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.