[PATCH 02 of 15 V2] bookmark: add methods to binary encode and decode bookmark values
boris.feld at octobus.net
Thu Nov 16 12:59:24 EST 2017
On Mon, 2017-11-13 at 18:30 -0500, Augie Fackler wrote:
> > On Nov 13, 2017, at 05:16, Boris Feld <boris.feld at octobus.net>
> > wrote:
> > On Fri, 2017-11-10 at 17:35 -0500, Augie Fackler wrote:
> > > (+indygreg, who also is a formats enthusiast)
> > >
> > > On Thu, Nov 02, 2017 at 02:17:59PM +0100, Boris Feld wrote:
> > > > # HG changeset patch
> > > > # User Boris Feld <boris.feld at octobus.net>
> > > > # Date 1508072395 -7200
> > > > # Sun Oct 15 14:59:55 2017 +0200
> > > > # Node ID 4d0c6772a81aa1e2b25f32f944563db1f33fd327
> > > > # Parent 8c9a9eecdcd61401a1604a08a5272f7dabd4b912
> > > > # EXP-Topic b2.bookmarks
> > > > # Available At https://bitbucket.org/octobus/mercurial-devel/
> > > > # hg pull https://bitbucket.org/octobus/mercurial-
> > > > deve
> > > > l/ -r 4d0c6772a81a
> > > > bookmark: add methods to binary encode and decode bookmark
> > > > values
> > > >
> > > > Coming new bundle2 parts related to bookmark will use a binary
> > > > encoding. It
> > > > encodes a series of '(bookmark, node)' pairs. Bookmark name has
> > > > a
> > > > high enough
> > > > size limit to not be affected by issue5165. (64K length, we are
> > > > well covered)
> > >
> > > I'm not thrilled here. Could we do some sort of varint encoding,
> > > which
> > > would be generally useful going forward for a variety of things,
> > > rather than just shrugging and setting a limit which we (today)
> > > deem
> > > absurdly large?
> > We are not sure what the value of that would be. Bundle2 makes it
> > very
> > easy to move to a newer encoding version if needed.
> I mean, not really. We have to add a b2cap, and then we've got to
> maintain multiple versions forever. It's kind of a pain. A varint
> encoding wouldn't be hard, and will probably take O(an hour) to code
> up. I'd entertain that much more eagerly than just dropping a uint64
> in here and asserting it should be big enough forever.
> > We are already
> > doing so for changegroup and obsmarkers.
> > The biggest bookmark we have seen was about 800 characters, that
> > should
> > leave us some room for the future without blocking us.
> That you know about. I seem to remember violent arguments that 255
> bytes should be more than enough. I agree a 64k bookmark sounds
> totally insane, but what if I wanted to put base64'd test result
> metadata in a bookmark?
> > >
> > > I agree that practically speaking, nobody should have a 64k
> > > bookmark,
> > > but we can do better. We also know that \0 shouldn't appear in a
> > > bookmark name, so we could just make the format be
> > > null-terminated. I'd much rather we get in the practice of
> > > deliberately crafting formats that are maximally flexible and
> > > reusable.
> > That would be significantly harder to parse since we would need to
> > search for the '\0' in the stream. We do not think it brings much
> > value
> > either since any new data we sneak into the last field would have
> > to be
> > understood by the receiver. If we need to negotiate for such
> > capabilities, this is equivalent to rolling out a new encoding.
> Then let's explore the varint option. Here's a rough sketch of a
> import io
> src = io.BytesIO(b'\xff\x01')
> cur = ord(src.read(1))
> i = cur & 0x7f
> while cur & 0x80:
> i = i << 7
> cur = ord(src.read(1))
> i += cur & 0x7f
> assert 0b11111110000001 == i
> Gives you 7 bits of value per byte transmitted, which means that
> we'll get most realistic bookmark lengths in two bytes of "here's how
> much string" (probably one byte for the vast majority of ascii
> I'd much rather we just build up the infrastructure for stuff like
> this _now_ and start using it, rather than continue to punt and shove
> massive uint64 fields in and then pray it'll be enough for all
> eternity. It does mean we can't use struct anymore, but that seems
> like a comparatively small price to pay for being able to trivially
> make this future proof.
> (As before, if other reviewers think I'm being too hard on this,
> please speak up.)
The current exchange implementation is already putting some soft
limits, the HTTP header size limit (usually around 8K). If people tried
exchanging more than 8K bookmarks over HTTP, they would have seen an
error and we would have seen tickets on Bugzilla.
Bookmarks are labels on changesets to help track lines of development,
so encoding the results of the CI as bookmark seems to fall outside
what bookmarks are designed for. As such, we think we should not
support this use-case, at least not with bookmarks.
If we want bookmarks larger than 64K, we will need to fix several other
problems in order to have decent performance and UI before supporting
bigger bookmarks decently.
As bookmarks are labels, they are displayed in various UI, both in
Mercurial and third-parties interfaces (GUI, hosting). I'm pretty sure
these interfaces were not designed for such longs bookmarks. And
designing an interface for an arbitrary-length bookmark name is not an
easy task. I tested adding a 64K bookmark in one of my repository and
thg crashed, I cannot view this repository even after removing the
bookmark and restarting thg.
Also, as they are labels, they are loaded very often. Arbitrary-length
bookmark name will require adjustments on the loading bookmark code and
to be very careful with memory usage.
We would prefer not to block this bugfix on testing varint, if a real
use-case appears for bigger bookmark, we would be happy and will take
care of updating the needed code.
The varint ideas seem interesting on its own, but we don't think we
should use it here, the extra complexity doesn't seem worthwhile in
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