posix: always seek to EOF when opening a file in append mode
Python 3 already does this, so skip it there.
Consider the program:
#include <stdio.h>
int main() {
FILE *f = fopen("narf", "w");
fprintf(f, "narf\n");
fclose(f);
f = fopen("narf", "a");
printf("%ld\n", ftell(f));
fprintf(f, "troz\n");
printf("%ld\n", ftell(f));
return 0;
}
on macOS, FreeBSD, and Linux with glibc, this program prints
5
10
but on musl libc (Alpine Linux and probably others) this prints
0
10
By my reading of
https://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html
this is technically correct, specifically:
> Opening a file with append mode (a as the first character in the
> mode argument) shall cause all subsequent writes to the file to be
> forced to the then current end-of-file, regardless of intervening
> calls to fseek().
in other words, the file position doesn't really matter in append-mode
files, and we can't depend on it being at all meaningful unless we
perform a seek() before tell() after open(..., 'a'). Experimentally
after a .write() we can do a .tell() and it'll always be reasonable,
but I'm unclear from reading the specification if that's a smart thing
to rely on. This matches what we do on Windows and what Python 3 does
for free, so let's just be consistent. Thanks to Yuya for the idea.
from __future__ import absolute_import, print_function
import io
import unittest
from mercurial import (
util,
)
class CappedReaderTests(unittest.TestCase):
def testreadfull(self):
source = io.BytesIO(b'x' * 100)
reader = util.cappedreader(source, 10)
res = reader.read(10)
self.assertEqual(res, b'x' * 10)
self.assertEqual(source.tell(), 10)
source.seek(0)
reader = util.cappedreader(source, 15)
res = reader.read(16)
self.assertEqual(res, b'x' * 15)
self.assertEqual(source.tell(), 15)
source.seek(0)
reader = util.cappedreader(source, 100)
res = reader.read(100)
self.assertEqual(res, b'x' * 100)
self.assertEqual(source.tell(), 100)
source.seek(0)
reader = util.cappedreader(source, 50)
res = reader.read()
self.assertEqual(res, b'x' * 50)
self.assertEqual(source.tell(), 50)
source.seek(0)
def testreadnegative(self):
source = io.BytesIO(b'x' * 100)
reader = util.cappedreader(source, 20)
res = reader.read(-1)
self.assertEqual(res, b'x' * 20)
self.assertEqual(source.tell(), 20)
source.seek(0)
reader = util.cappedreader(source, 100)
res = reader.read(-1)
self.assertEqual(res, b'x' * 100)
self.assertEqual(source.tell(), 100)
source.seek(0)
def testreadmultiple(self):
source = io.BytesIO(b'x' * 100)
reader = util.cappedreader(source, 10)
for i in range(10):
res = reader.read(1)
self.assertEqual(res, b'x')
self.assertEqual(source.tell(), i + 1)
self.assertEqual(source.tell(), 10)
res = reader.read(1)
self.assertEqual(res, b'')
self.assertEqual(source.tell(), 10)
source.seek(0)
reader = util.cappedreader(source, 45)
for i in range(4):
res = reader.read(10)
self.assertEqual(res, b'x' * 10)
self.assertEqual(source.tell(), (i + 1) * 10)
res = reader.read(10)
self.assertEqual(res, b'x' * 5)
self.assertEqual(source.tell(), 45)
def readlimitpasteof(self):
source = io.BytesIO(b'x' * 100)
reader = util.cappedreader(source, 1024)
res = reader.read(1000)
self.assertEqual(res, b'x' * 100)
self.assertEqual(source.tell(), 100)
res = reader.read(1000)
self.assertEqual(res, b'')
self.assertEqual(source.tell(), 100)
if __name__ == '__main__':
import silenttestrunner
silenttestrunner.main(__name__)