bpo-41002: Optimize HTTPResponse.read with a given amount (GH-20943)

I've done the implementation for both non-chunked and chunked reads. I haven't benchmarked chunked reads because I don't currently have a convenient way to generate a high-bandwidth chunked stream, but I don't see any reason that it shouldn't enjoy the same benefits that the non-chunked case does. I've used the benchmark attached to the bpo bug to verify that performance now matches the unsized read case.

Automerge-Triggered-By: @methane

1 files changed, 25 insertions, 10 deletions

diff --git a/Lib/http/client.py b/Lib/http/client.py
index 019380a..500230d 100644
--- a/Lib/http/client.py
+++ b/Lib/http/client.py
@@ -448,18 +448,25 @@ class HTTPResponse(io.BufferedIOBase):
             self._close_conn()
             return b""
 
+        if self.chunked:
+            return self._read_chunked(amt)
+
         if amt is not None:
-            # Amount is given, implement using readinto
-            b = bytearray(amt)
-            n = self.readinto(b)
-            return memoryview(b)[:n].tobytes()
+            if self.length is not None and amt > self.length:
+                # clip the read to the "end of response"
+                amt = self.length
+            s = self.fp.read(amt)
+            if not s and amt:
+                # Ideally, we would raise IncompleteRead if the content-length
+                # wasn't satisfied, but it might break compatibility.
+                self._close_conn()
+            elif self.length is not None:
+                self.length -= len(s)
+                if not self.length:
+                    self._close_conn()
+            return s
         else:
             # Amount is not given (unbounded read) so we must check self.length
-            # and self.chunked
-
-            if self.chunked:
-                return self._readall_chunked()
-
             if self.length is None:
                 s = self.fp.read()
             else:
@@ -560,7 +567,7 @@ class HTTPResponse(io.BufferedIOBase):
             self.chunk_left = chunk_left
         return chunk_left
 
-    def _readall_chunked(self):
+    def _read_chunked(self, amt=None):
         assert self.chunked != _UNKNOWN
         value = []
         try:
@@ -568,7 +575,15 @@ class HTTPResponse(io.BufferedIOBase):
                 chunk_left = self._get_chunk_left()
                 if chunk_left is None:
                     break
+
+                if amt is not None and amt <= chunk_left:
+                    value.append(self._safe_read(amt))
+                    self.chunk_left = chunk_left - amt
+                    break
+
                 value.append(self._safe_read(chunk_left))
+                if amt is not None:
+                    amt -= chunk_left
                 self.chunk_left = 0
             return b''.join(value)
         except IncompleteRead: