gh-129005: Align FileIO.readall between _pyio and _io

Lib/_pyio.py

@@ -1454,6 +1454,17 @@ def write(self, b):
  return BufferedWriter.write(self, b)
 
 
+def _new_buffersize(bytes_read):
+ # Parallels _io/fileio.c new_buffersize
+ if bytes_read > 65536:
+ addend = bytes_read >> 3
+ else:
+ addend = 256 + bytes_read
+ if addend < DEFAULT_BUFFER_SIZE:
+ addend = DEFAULT_BUFFER_SIZE
+ return bytes_read + addend
+
+
 class FileIO(RawIOBase):
  _fd = -1
  _created = False
@@ -1672,22 +1683,20 @@ def readall(self):
  except OSError:
  pass
 
- result = bytearray()
- while True:
- if len(result) >= bufsize:
- bufsize = len(result)
- bufsize += max(bufsize, DEFAULT_BUFFER_SIZE)
- n = bufsize - len(result)
- try:
- chunk = os.read(self._fd, n)
- except BlockingIOError:
- if result:
- break
+ result = bytearray(bufsize)
+ bytes_read = 0
+ try:
+ while n := os.readinto(self._fd, memoryview(result)[bytes_read:]):
+ bytes_read += n
+ if bytes_read >= len(result):
+ result.resize(_new_buffersize(bytes_read))
+ except BlockingIOError:
+ if not bytes_read:
  return None
- if not chunk: # reached the end of the file
- break
- result += chunk
 
+ assert len(result) - bytes_read >= 1, \
+ "os.readinto buffer size 0 will result in erroneous EOF / returns 0"
+ result.resize(bytes_read)
  return bytes(result)
 
  def readinto(self, buffer):

Misc/NEWS.d/next/Library/2025-02-05-13-19-15.gh-issue-129005.Sb69L_.rst

@@ -0,0 +1,2 @@
+``_pyio.FileIO.readall()`` now allocates, resizes, and fills a data buffer
+using the same algorithm ``_io.FileIO.readall()`` uses.