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

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:

Lib/test/test_httplib.py

@@ -569,6 +569,33 @@ class BasicTest(TestCase):
         resp.close()
         self.assertTrue(resp.closed)
 
+    def test_partial_reads_past_end(self):
+        # if we have Content-Length, clip reads to the end
+        body = "HTTP/1.1 200 Ok\r\nContent-Length: 4\r\n\r\nText"
+        sock = FakeSocket(body)
+        resp = client.HTTPResponse(sock)
+        resp.begin()
+        self.assertEqual(resp.read(10), b'Text')
+        self.assertTrue(resp.isclosed())
+        self.assertFalse(resp.closed)
+        resp.close()
+        self.assertTrue(resp.closed)
+
+    def test_partial_readintos_past_end(self):
+        # if we have Content-Length, clip readintos to the end
+        body = "HTTP/1.1 200 Ok\r\nContent-Length: 4\r\n\r\nText"
+        sock = FakeSocket(body)
+        resp = client.HTTPResponse(sock)
+        resp.begin()
+        b = bytearray(10)
+        n = resp.readinto(b)
+        self.assertEqual(n, 4)
+        self.assertEqual(bytes(b)[:4], b'Text')
+        self.assertTrue(resp.isclosed())
+        self.assertFalse(resp.closed)
+        resp.close()
+        self.assertTrue(resp.closed)
+
     def test_partial_reads_no_content_length(self):
         # when no length is present, the socket should be gracefully closed when
         # all data was read

Misc/NEWS.d/next/Library/2020-06-17-17-26-24.bpo-41002.NPBItE.rst

@@ -0,0 +1 @@
+Improve performance of HTTPResponse.read with a given amount. Patch by Bruce Merry.