bpo-32410: Avoid blocking on file IO in sendfile fallback code (GH-7172)

2 files changed, 9 insertions, 2 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index a0243f5..ffd2513 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -800,7 +800,10 @@ class BaseEventLoop(events.AbstractEventLoop):
     async def _sock_sendfile_fallback(self, sock, file, offset, count):
         if offset:
             file.seek(offset)
-        blocksize = min(count, 16384) if count else 16384
+        blocksize = (
+            min(count, constants.SENDFILE_FALLBACK_READBUFFER_SIZE)
+            if count else constants.SENDFILE_FALLBACK_READBUFFER_SIZE
+        )
         buf = bytearray(blocksize)
         total_sent = 0
         try:
@@ -810,7 +813,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                     if blocksize <= 0:
                         break
                 view = memoryview(buf)[:blocksize]
-                read = file.readinto(view)
+                read = await self.run_in_executor(None, file.readinto, view)
                 if not read:
                     break  # EOF
                 await self.sock_sendall(sock, view)
diff --git a/Lib/asyncio/constants.py b/Lib/asyncio/constants.py
index 739b0a7..d7ba496 100644
--- a/Lib/asyncio/constants.py
+++ b/Lib/asyncio/constants.py
@@ -14,6 +14,10 @@ DEBUG_STACK_DEPTH = 10
 # Number of seconds to wait for SSL handshake to complete
 SSL_HANDSHAKE_TIMEOUT = 10.0
 
+# Used in sendfile fallback code.  We use fallback for platforms
+# that don't support sendfile, or for TLS connections.
+SENDFILE_FALLBACK_READBUFFER_SIZE = 1024 * 256
+
 # The enum should be here to break circular dependencies between
 # base_events and sslproto
 class _SendfileMode(enum.Enum):