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
# enable bundle2 in advance
$ cat << EOF >> $HGRCPATH
> [format]
> usegeneraldelta=yes
> EOF
$ mkdir part1
$ cd part1
$ hg init
$ echo a > a
$ hg add a
$ hg commit -m "1"
$ hg status
$ hg copy a b
$ hg --config ui.portablefilenames=abort copy a con.xml
abort: filename contains 'con', which is reserved on Windows: con.xml
[255]
$ hg status
A b
$ hg sum
parent: 0:c19d34741b0a tip
1
branch: default
commit: 1 copied
update: (current)
phases: 1 draft
$ hg --debug commit -m "2"
committing files:
b
b: copy a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
committing manifest
committing changelog
updating the branch cache
committed changeset 1:93580a2c28a50a56f63526fb305067e6fbf739c4
we should see two history entries
$ hg history -v
changeset: 1:93580a2c28a5
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
files: b
description:
2
changeset: 0:c19d34741b0a
user: test
date: Thu Jan 01 00:00:00 1970 +0000
files: a
description:
1
we should see one log entry for a
$ hg log a
changeset: 0:c19d34741b0a
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: 1
this should show a revision linked to changeset 0
$ hg debugindex a
rev linkrev nodeid p1 p2
0 0 b789fdd96dc2 000000000000 000000000000
we should see one log entry for b
$ hg log b
changeset: 1:93580a2c28a5
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: 2
this should show a revision linked to changeset 1
$ hg debugindex b
rev linkrev nodeid p1 p2
0 1 37d9b5d994ea 000000000000 000000000000
this should show the rename information in the metadata
$ hg debugdata b 0 | head -3 | tail -2
copy: a
copyrev: b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ md5sum.py .hg/store/data/b.i
44913824c8f5890ae218f9829535922e .hg/store/data/b.i
$ hg cat b > bsum
$ md5sum.py bsum
60b725f10c9c85c70d97880dfe8191b3 bsum
$ hg cat a > asum
$ md5sum.py asum
60b725f10c9c85c70d97880dfe8191b3 asum
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
2 files, 2 changesets, 2 total revisions
$ cd ..
$ mkdir part2
$ cd part2
$ hg init
$ echo foo > foo
should fail - foo is not managed
$ hg mv foo bar
foo: not copying - file is not managed
abort: no files to copy
[255]
$ hg st -A
? foo
$ hg add foo
dry-run; print a warning that this is not a real copy; foo is added
$ hg mv --dry-run foo bar
foo has not been committed yet, so no copy data will be stored for bar.
$ hg st -A
A foo
should print a warning that this is not a real copy; bar is added
$ hg mv foo bar
foo has not been committed yet, so no copy data will be stored for bar.
$ hg st -A
A bar
should print a warning that this is not a real copy; foo is added
$ hg cp bar foo
bar has not been committed yet, so no copy data will be stored for foo.
$ hg rm -f bar
$ rm bar
$ hg st -A
A foo
$ hg commit -m1
moving a missing file
$ rm foo
$ hg mv foo foo3
foo: deleted in working directory
foo3 does not exist!
$ hg up -qC .
copy --after to a nonexistent target filename
$ hg cp -A foo dummy
foo: not recording copy - dummy does not exist
dry-run; should show that foo is clean
$ hg copy --dry-run foo bar
$ hg st -A
C foo
should show copy
$ hg copy foo bar
$ hg st -C
A bar
foo
shouldn't show copy
$ hg commit -m2
$ hg st -C
should match
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 2ed2a3912a0b 000000000000 000000000000
$ hg debugrename bar
bar renamed from foo:2ed2a3912a0b24502043eae84ee4b279c18b90dd
$ echo bleah > foo
$ echo quux > bar
$ hg commit -m3
should not be renamed
$ hg debugrename bar
bar not renamed
$ hg copy -f foo bar
should show copy
$ hg st -C
M bar
foo
XXX: filtering lfilesrepo.status() in 3.3-rc causes the copy source to not be
displayed.
$ hg st -C --config extensions.largefiles=
The fsmonitor extension is incompatible with the largefiles extension and has been disabled. (fsmonitor !)
M bar
foo
$ hg commit -m3
should show no parents for tip
$ hg debugindex bar
rev linkrev nodeid p1 p2
0 1 7711d36246cc 000000000000 000000000000
1 2 bdf70a2b8d03 7711d36246cc 000000000000
2 3 b2558327ea8d 000000000000 000000000000
should match
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 2ed2a3912a0b 000000000000 000000000000
1 2 dd12c926cf16 2ed2a3912a0b 000000000000
$ hg debugrename bar
bar renamed from foo:dd12c926cf165e3eb4cf87b084955cb617221c17
should show no copies
$ hg st -C
copy --after on an added file
$ cp bar baz
$ hg add baz
$ hg cp -A bar baz
$ hg st -C
A baz
bar
foo was clean:
$ hg st -AC foo
C foo
Trying to copy on top of an existing file fails,
$ hg copy -A bar foo
foo: not overwriting - file already committed
(hg copy --after --force to replace the file by recording a copy)
same error without the --after, so the user doesn't have to go through
two hints:
$ hg copy bar foo
foo: not overwriting - file already committed
(hg copy --force to replace the file by recording a copy)
but it's considered modified after a copy --after --force
$ hg copy -Af bar foo
$ hg st -AC foo
M foo
bar
The hint for a file that exists but is not in file history doesn't
mention --force:
$ touch xyzzy
$ hg cp bar xyzzy
xyzzy: not overwriting - file exists
(hg copy --after to record the copy)
$ cd ..