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.

[package]
name = "hgcli"
version = "0.1.0"
authors = ["Gregory Szorc <gregory.szorc@gmail.com>"]
license = "GPL-2.0"

build = "build.rs"

[[bin]]
name = "hg"
path = "src/main.rs"

[features]
# localdev: detect Python in PATH and use files from source checkout.
default = ["localdev"]
localdev = []

[dependencies]
libc = "0.2.34"

# We currently use a custom build of cpython and python27-sys with the
# following changes:
# * GILGuard call of prepare_freethreaded_python() is removed.
# TODO switch to official release when our changes are incorporated.
[dependencies.cpython]
version = "0.1"
default-features = false
features = ["python27-sys"]
git = "https://github.com/indygreg/rust-cpython.git"
rev = "c90d65cf84abfffce7ef54476bbfed56017a2f52"

[dependencies.python27-sys]
version = "0.1.2"
git = "https://github.com/indygreg/rust-cpython.git"
rev = "c90d65cf84abfffce7ef54476bbfed56017a2f52"