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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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bundles.txt
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A bundle is a container for repository data.
Bundles are used as standalone files as well as the interchange format
over the wire protocol used when two Mercurial peers communicate with
each other.
Headers
=======
Bundles produced since Mercurial 0.7 (September 2005) have a 4 byte
header identifying the major bundle type. The header always begins with
``HG`` and the follow 2 bytes indicate the bundle type/version. Some
bundle types have additional data after this 4 byte header.
The following sections describe each bundle header/type.
HG10
----
``HG10`` headers indicate a *changegroup bundle*. This is the original
bundle format, so it is sometimes referred to as *bundle1*. It has been
present since version 0.7 (released September 2005).
This header is followed by 2 bytes indicating the compression algorithm
used for data that follows. All subsequent data following this
compression identifier is compressed according to the algorithm/method
specified.
Supported algorithms include the following.
``BZ``
*bzip2* compression.
Bzip2 compressors emit a leading ``BZ`` header. Mercurial uses this
leading ``BZ`` as part of the bundle header. Therefore consumers
of bzip2 bundles need to *seed* the bzip2 decompressor with ``BZ`` or
seek the input stream back to the beginning of the algorithm component
of the bundle header so that decompressor input is valid. This behavior
is unique among supported compression algorithms.
Supported since version 0.7 (released December 2006).
``GZ``
*zlib* compression.
Supported since version 0.9.2 (released December 2006).
``UN``
*Uncompressed* or no compression. Unmodified changegroup data follows.
Supported since version 0.9.2 (released December 2006).
3rd party extensions may implement their own compression. However, no
authority reserves values for their compression algorithm identifiers.
HG2X
----
``HG2X`` headers (where ``X`` is any value) denote a *bundle2* bundle.
Bundle2 bundles are a container format for various kinds of repository
data and capabilities, beyond changegroup data (which was the only data
supported by ``HG10`` bundles.
``HG20`` is currently the only defined bundle2 version.
The ``HG20`` format is documented at :hg:`help internals.bundle2`.
Initial ``HG20`` support was added in Mercurial 3.0 (released May
2014). However, bundle2 bundles were hidden behind an experimental flag
until version 3.5 (released August 2015), when they were enabled in the
wire protocol. Various commands (including ``hg bundle``) did not
support generating bundle2 files until Mercurial 3.6 (released November
2015).
HGS1
----
*Experimental*
A ``HGS1`` header indicates a *streaming clone bundle*. This is a bundle
that contains raw revlog data from a repository store. (Typically revlog
data is exchanged in the form of changegroups.)
The purpose of *streaming clone bundles* are to *clone* repository data
very efficiently.
The ``HGS1`` header is always followed by 2 bytes indicating a
compression algorithm of the data that follows. Only ``UN``
(uncompressed data) is currently allowed.
``HGS1UN`` support was added as an experimental feature in version 3.6
(released November 2015) as part of the initial offering of the *clone
bundles* feature.