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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.

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test-issue1306.t
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https://bz.mercurial-scm.org/1306
Initialize remote repo with branches:
$ hg init remote
$ cd remote
$ echo a > a
$ hg ci -Ama
adding a
$ hg branch br
marked working directory as branch br
(branches are permanent and global, did you want a bookmark?)
$ hg ci -Amb
$ echo c > c
$ hg ci -Amc
adding c
$ hg log
changeset: 2:ae3d9c30ec50
branch: br
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: c
changeset: 1:3f7f930ca414
branch: br
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: b
changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
$ cd ..
Try cloning -r branch:
$ hg clone -rbr remote local1
adding changesets
adding manifests
adding file changes
added 3 changesets with 2 changes to 2 files
new changesets cb9a9f314b8b:ae3d9c30ec50
updating to branch br
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg -R local1 parents
changeset: 2:ae3d9c30ec50
branch: br
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: c
Try cloning -rother clone#branch:
$ hg clone -r0 remote#br local2
adding changesets
adding manifests
adding file changes
added 3 changesets with 2 changes to 2 files
new changesets cb9a9f314b8b:ae3d9c30ec50
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg -R local2 parents
changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
Try cloning -r1 clone#branch:
$ hg clone -r1 remote#br local3
adding changesets
adding manifests
adding file changes
added 3 changesets with 2 changes to 2 files
new changesets cb9a9f314b8b:ae3d9c30ec50
updating to branch br
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg -R local3 parents
changeset: 1:3f7f930ca414
branch: br
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: b