[3.14] gh-70765: avoid waiting for HTTP headers when parsing HTTP/0.9 requests (GH-139514) (#139600)

(cherry picked from commit 13dc2fde8c)
(cherry picked from commit 1fe89d324e)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/http/server.py

@@ -324,6 +324,7 @@ def parse_request(self):
         error response has already been sent back.
 
         """
+        is_http_0_9 = False
         self.command = None  # set in case of error on the first line
         self.request_version = version = self.default_request_version
         self.close_connection = True
@@ -381,6 +382,7 @@ def parse_request(self):
                     HTTPStatus.BAD_REQUEST,
                     "Bad HTTP/0.9 request type (%r)" % command)
                 return False
+            is_http_0_9 = True
         self.command, self.path = command, path
 
         # gh-87389: The purpose of replacing '//' with '/' is to protect
@@ -390,6 +392,11 @@ def parse_request(self):
         if self.path.startswith('//'):
             self.path = '/' + self.path.lstrip('/')  # Reduce to a single /
 
+        # For HTTP/0.9, headers are not expected at all.
+        if is_http_0_9:
+            self.headers = {}
+            return True
+
         # Examine the headers and look for a Connection directive.
         try:
             self.headers = http.client.parse_headers(self.rfile,