gh-119451: Fix a potential denial of service in http.client (GH-119454)

Reading the whole body of the HTTP response could cause OOM if
the Content-Length value is too large even if the server does not send
a large amount of data. Now the HTTP client reads large data by chunks,
therefore the amount of consumed memory is proportional to the amount
of sent data.

Lib/http/client.py

@@ -111,6 +111,11 @@
 _MAXLINE = 65536
 _MAXHEADERS = 100
 
+# Data larger than this will be read in chunks, to prevent extreme
+# overallocation.
+_MIN_READ_BUF_SIZE = 1 << 20
+
+
 # Header name/value ABNF (http://tools.ietf.org/html/rfc7230#section-3.2)
 #
 # VCHAR          = %x21-7E
@@ -642,10 +647,25 @@ def _safe_read(self, amt):
         reading. If the bytes are truly not available (due to EOF), then the
         IncompleteRead exception can be used to detect the problem.
         """
-        data = self.fp.read(amt)
-        if len(data) < amt:
-            raise IncompleteRead(data, amt-len(data))
-        return data
+        cursize = min(amt, _MIN_READ_BUF_SIZE)
+        data = self.fp.read(cursize)
+        if len(data) >= amt:
+            return data
+        if len(data) < cursize:
+            raise IncompleteRead(data, amt - len(data))
+
+        data = io.BytesIO(data)
+        data.seek(0, 2)
+        while True:
+            # This is a geometric increase in read size (never more than
+            # doubling out the current length of data per loop iteration).
+            delta = min(cursize, amt - cursize)
+            data.write(self.fp.read(delta))
+            if data.tell() >= amt:
+                return data.getvalue()
+            cursize += delta
+            if data.tell() < cursize:
+                raise IncompleteRead(data.getvalue(), amt - data.tell())
 
     def _safe_readinto(self, b):
         """Same as _safe_read, but for reading into a buffer."""