():
+            await start_trace(options.url, {"__location__": "library_client"})
+            try:
                 async for chunk in await func(**body):
                     data = json.dumps(convert_pydantic_to_json_value(chunk))
                     sse_event = f"data: {data}\n\n"
                     yield sse_event.encode("utf-8")
-
-        try:
-            wrapped_gen = preserve_contexts_async_generator(gen(), [CURRENT_TRACE_CONTEXT, PROVIDER_DATA_VAR])
             finally:
                 await end_trace()
 
+        wrapped_gen = preserve_contexts_async_generator(gen(), [CURRENT_TRACE_CONTEXT, PROVIDER_DATA_VAR])
+
         mock_response = httpx.Response(
             status_code=httpx.codes.OK,
             content=wrapped_gen,

llama_stack/distribution/utils/context.py

@@ -14,19 +14,19 @@ def preserve_contexts_async_generator(
     gen: AsyncGenerator[T, None], context_vars: List[ContextVar]
 ) -> AsyncGenerator[T, None]:
     """
-    Wraps an async generator to preserve both tracing and headers context variables across iterations.
-    This is needed because we start a new asyncio event loop for each request, and we need to preserve the context
-    across the event loop boundary.
+    Wraps an async generator to preserve context variables across iterations.
+    This is needed because we start a new asyncio event loop for each streaming request,
+    and we need to preserve the context across the event loop boundary.
     """
-    context_values = [context_var.get() for context_var in context_vars]
 
     async def wrapper():
         while True:
-            for context_var, context_value in zip(context_vars, context_values, strict=False):
-                _ = context_var.set(context_value)
             try:
                 item = await gen.__anext__()
+                context_values = {context_var.name: context_var.get() for context_var in context_vars}
                 yield item
+                for context_var in context_vars:
+                    _ = context_var.set(context_values[context_var.name])
             except StopAsyncIteration:
                 break
 

llama_stack/distribution/utils/tests/test_context.py

@@ -0,0 +1,155 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+from contextvars import ContextVar
+
+import pytest
+
+from llama_stack.distribution.utils.context import preserve_contexts_async_generator
+
+
+@pytest.mark.asyncio
+async def test_preserve_contexts_with_exception():
+    # Create context variable
+    context_var = ContextVar("exception_var", default="initial")
+    token = context_var.set("start_value")
+
+    # Create an async generator that raises an exception
+    async def exception_generator():
+        yield context_var.get()
+        context_var.set("modified")
+        raise ValueError("Test exception")
+        yield None  # This will never be reached
+
+    # Wrap the generator
+    wrapped_gen = preserve_contexts_async_generator(exception_generator(), [context_var])
+
+    # First iteration should work
+    value = await wrapped_gen.__anext__()
+    assert value == "start_value"
+
+    # Second iteration should raise the exception
+    with pytest.raises(ValueError, match="Test exception"):
+        await wrapped_gen.__anext__()
+
+    # Clean up
+    context_var.reset(token)
+
+
+@pytest.mark.asyncio
+async def test_preserve_contexts_empty_generator():
+    # Create context variable
+    context_var = ContextVar("empty_var", default="initial")
+    token = context_var.set("value")
+
+    # Create an empty async generator
+    async def empty_generator():
+        if False:  # This condition ensures the generator yields nothing
+            yield None
+
+    # Wrap the generator
+    wrapped_gen = preserve_contexts_async_generator(empty_generator(), [context_var])
+
+    # The generator should raise StopAsyncIteration immediately
+    with pytest.raises(StopAsyncIteration):
+        await wrapped_gen.__anext__()
+
+    # Context variable should remain unchanged
+    assert context_var.get() == "value"
+
+    # Clean up
+    context_var.reset(token)
+
+
+@pytest.mark.asyncio
+async def test_preserve_contexts_across_event_loops():
+    """
+    Test that context variables are preserved across event loop boundaries with nested generators.
+    This simulates the real-world scenario where:
+    1. A new event loop is created for each streaming request
+    2. The async generator runs inside that loop
+    3. There are multiple levels of nested generators
+    4. Context needs to be preserved across these boundaries
+    """
+    # Create context variables
+    request_id = ContextVar("request_id", default=None)
+    user_id = ContextVar("user_id", default=None)
+
+    # Set initial values
+
+    # Results container to verify values across thread boundaries
+    results = []
+
+    # Inner-most generator (level 2)
+    async def inner_generator():
+        # Should have the context from the outer scope
+        yield (1, request_id.get(), user_id.get())
+
+        # Modify one context variable
+        user_id.set("user-modified")
+
+        # Should reflect the modification
+        yield (2, request_id.get(), user_id.get())
+
+    # Middle generator (level 1)
+    async def middle_generator():
+        inner_gen = inner_generator()
+
+        # Forward the first yield from inner
+        item = await inner_gen.__anext__()
+        yield item
+
+        # Forward the second yield from inner
+        item = await inner_gen.__anext__()
+        yield item
+
+        request_id.set("req-modified")
+
+        # Add our own yield with both modified variables
+        yield (3, request_id.get(), user_id.get())
+
+    # Function to run in a separate thread with a new event loop
+    def run_in_new_loop():
+        # Create a new event loop for this thread
+        loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(loop)
+
+        try:
+            # Outer generator (runs in the new loop)
+            async def outer_generator():
+                request_id.set("req-12345")
+                user_id.set("user-6789")
+                # Wrap the middle generator
+                wrapped_gen = preserve_contexts_async_generator(middle_generator(), [request_id, user_id])
+
+                # Process all items from the middle generator
+                async for item in wrapped_gen:
+                    # Store results for verification
+                    results.append(item)
+
+            # Run the outer generator in the new loop
+            loop.run_until_complete(outer_generator())
+        finally:
+            loop.close()
+
+    # Run the generator chain in a separate thread with a new event loop
+    with ThreadPoolExecutor(max_workers=1) as executor:
+        future = executor.submit(run_in_new_loop)
+        future.result()  # Wait for completion
+
+    # Verify the results
+    assert len(results) == 3
+
+    # First yield should have original values
+    assert results[0] == (1, "req-12345", "user-6789")
+
+    # Second yield should have modified user_id
+    assert results[1] == (2, "req-12345", "user-modified")
+
+    # Third yield should have both modified values
+    assert results[2] == (3, "req-modified", "user-modified")