Port Datasette from Sanic to ASGI + Uvicorn (#518)

Datasette now uses ASGI internally, and no longer depends on Sanic.

It now uses Uvicorn as the underlying HTTP server.

This was thirteen months in the making... for full details see the issue:

https://github.com/simonw/datasette/issues/272

And for a full sequence of commits plus commentary, see the pull request:

https://github.com/simonw/datasette/pull/518

datasette/tracer.py

@@ -1,6 +1,7 @@
 import asyncio
 from contextlib import contextmanager
 import time
+import json
 import traceback
 
 tracers = {}
@@ -32,15 +33,15 @@ def trace(type, **kwargs):
     start = time.time()
     yield
     end = time.time()
-    trace = {
+    trace_info = {
         "type": type,
         "start": start,
         "end": end,
         "duration_ms": (end - start) * 1000,
         "traceback": traceback.format_list(traceback.extract_stack(limit=6)[:-3]),
     }
-    trace.update(kwargs)
-    tracer.append(trace)
+    trace_info.update(kwargs)
+    tracer.append(trace_info)
 
 
 @contextmanager
@@ -53,3 +54,77 @@ def capture_traces(tracer):
     tracers[task_id] = tracer
     yield
     del tracers[task_id]
+
+
+class AsgiTracer:
+    # If the body is larger than this we don't attempt to append the trace
+    max_body_bytes = 1024 * 256  # 256 KB
+
+    def __init__(self, app):
+        self.app = app
+
+    async def __call__(self, scope, receive, send):
+        if b"_trace=1" not in scope.get("query_string", b"").split(b"&"):
+            await self.app(scope, receive, send)
+            return
+        trace_start = time.time()
+        traces = []
+
+        accumulated_body = b""
+        size_limit_exceeded = False
+        response_headers = []
+
+        async def wrapped_send(message):
+            nonlocal accumulated_body, size_limit_exceeded, response_headers
+            if message["type"] == "http.response.start":
+                response_headers = message["headers"]
+                await send(message)
+                return
+
+            if message["type"] != "http.response.body" or size_limit_exceeded:
+                await send(message)
+                return
+
+            # Accumulate body until the end or until size is exceeded
+            accumulated_body += message["body"]
+            if len(accumulated_body) > self.max_body_bytes:
+                await send(
+                    {
+                        "type": "http.response.body",
+                        "body": accumulated_body,
+                        "more_body": True,
+                    }
+                )
+                size_limit_exceeded = True
+                return
+
+            if not message.get("more_body"):
+                # We have all the body - modify it and send the result
+                # TODO: What to do about Content-Type or other cases?
+                trace_info = {
+                    "request_duration_ms": 1000 * (time.time() - trace_start),
+                    "sum_trace_duration_ms": sum(t["duration_ms"] for t in traces),
+                    "num_traces": len(traces),
+                    "traces": traces,
+                }
+                try:
+                    content_type = [
+                        v.decode("utf8")
+                        for k, v in response_headers
+                        if k.lower() == b"content-type"
+                    ][0]
+                except IndexError:
+                    content_type = ""
+                if "text/html" in content_type and b"</body>" in accumulated_body:
+                    extra = json.dumps(trace_info, indent=2)
+                    extra_html = "<pre>{}</pre></body>".format(extra).encode("utf8")
+                    accumulated_body = accumulated_body.replace(b"</body>", extra_html)
+                elif "json" in content_type and accumulated_body.startswith(b"{"):
+                    data = json.loads(accumulated_body.decode("utf8"))
+                    if "_trace" not in data:
+                        data["_trace"] = trace_info
+                    accumulated_body = json.dumps(data).encode("utf8")
+                await send({"type": "http.response.body", "body": accumulated_body})
+
+        with capture_traces(traces):
+            await self.app(scope, receive, wrapped_send)