From 96078b08952d0fd992dfd6c79b8dbd044665c469 Mon Sep 17 00:00:00 2001
From: Cody Maloney <cmaloney@theoreticalchaos.com>
Date: Tue, 28 Jan 2025 21:04:11 -0800
Subject: [PATCH 1/2] gh-129005: Align FileIO.readall allocation

Both now use a pre-allocated buffer of length `bufsize`, fill it using
a readinto, and have matching "expand buffer" logic.

On my machine this takes:

`./python -m test -M8g -uall test_largefile -m test_large_read -v`
from ~3.7 seconds to ~3.4 seconds
---
 Lib/_pyio.py                                  | 27 ++++++++++++-------
 ...-01-28-21-22-44.gh-issue-129005.h57i9j.rst |  1 +
 2 files changed, 19 insertions(+), 9 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 023478aa78c6a0..76a27910da4d5f 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1674,22 +1674,31 @@ def readall(self):
                 except OSError:
                     pass
 
-        result = bytearray()
+        result = bytearray(bufsize)
+        bytes_read = 0
         while True:
-            if len(result) >= bufsize:
-                bufsize = len(result)
-                bufsize += max(bufsize, DEFAULT_BUFFER_SIZE)
-            n = bufsize - len(result)
+            if bytes_read >= bufsize:
+                # Parallels _io/fileio.c new_buffersize
+                if bufsize > 65536:
+                    addend = bufsize >> 3
+                else:
+                    addend = bufsize + 256
+                if addend < DEFAULT_BUFFER_SIZE:
+                    addend = DEFAULT_BUFFER_SIZE
+                bufsize += addend
+                result[bytes_read:bufsize] = b'\0'
+            assert bufsize - bytes_read > 0, "Should always try and read at least one byte"
             try:
-                chunk = os.read(self._fd, n)
+                n = os.readinto(self._fd, memoryview(result)[bytes_read:])
             except BlockingIOError:
-                if result:
+                if bytes_read > 0:
                     break
                 return None
-            if not chunk: # reached the end of the file
+            if n == 0:  # reached the end of the file
                 break
-            result += chunk
+            bytes_read += n
 
+        del result[bytes_read:]
         return bytes(result)
 
     def readinto(self, buffer):
diff --git a/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst b/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst
new file mode 100644
index 00000000000000..91ecb1d237278b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst
@@ -0,0 +1 @@
+``_pyio.FileIO.readall`` now allocates, resizes, and fills a data buffer using the same algorithm ``_io.FileIO.readall`` uses.

From 044ec508ec72f88ee5a9d050563d5084cc4be66c Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Thu, 30 Jan 2025 11:49:13 +0100
Subject: [PATCH 2/2] Update
 Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst

---
 .../Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst     | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst b/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst
index 91ecb1d237278b..c76fb05e196f87 100644
--- a/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst
+++ b/Misc/NEWS.d/next/Library/2025-01-28-21-22-44.gh-issue-129005.h57i9j.rst
@@ -1 +1,2 @@
-``_pyio.FileIO.readall`` now allocates, resizes, and fills a data buffer using the same algorithm ``_io.FileIO.readall`` uses.
+``_pyio.FileIO.readall()`` now allocates, resizes, and fills a data buffer using
+the same algorithm ``_io.FileIO.readall()`` uses.