add constant MAXIMUM_BUFFER_SIZE

Author: rmmancom

Lib/_pyio.py

@@ -23,8 +23,10 @@
     valid_seek_flags.add(os.SEEK_HOLE)
     valid_seek_flags.add(os.SEEK_DATA)
 
-# open() uses max(st_blksize, io.DEFAULT_BUFFER_SIZE) when st_blksize is available
+# open() uses max(min(blocksize, MAXIMUM_BUFFER_SIZE), DEFAULT_BUFFER_SIZE)
+# when the device block size is available.
 DEFAULT_BUFFER_SIZE = 128 * 1024  # bytes
+MAXIMUM_BUFFER_SIZE = 8192 * 1024  # bytes
 
 # NOTE: Base classes defined here are registered with the "official" ABCs
 # defined in io.py. We don't use real inheritance though, because we don't want
@@ -125,7 +127,7 @@ def open(file, mode="r", buffering=-1, encoding=None, errors=None,
 
    * Binary files are buffered in fixed-size chunks; the size of the buffer
      is the maximum of the DEFAULT_BUFFER_SIZE and the device block size.
-     On most systems, the buffer will typically be 131072 bytes long.
+     On most systems, the buffer will typically be 128 kilobytes long.
 
     * "Interactive" text files (files for which isatty() returns True)
       use line buffering.  Other text files use the policy described above
@@ -241,7 +243,7 @@ def open(file, mode="r", buffering=-1, encoding=None, errors=None,
             buffering = -1
             line_buffering = True
         if buffering < 0:
-            buffering = max(min(raw._blksize, 8192 * 1024), DEFAULT_BUFFER_SIZE)
+            buffering = max(min(raw._blksize, MAXIMUM_BUFFER_SIZE), DEFAULT_BUFFER_SIZE)
         if buffering < 0:
             raise ValueError("invalid buffering size")
         if buffering == 0:

Modules/_io/_iomodule.c

@@ -60,8 +60,12 @@ PyDoc_STRVAR(module_doc,
 "DEFAULT_BUFFER_SIZE\n"
 "\n"
 "   An int containing the default buffer size used by the module's buffered\n"
-"   I/O classes. open() uses the file's blksize (as obtained by os.stat) if\n"
-"   possible.\n"
+"   I/O classes.\n"
+"\n"
+"MAXIMUM_BUFFER_SIZE\n"
+"\n"
+"   An int containing the maximum buffer size used by the module's buffered\n"
+"   I/O classes.\n"
     );
 
 
@@ -132,8 +136,9 @@ the size of a fixed-size chunk buffer.  When no buffering argument is
 given, the default buffering policy works as follows:
 
 * Binary files are buffered in fixed-size chunks; the size of the buffer
- is the maximum of the DEFAULT_BUFFER_SIZE and the device block size.
- On most systems, the buffer will typically be 131072 bytes long.
+ is max(min(blocksize, MAXIMUM_BUFFER_SIZE), DEFAULT_BUFFER_SIZE)
+ when the device block size is available.
+ On most systems, the buffer will typically be 128 kilobytes long.
 
 * "Interactive" text files (files for which isatty() returns True)
   use line buffering.  Other text files use the policy described above
@@ -199,7 +204,7 @@ static PyObject *
 _io_open_impl(PyObject *module, PyObject *file, const char *mode,
               int buffering, const char *encoding, const char *errors,
               const char *newline, int closefd, PyObject *opener)
-/*[clinic end generated code: output=aefafc4ce2b46dc0 input=105f6f1cb63368c4]*/
+/*[clinic end generated code: output=aefafc4ce2b46dc0 input=e1e2d41c6e922cbe]*/
 {
     size_t i;
 
@@ -367,14 +372,7 @@ _io_open_impl(PyObject *module, PyObject *file, const char *mode,
         if (blksize_obj == NULL)
             goto error;
         buffering = PyLong_AsLong(blksize_obj);
-        if (buffering > 8192 * 1024)
-        {
-            buffering = 8192 * 1024;
-        }
-        if (buffering < DEFAULT_BUFFER_SIZE)
-        {
-            buffering = DEFAULT_BUFFER_SIZE;
-        }
+        buffering = Py_MAX(Py_MIN(buffering, MAXIMUM_BUFFER_SIZE), DEFAULT_BUFFER_SIZE);
         Py_DECREF(blksize_obj);
         if (buffering == -1 && PyErr_Occurred())
             goto error;

Modules/_io/_iomodule.h

@@ -79,6 +79,7 @@ extern Py_ssize_t _PyIO_find_line_ending(
 extern int _PyIO_trap_eintr(void);
 
 #define DEFAULT_BUFFER_SIZE (128 * 1024)  /* bytes */
+#define MAXIMUM_BUFFER_SIZE (8192 * 1024)  /* bytes */
 
 /*
  * Offset type for positioning.