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posix: always seek to EOF when opening a file in append mode...
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.
Gregory Szorc - - Load All Authors

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test-cappedreader.py
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/ tests / test-cappedreader.py
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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__)