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.

--- a/mercurial/posix.py	Wed Jul 03 10:06:39 2019 +0800
+++ b/mercurial/posix.py	Mon Jul 08 13:12:20 2019 -0400
@@ -30,7 +30,6 @@
 
 osutil = policy.importmod(r'osutil')
 
-posixfile = open
 normpath = os.path.normpath
 samestat = os.path.samestat
 try:
@@ -52,6 +51,19 @@
 umask = os.umask(0)
 os.umask(umask)
 
+if not pycompat.ispy3:
+    def posixfile(name, mode=r'r', buffering=-1):
+        fp = open(name, mode=mode, buffering=buffering)
+        # The position when opening in append mode is implementation defined, so
+        # make it consistent by always seeking to the end.
+        if r'a' in mode:
+            fp.seek(0, os.SEEK_END)
+        return fp
+else:
+    # The underlying file object seeks as required in Python 3:
+    # https://github.com/python/cpython/blob/v3.7.3/Modules/_io/fileio.c#L474
+    posixfile = open
+
 def split(p):
     '''Same as posixpath.split, but faster