wireproto: add streams to frame-based protocol
Previously, the frame-based protocol was just a series of frames,
with each frame associated with a request ID.
In order to scale the protocol, we'll want to enable the use of
compression. While it is possible to enable compression at the
socket/pipe level, this has its disadvantages. The big one is it
undermines the point of frames being standalone, atomic units that
can be read and written: if you add compression above the framing
protocol, you are back to having a stream-based protocol as opposed
to something frame-based.
So in order to preserve frames, compression needs to occur at
the frame payload level.
Compressing each frame's payload individually will limit compression
ratios because the window size of the compressor will be limited
by the max frame size, which is 32-64kb as currently defined. It
will also add CPU overhead, as it is more efficient for compressors
to operate on fewer, larger blocks of data than more, smaller blocks.
So compressing each frame independently is out.
This means we need to compress each frame's payload as if it is part
of a larger stream.
The simplest approach is to have 1 stream per connection. This
could certainly work. However, it has disadvantages (documented below).
We could also have 1 stream per RPC/command invocation. (This is the
model HTTP/2 goes with.) This also has disadvantages.
The main disadvantage to one global stream is that it has the very
real potential to create CPU bottlenecks doing compression. Networks
are only getting faster and the performance of single CPU cores has
been relatively flat. Newer compression formats like zstandard offer
better CPU cycle efficiency than predecessors like zlib. But it still
all too common to saturate your CPU with compression overhead long
before you saturate the network pipe.
The main disadvantage with streams per request is that you can't
reap the benefits of the compression context for multiple requests.
For example, if you send 1000 RPC requests (or HTTP/2 requests for
that matter), the response to each would have its own compression
context. The overall size of the raw responses would be larger because
compression contexts wouldn't be able to reference data from another
request or response.
The approach for streams as implemented in this commit is to support
N streams per connection and for streams to potentially span requests
and responses. As explained by the added internals docs, this
facilitates servers and clients delegating independent streams and
compression to independent threads / CPU cores. This helps alleviate
the CPU bottleneck of compression. This design also allows compression
contexts to be reused across requests/responses. This can result in
improved compression ratios and less overhead for compressors and
decompressors having to build new contexts.
Another feature that was defined was the ability for individual frames
within a stream to declare whether that individual frame's payload
uses the content encoding (read: compression) defined by the stream.
The idea here is that some servers may serve data from a combination
of caches and dynamic resolution. Data coming from caches may be
pre-compressed. We want to facilitate servers being able to essentially
stream bytes from caches to the wire with minimal overhead. Being
able to mix and match with frames are compressed within a stream
enables these types of advanced server functionality.
This commit defines the new streams mechanism. Basic code for
supporting streams in frames has been added. But that code is
seriously lacking and doesn't fully conform to the defined protocol.
For example, we don't close any streams. And support for content
encoding within streams is not yet implemented. The change was
rather invasive and I didn't think it would be reasonable to implement
the entire feature in a single commit.
For the record, I would have loved to reuse an existing multiplexing
protocol to build the new wire protocol on top of. However, I couldn't
find a protocol that offers the performance and scaling characteristics
that I desired. Namely, it should support multiple compression
contexts to facilitate scaling out to multiple CPU cores and
compression contexts should be able to live longer than single RPC
requests. HTTP/2 *almost* fits the bill. But the semantics of HTTP
message exchange state that streams can only live for a single
request-response. We /could/ tunnel on top of HTTP/2 streams and
frames with HEADER and DATA frames. But there's no guarantee that
HTTP/2 libraries and proxies would allow us to use HTTP/2 streams
and frames without the HTTP message exchange semantics defined in
RFC 7540 Section 8. Other RPC protocols like gRPC tunnel are built
on top of HTTP/2 and thus preserve its semantics of stream per
RPC invocation. Even QUIC does this. We could attempt to invent a
higher-level stream that spans HTTP/2 streams. But this would be
violating HTTP/2 because there is no guarantee that HTTP/2 streams
are routed to the same server. The best we can do - which is what
this protocol does - is shoehorn all request and response data into
a single HTTP message and create streams within. At that point, we've
defined a Content-Type in HTTP parlance. It just so happens our
media type can also work as a standalone, stream-based protocol,
without leaning on HTTP or similar protocol.
Differential Revision: https://phab.mercurial-scm.org/D2907
Testing changing branch on commits
==================================
Setup
$ cat >> $HGRCPATH << EOF
> [alias]
> glog = log -G -T "{rev}:{node|short} {desc}\n{branch} ({bookmarks})"
> [experimental]
> evolution = createmarkers
> [extensions]
> rebase=
> EOF
$ hg init repo
$ cd repo
$ for ch in a b c d e; do echo foo >> $ch; hg ci -Aqm "Added "$ch; done
$ hg glog
@ 4:aa98ab95a928 Added e
| default ()
o 3:62615734edd5 Added d
| default ()
o 2:28ad74487de9 Added c
| default ()
o 1:29becc82797a Added b
| default ()
o 0:18d04c59bb5d Added a
default ()
$ hg branches
default 4:aa98ab95a928
Try without passing a new branch name
$ hg branch -r .
abort: no branch name specified for the revisions
[255]
Setting an invalid branch name
$ hg branch -r . a:b
abort: ':' cannot be used in a name
[255]
$ hg branch -r . tip
abort: the name 'tip' is reserved
[255]
$ hg branch -r . 1234
abort: cannot use an integer as a name
[255]
Change on non-linear set of commits
$ hg branch -r 2 -r 4 foo
abort: cannot change branch of non-linear revisions
[255]
Change in middle of the stack (linear commits)
$ hg branch -r 1::3 foo
abort: cannot change branch of changeset with children
[255]
Change with dirty working directory
$ echo bar > a
$ hg branch -r . foo
abort: uncommitted changes
[255]
$ hg revert --all
reverting a
Change on empty revision set
$ hg branch -r 'draft() - all()' foo
abort: empty revision set
[255]
Changing branch on linear set of commits from head
Without obsmarkers
$ hg branch -r 3:4 foo --config experimental.evolution=!
changed branch on 2 changesets
saved backup bundle to $TESTTMP/repo/.hg/strip-backup/62615734edd5-e86bd13a-branch-change.hg
$ hg glog
@ 4:3938acfb5c0f Added e
| foo ()
o 3:9435da006bdc Added d
| foo ()
o 2:28ad74487de9 Added c
| default ()
o 1:29becc82797a Added b
| default ()
o 0:18d04c59bb5d Added a
default ()
$ hg branches
foo 4:3938acfb5c0f
default 2:28ad74487de9 (inactive)
With obsmarkers
$ hg branch -r 3::4 bar
changed branch on 2 changesets
$ hg glog
@ 6:7c1991464886 Added e
| bar ()
o 5:1ea05e93925f Added d
| bar ()
o 2:28ad74487de9 Added c
| default ()
o 1:29becc82797a Added b
| default ()
o 0:18d04c59bb5d Added a
default ()
$ hg branches
bar 6:7c1991464886
default 2:28ad74487de9 (inactive)
Change branch name to an existing branch
$ hg branch -r . default
abort: a branch of the same name already exists
[255]
Changing on a branch head which is not topological head
$ hg branch -r 2 stable
abort: cannot change branch of changeset with children
[255]
Enabling the allowunstable config and trying to change branch on a branch head
which is not a topological head
$ echo "[experimental]" >> .hg/hgrc
$ echo "evolution.allowunstable=yes" >> .hg/hgrc
$ hg branch -r 2 foo
changed branch on 1 changesets
2 new orphan changesets
Changing branch of an obsoleted changeset
$ hg branch -r 4 foobar
abort: hidden revision '4' was rewritten as: 7c1991464886!
(use --hidden to access hidden revisions)
[255]
$ hg branch -r 4 --hidden foobar
abort: cannot change branch of a obsolete changeset
[255]
Make sure bookmark movement is correct
$ hg bookmark b1
$ hg glog -r '.^::'
@ 6:7c1991464886 Added e
| bar (b1)
* 5:1ea05e93925f Added d
| bar ()
~
$ hg branch -r '(.^)::' wat --debug
changing branch of '1ea05e93925f806d875a2163f9b76764be644636' from 'bar' to 'wat'
committing files:
d
committing manifest
committing changelog
new node id is 343660ccab7400da637bd6a211d07f413536d718
changing branch of '7c19914648869f5b02fc7fed31ddee9783fdd680' from 'bar' to 'wat'
committing files:
e
committing manifest
committing changelog
new node id is de1404b45a69f8cc6437d7679033ee33e9efb4ba
moving bookmarks ['b1'] from 7c19914648869f5b02fc7fed31ddee9783fdd680 to de1404b45a69f8cc6437d7679033ee33e9efb4ba
resolving manifests
branchmerge: False, force: False, partial: False
ancestor: 7c1991464886, local: 7c1991464886+, remote: de1404b45a69
starting 4 threads for background file closing (?)
changed branch on 2 changesets
updating the branch cache
invalid branchheads cache (served): tip differs
$ hg glog -r '(.^)::'
@ 9:de1404b45a69 Added e
| wat (b1)
* 8:343660ccab74 Added d
| wat ()
~
Make sure phase handling is correct
$ echo foo >> bar
$ hg ci -Aqm "added bar" --secret
1 new orphan changesets
$ hg glog -r .
@ 10:8ad1294c1660 added bar
| wat (b1)
~
$ hg branch -r . secret
changed branch on 1 changesets
$ hg phase -r .
11: secret
$ hg branches
secret 11:38a9b2d53f98
foo 7:8a4729a5e2b8
wat 9:de1404b45a69 (inactive)
default 2:28ad74487de9 (inactive)
$ hg branch
secret
Changing branch of another head, different from one on which we are
$ hg glog
@ 11:38a9b2d53f98 added bar
| secret (b1)
* 9:de1404b45a69 Added e
| wat ()
* 8:343660ccab74 Added d
| wat ()
| o 7:8a4729a5e2b8 Added c
| | foo ()
x | 2:28ad74487de9 Added c
|/ default ()
o 1:29becc82797a Added b
| default ()
o 0:18d04c59bb5d Added a
default ()
$ hg branch
secret
$ hg branch -r 7 foobar
changed branch on 1 changesets
The current branch must be preserved
$ hg branch
secret
Changing branch on multiple heads at once
$ hg rebase -s 8 -d 12 --keepbranches -q
$ hg rebase -s 14 -d 1 --keepbranches -q
$ hg branch -r 0: stable
changed branch on 6 changesets
$ hg glog
@ 23:6a5ddbcfb870 added bar
| stable (b1)
o 22:baedc6e98a67 Added e
| stable ()
| o 21:99ac7bf8aad1 Added d
| | stable ()
| o 20:0ecb4d39c4bd Added c
|/ stable ()
o 19:fd45b986b109 Added b
| stable ()
o 18:204d2769eca2 Added a
stable ()
$ hg branches
stable 23:6a5ddbcfb870
$ hg branch
stable
Changing to same branch is no-op
$ hg branch -r 19::21 stable
changed branch on 0 changesets
Changing branch name to existing branch name if the branch of parent of root of
revs is same as the new branch name
$ hg branch -r 20::21 bugfix
changed branch on 2 changesets
$ hg glog
o 25:714defe1cf34 Added d
| bugfix ()
o 24:98394def28fc Added c
| bugfix ()
| @ 23:6a5ddbcfb870 added bar
| | stable (b1)
| o 22:baedc6e98a67 Added e
|/ stable ()
o 19:fd45b986b109 Added b
| stable ()
o 18:204d2769eca2 Added a
stable ()
$ hg branch -r 24:25 stable
changed branch on 2 changesets
$ hg glog
o 27:4ec342341562 Added d
| stable ()
o 26:83f48859c2de Added c
| stable ()
| @ 23:6a5ddbcfb870 added bar
| | stable (b1)
| o 22:baedc6e98a67 Added e
|/ stable ()
o 19:fd45b986b109 Added b
| stable ()
o 18:204d2769eca2 Added a
stable ()
Testing on merge
$ hg merge -r 26
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg branch -r . abcd
abort: outstanding uncommitted merge
[255]
$ hg ci -m "Merge commit"
$ hg branch -r '(.^)::' def
abort: cannot change branch of a merge commit
[255]
Changing branch on public changeset
$ hg phase -r 27 -p
$ hg branch -r 27 def
abort: cannot change branch of public changesets
(see 'hg help phases' for details)
[255]