tests: Use asyncio to wait for nghttpd to start

Seems like the tests may fail because we don't wait for nghttpd (the
HTTP2 server used for testing) to actually start listening.
This commit uses asyncio to launch nghttpd and read its stdout until it
starts listening.

tests/test_impersonate.py

@@ -2,6 +2,7 @@ import os
 import io
 import re
 import sys
+import asyncio
 import logging
 import subprocess
 import tempfile
@@ -241,20 +242,69 @@ class TestImpersonation:
         p.terminate()
         p.wait(timeout=10)
 
+    async def _read_proc_output(self, proc, timeout):
+        """Read an async process' output until timeout is reached"""
+        data = bytes()
+        loop = asyncio.get_running_loop()
+        start_time = loop.time()
+        passed = loop.time() - start_time
+        while passed < timeout:
+            try:
+                data += await asyncio.wait_for(
+                    proc.stdout.readline(), timeout=timeout - passed
+                )
+            except asyncio.TimeoutError:
+                pass
+            passed = loop.time() - start_time
+        return data
+
+    async def _wait_nghttpd(self, proc):
+        """Wait for nghttpd to start listening on its designated port"""
+        data = bytes()
+        while data is not None:
+            data = await proc.stdout.readline()
+            if not data:
+                # Process terminated
+                return False
+
+            line = data.decode("utf-8").rstrip()
+            if "listen 0.0.0.0:8443" in line:
+                return True
+
+        return False
+
     @pytest.fixture
-    def nghttpd(self):
-        """Initiailize an HTTP/2 server"""
+    async def nghttpd(self):
+        """Initiailize an HTTP/2 server.
+        The returned object is an asyncio.subprocess.Process object,
+        so async methods must be used with it.
+        """
         logging.debug(f"Running nghttpd on :8443")
 
-        p = subprocess.Popen([
+        # Launch nghttpd and wait for it to start listening.
+
+        proc = await asyncio.create_subprocess_exec(
             "nghttpd", "-v",
-            "8443", "ssl/server.key", "ssl/server.crt"
-        ], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+            "8443", "ssl/server.key", "ssl/server.crt",
+            stdout=asyncio.subprocess.PIPE,
+            stderr=asyncio.subprocess.PIPE
+        )
 
-        yield p
+        try:
+            # Wait up to 3 seconds for nghttpd to start.
+            # Otherwise fail.
+            started = await asyncio.wait_for(
+                self._wait_nghttpd(proc), timeout=3
+            )
+            if not started:
+                raise Exception("nghttpd failed to start on time")
+        except asyncio.TimeoutError:
+            raise Exception("nghttpd failed to start on time")
 
-        p.terminate()
-        p.wait(timeout=10)
+        yield proc
+
+        proc.terminate()
+        await proc.wait()
 
     def _set_ld_preload(self, env_vars, lib):
         if sys.platform.startswith("linux"):
@@ -422,18 +472,19 @@ class TestImpersonation:
         equals, msg =  sig.equals(expected_sig, reason=True)
         assert equals, msg
 
+    @pytest.mark.asyncio
     @pytest.mark.parametrize(
         "curl_binary, env_vars, ld_preload, expected_signature",
         CURL_BINARIES_AND_SIGNATURES
     )
-    def test_http2_headers(self,
-                           pytestconfig,
-                           nghttpd,
-                           curl_binary,
-                           env_vars,
-                           ld_preload,
-                           browser_signatures,
-                           expected_signature):
+    async def test_http2_headers(self,
+                                 pytestconfig,
+                                 nghttpd,
+                                 curl_binary,
+                                 env_vars,
+                                 ld_preload,
+                                 browser_signatures,
+                                 expected_signature):
         curl_binary = os.path.join(
             pytestconfig.getoption("install_dir"), "bin", curl_binary
         )
@@ -453,17 +504,8 @@ class TestImpersonation:
                              extra_args=["-k"],
                              url="https://localhost:8443")
         assert ret == 0
-        try:
-            output, stderr = nghttpd.communicate(timeout=2)
 
-            # If nghttpd finished running before timeout, it's likely it failed
-            # with an error.
-            assert nghttpd.returncode == 0, \
-                (f"nghttpd failed with error code {nghttpd.returncode}, "
-                 f"stderr: {stderr}")
-        except subprocess.TimeoutExpired:
-            nghttpd.kill()
-            output, stderr = nghttpd.communicate(timeout=3)
+        output = await self._read_proc_output(nghttpd, timeout=2)
 
         assert len(output) > 0
         pseudo_headers, headers = self._parse_nghttpd2_output(output)