worker: implement _blockingreader.readinto() (issue6444)

The core logic for readinto() was already implemented in read(), so this is
mostly extracting that code into its own method.

Another fix for issue6444 was committed to the stable branch: 2fe4efaa59af.
That is a minimal fix that implements readinto() only on Python versions that
require readinto() (3.8.0 and 3.8.1), which is the right approach for the
stable branch. However, I think that this changeset has its value. It improves
performance in cases when pickle can use readinto(), it reduces code
duplication compared to the other patch, and by defining readinto() on all
Python versions, it makes behavior more consistent across all Python versions.
This changesets reverts the other change.

mercurial/worker.py

@@ -79,35 +79,12 b' class _blockingreader:'
     def __init__(self, wrapped):
         self._wrapped = wrapped
 
-    # Do NOT implement readinto() by making it delegate to
-    # _wrapped.readinto(), since that is unbuffered. The unpickler is fine
-    # with just read() and readline(), so we don't need to implement it.
-
-    if (3, 8, 0) <= sys.version_info[:3] < (3, 8, 2):
-
-        # This is required for python 3.8, prior to 3.8.2.  See issue6444.
-        def readinto(self, b):
-            pos = 0
-            size = len(b)
-
-            while pos < size:
-                ret = self._wrapped.readinto(b[pos:])
-                if not ret:
-                    break
-                pos += ret
-
-            return pos
-
     def readline(self):
         return self._wrapped.readline()
 
-    # issue multiple reads until size is fulfilled (or EOF is encountered)
-    def read(self, size=-1):
-        if size < 0:
-            return self._wrapped.readall()
-
-        buf = bytearray(size)
+    def readinto(self, buf):
         pos = 0
+        size = len(buf)
 
         with memoryview(buf) as view:
             while pos < size:
@@ -117,7 +94,16 b' class _blockingreader:'
                     break
                 pos += ret
 
-        del buf[pos:]
+        return pos
+
+    # issue multiple reads until size is fulfilled (or EOF is encountered)
+    def read(self, size=-1):
+        if size < 0:
+            return self._wrapped.readall()
+
+        buf = bytearray(size)
+        n_read = self.readinto(buf)
+        del buf[n_read:]
         return bytes(buf)