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gh-120754: Refactor I/O modules to stash whole stat result rather tha…
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…n individual members (#123412)

Multiple places in the I/O stack optimize common cases by using the
information from stat. Currently individual members are extracted from
the stat and stored into the fileio struct. Refactor the code to store
the whole stat struct instead.

Parallels the changes to _io. The `stat` Python object doesn't allow
changing members, so rather than modifying estimated_size, just clear
the value.
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@cmaloney
cmaloney authored Sep 18, 2024
1 parent 96f619f commit 8b6c7c7
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Showing 2 changed files with 81 additions and 46 deletions.
44 changes: 24 additions & 20 deletions Lib/_pyio.py
View file
Original file line number Diff line number Diff line change
Expand Up @@ -242,14 +242,7 @@ def open(file, mode="r", buffering=-1, encoding=None, errors=None,
buffering = -1
line_buffering = True
if buffering < 0:
buffering = DEFAULT_BUFFER_SIZE
try:
bs = os.fstat(raw.fileno()).st_blksize
except (OSError, AttributeError):
pass
else:
if bs > 1:
buffering = bs
buffering = raw._blksize
if buffering < 0:
raise ValueError("invalid buffering size")
if buffering == 0:
Expand Down Expand Up @@ -1565,19 +1558,15 @@ def __init__(self, file, mode='r', closefd=True, opener=None):
os.set_inheritable(fd, False)

self._closefd = closefd
fdfstat = os.fstat(fd)
self._stat_atopen = os.fstat(fd)
try:
if stat.S_ISDIR(fdfstat.st_mode):
if stat.S_ISDIR(self._stat_atopen.st_mode):
raise IsADirectoryError(errno.EISDIR,
os.strerror(errno.EISDIR), file)
except AttributeError:
# Ignore the AttributeError if stat.S_ISDIR or errno.EISDIR
# don't exist.
pass
self._blksize = getattr(fdfstat, 'st_blksize', 0)
if self._blksize <= 1:
self._blksize = DEFAULT_BUFFER_SIZE
self._estimated_size = fdfstat.st_size

if _setmode:
# don't translate newlines (\r\n <=> \n)
Expand Down Expand Up @@ -1623,6 +1612,17 @@ def __repr__(self):
return ('<%s name=%r mode=%r closefd=%r>' %
(class_name, name, self.mode, self._closefd))

@property
def _blksize(self):
if self._stat_atopen is None:
return DEFAULT_BUFFER_SIZE

blksize = getattr(self._stat_atopen, "st_blksize", 0)
# WASI sets blsize to 0
if not blksize:
return DEFAULT_BUFFER_SIZE
return blksize

def _checkReadable(self):
if not self._readable:
raise UnsupportedOperation('File not open for reading')
Expand Down Expand Up @@ -1655,16 +1655,20 @@ def readall(self):
"""
self._checkClosed()
self._checkReadable()
if self._estimated_size <= 0:
if self._stat_atopen is None or self._stat_atopen.st_size <= 0:
bufsize = DEFAULT_BUFFER_SIZE
else:
bufsize = self._estimated_size + 1
# In order to detect end of file, need a read() of at least 1
# byte which returns size 0. Oversize the buffer by 1 byte so the
# I/O can be completed with two read() calls (one for all data, one
# for EOF) without needing to resize the buffer.
bufsize = self._stat_atopen.st_size + 1

if self._estimated_size > 65536:
if self._stat_atopen.st_size > 65536:
try:
pos = os.lseek(self._fd, 0, SEEK_CUR)
if self._estimated_size >= pos:
bufsize = self._estimated_size - pos + 1
if self._stat_atopen.st_size >= pos:
bufsize = self._stat_atopen.st_size - pos + 1
except OSError:
pass

Expand Down Expand Up @@ -1742,7 +1746,7 @@ def truncate(self, size=None):
if size is None:
size = self.tell()
os.ftruncate(self._fd, size)
self._estimated_size = size
self._stat_atopen = None
return size

def close(self):
Expand Down
83 changes: 57 additions & 26 deletions Modules/_io/fileio.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -74,8 +74,13 @@ typedef struct {
signed int seekable : 2; /* -1 means unknown */
unsigned int closefd : 1;
char finalizing;
unsigned int blksize;
Py_off_t estimated_size;
/* Stat result which was grabbed at file open, useful for optimizing common
File I/O patterns to be more efficient. This is only guidance / an
estimate, as it is subject to Time-Of-Check to Time-Of-Use (TOCTOU)
issues / bugs. Both the underlying file descriptor and file may be
modified outside of the fileio object / Python (ex. gh-90102, GH-121941,
gh-109523). */
struct _Py_stat_struct *stat_atopen;
PyObject *weakreflist;
PyObject *dict;
} fileio;
Expand Down Expand Up @@ -199,8 +204,7 @@ fileio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
self->writable = 0;
self->appending = 0;
self->seekable = -1;
self->blksize = 0;
self->estimated_size = -1;
self->stat_atopen = NULL;
self->closefd = 1;
self->weakreflist = NULL;
}
Expand Down Expand Up @@ -256,7 +260,6 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
#elif !defined(MS_WINDOWS)
int *atomic_flag_works = NULL;
#endif
struct _Py_stat_struct fdfstat;
int fstat_result;
int async_err = 0;

Expand Down Expand Up @@ -454,9 +457,13 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
#endif
}

self->blksize = DEFAULT_BUFFER_SIZE;
self->stat_atopen = PyMem_New(struct _Py_stat_struct, 1);
if (self->stat_atopen == NULL) {
PyErr_NoMemory();
goto error;
}
Py_BEGIN_ALLOW_THREADS
fstat_result = _Py_fstat_noraise(self->fd, &fdfstat);
fstat_result = _Py_fstat_noraise(self->fd, self->stat_atopen);
Py_END_ALLOW_THREADS
if (fstat_result < 0) {
/* Tolerate fstat() errors other than EBADF. See Issue #25717, where
Expand All @@ -471,25 +478,21 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
#endif
goto error;
}

PyMem_Free(self->stat_atopen);
self->stat_atopen = NULL;
}
else {
#if defined(S_ISDIR) && defined(EISDIR)
/* On Unix, open will succeed for directories.
In Python, there should be no file objects referring to
directories, so we need a check. */
if (S_ISDIR(fdfstat.st_mode)) {
if (S_ISDIR(self->stat_atopen->st_mode)) {
errno = EISDIR;
PyErr_SetFromErrnoWithFilenameObject(PyExc_OSError, nameobj);
goto error;
}
#endif /* defined(S_ISDIR) */
#ifdef HAVE_STRUCT_STAT_ST_BLKSIZE
if (fdfstat.st_blksize > 1)
self->blksize = fdfstat.st_blksize;
#endif /* HAVE_STRUCT_STAT_ST_BLKSIZE */
if (fdfstat.st_size < PY_SSIZE_T_MAX) {
self->estimated_size = (Py_off_t)fdfstat.st_size;
}
}

#if defined(MS_WINDOWS) || defined(__CYGWIN__)
Expand Down Expand Up @@ -521,6 +524,10 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
internal_close(self);
_PyErr_ChainExceptions1(exc);
}
if (self->stat_atopen != NULL) {
PyMem_Free(self->stat_atopen);
self->stat_atopen = NULL;
}

done:
#ifdef MS_WINDOWS
Expand Down Expand Up @@ -553,6 +560,10 @@ fileio_dealloc(fileio *self)
if (_PyIOBase_finalize((PyObject *) self) < 0)
return;
_PyObject_GC_UNTRACK(self);
if (self->stat_atopen != NULL) {
PyMem_Free(self->stat_atopen);
self->stat_atopen = NULL;
}
if (self->weakreflist != NULL)
PyObject_ClearWeakRefs((PyObject *) self);
(void)fileio_clear(self);
Expand Down Expand Up @@ -725,20 +736,27 @@ _io_FileIO_readall_impl(fileio *self)
return err_closed();
}

end = self->estimated_size;
if (self->stat_atopen != NULL && self->stat_atopen->st_size < _PY_READ_MAX) {
end = (Py_off_t)self->stat_atopen->st_size;
}
else {
end = -1;
}
if (end <= 0) {
/* Use a default size and resize as needed. */
bufsize = SMALLCHUNK;
}
else {
/* This is probably a real file, so we try to allocate a
buffer one byte larger than the rest of the file. If the
calculation is right then we should get EOF without having
to enlarge the buffer. */
/* This is probably a real file. */
if (end > _PY_READ_MAX - 1) {
bufsize = _PY_READ_MAX;
}
else {
/* In order to detect end of file, need a read() of at
least 1 byte which returns size 0. Oversize the buffer
by 1 byte so the I/O can be completed with two read()
calls (one for all data, one for EOF) without needing
to resize the buffer. */
bufsize = (size_t)end + 1;
}

Expand Down Expand Up @@ -1094,11 +1112,13 @@ _io_FileIO_truncate_impl(fileio *self, PyTypeObject *cls, PyObject *posobj)
return NULL;
}

/* Sometimes a large file is truncated. While estimated_size is used as a
estimate, that it is much larger than the actual size can result in a
significant over allocation and sometimes a MemoryError / running out of
memory. */
self->estimated_size = pos;
/* Since the file was truncated, its size at open is no longer accurate
as an estimate. Clear out the stat result, and rely on dynamic resize
code if a readall is requested. */
if (self->stat_atopen != NULL) {
PyMem_Free(self->stat_atopen);
self->stat_atopen = NULL;
}

return posobj;
}
Expand Down Expand Up @@ -1229,16 +1249,27 @@ get_mode(fileio *self, void *closure)
return PyUnicode_FromString(mode_string(self));
}

static PyObject *
get_blksize(fileio *self, void *closure)
{
#ifdef HAVE_STRUCT_STAT_ST_BLKSIZE
if (self->stat_atopen != NULL && self->stat_atopen->st_blksize > 1) {
return PyLong_FromLong(self->stat_atopen->st_blksize);
}
#endif /* HAVE_STRUCT_STAT_ST_BLKSIZE */
return PyLong_FromLong(DEFAULT_BUFFER_SIZE);
}

static PyGetSetDef fileio_getsetlist[] = {
{"closed", (getter)get_closed, NULL, "True if the file is closed"},
{"closefd", (getter)get_closefd, NULL,
"True if the file descriptor will be closed by close()."},
{"mode", (getter)get_mode, NULL, "String giving the file mode"},
{"_blksize", (getter)get_blksize, NULL, "Stat st_blksize if available"},
{NULL},
};

static PyMemberDef fileio_members[] = {
{"_blksize", Py_T_UINT, offsetof(fileio, blksize), 0},
{"_finalizing", Py_T_BOOL, offsetof(fileio, finalizing), 0},
{"__weaklistoffset__", Py_T_PYSSIZET, offsetof(fileio, weakreflist), Py_READONLY},
{"__dictoffset__", Py_T_PYSSIZET, offsetof(fileio, dict), Py_READONLY},
Expand Down

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