tests/test-http-protocol.t

localrepo: experimental support for non-zlib revlog compression

The final part of integrating the compression manager APIs into
revlog storage is the plumbing for repositories to advertise they
are using non-zlib storage and for revlogs to instantiate a non-zlib
compression engine.

The main intent of the compression manager work was to zstd all
of the things. Adding zstd to revlogs has proved to be more involved
than other places because revlogs are... special. Very small inputs
and the use of delta chains (which are themselves a form of
compression) are a completely different use case from streaming
compression, which bundles and the wire protocol employ. I've
conducted numerous experiments with zstd in revlogs and have yet
to formalize compression settings and a storage architecture that
I'm confident I won't regret later. In other words, I'm not yet
ready to commit to a new mechanism for using zstd - or any other
compression format - in revlogs.

That being said, having some support for zstd (and other compression
formats) in revlogs in core is beneficial. It can allow others to
conduct experiments.

This patch introduces *highly experimental* support for non-zlib
compression formats in revlogs. Introduced is a config option to
control which compression engine to use. Also introduced is a namespace
of "exp-compression-*" requirements to denote support for non-zlib
compression in revlogs. I've prefixed the namespace with "exp-"
(short for "experimental") because I'm not confident of the
requirements "schema" and in no way want to give the illusion of
supporting these requirements in the future. I fully intend to drop
support for these requirements once we figure out what we're doing
with zstd in revlogs.

A good portion of the patch is teaching the requirements system
about registered compression engines and passing the requested
compression engine as an opener option so revlogs can instantiate
the proper compression engine for new operations.

That's a verbose way of saying "we can now use zstd in revlogs!"

On an `hg pull` conversion of the mozilla-unified repo with no extra
redelta settings (like aggressivemergedeltas), we can see the impact
of zstd vs zlib in revlogs:

$ hg perfrevlogchunks -c
! chunk
! wall 2.032052 comb 2.040000 user 1.990000 sys 0.050000 (best of 5)
! wall 1.866360 comb 1.860000 user 1.820000 sys 0.040000 (best of 6)

! chunk batch
! wall 1.877261 comb 1.870000 user 1.860000 sys 0.010000 (best of 6)
! wall 1.705410 comb 1.710000 user 1.690000 sys 0.020000 (best of 6)

$ hg perfrevlogchunks -m
! chunk
! wall 2.721427 comb 2.720000 user 2.640000 sys 0.080000 (best of 4)
! wall 2.035076 comb 2.030000 user 1.950000 sys 0.080000 (best of 5)

! chunk batch
! wall 2.614561 comb 2.620000 user 2.580000 sys 0.040000 (best of 4)
! wall 1.910252 comb 1.910000 user 1.880000 sys 0.030000 (best of 6)

$ hg perfrevlog -c -d 1
! wall 4.812885 comb 4.820000 user 4.800000 sys 0.020000 (best of 3)
! wall 4.699621 comb 4.710000 user 4.700000 sys 0.010000 (best of 3)

$ hg perfrevlog -m -d 1000
! wall 34.252800 comb 34.250000 user 33.730000 sys 0.520000 (best of 3)
! wall 24.094999 comb 24.090000 user 23.320000 sys 0.770000 (best of 3)

Only modest wins for the changelog. But manifest reading is
significantly faster. What's going on?

One reason might be data volume. zstd decompresses faster. So given
more bytes, it will put more distance between it and zlib.

Another reason is size. In the current design, zstd revlogs are
*larger*:

debugcreatestreamclonebundle (size in bytes)
zlib: 1,638,852,492
zstd: 1,680,601,332

I haven't investigated this fully, but I reckon a significant cause of
larger revlogs is that the zstd frame/header has more bytes than
zlib's. For very small inputs or data that doesn't compress well, we'll
tend to store more uncompressed chunks than with zlib (because the
compressed size isn't smaller than original). This will make revlog
reading faster because it is doing less decompression.

Moving on to bundle performance:

$ hg bundle -a -t none-v2 (total CPU time)
zlib: 102.79s
zstd:  97.75s

So, marginal CPU decrease for reading all chunks in all revlogs
(this is somewhat disappointing).

$ hg bundle -a -t <engine>-v2 (total CPU time)
zlib: 191.59s
zstd: 115.36s

This last test effectively measures the difference between zlib->zlib
and zstd->zstd for revlogs to bundle. This is a rough approximation of
what a server does during `hg clone`.

There are some promising results for zstd. But not enough for me to
feel comfortable advertising it to users. We'll get there...

  $ cat >> $HGRCPATH << EOF
  > [web]
  > push_ssl = false
  > allow_push = *
  > EOF

  $ hg init server
  $ cd server
  $ touch a
  $ hg -q commit -A -m initial
  $ cd ..

  $ hg -R server serve -p $HGPORT -d --pid-file hg.pid
  $ cat hg.pid >> $DAEMON_PIDS

compression formats are advertised in compression capability

#if zstd
  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
  compression=zstd,zlib
#else
  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
  compression=ZL
#endif

  $ killdaemons.py

server.compressionengines can replace engines list wholesale

  $ hg --config server.compressionengines=none -R server serve -p $HGPORT -d --pid-file hg.pid
  $ cat hg.pid > $DAEMON_PIDS
  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
  compression=none

  $ killdaemons.py

Order of engines can also change

  $ hg --config server.compressionengines=none,zlib -R server serve -p $HGPORT -d --pid-file hg.pid
  $ cat hg.pid > $DAEMON_PIDS
  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=capabilities' | tr ' ' '\n' | grep compression
  compression=none,zlib

  $ killdaemons.py

Start a default server again

  $ hg -R server serve -p $HGPORT -d --pid-file hg.pid
  $ cat hg.pid > $DAEMON_PIDS

Server should send application/mercurial-0.1 to clients if no Accept is used

  $ get-with-headers.py --headeronly 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' -
  200 Script output follows
  content-type: application/mercurial-0.1
  date: * (glob)
  server: * (glob)
  transfer-encoding: chunked

Server should send application/mercurial-0.1 when client says it wants it

  $ get-with-headers.py --hgproto '0.1' --headeronly 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' -
  200 Script output follows
  content-type: application/mercurial-0.1
  date: * (glob)
  server: * (glob)
  transfer-encoding: chunked

Server should send application/mercurial-0.2 when client says it wants it

  $ get-with-headers.py --hgproto '0.2' --headeronly 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' -
  200 Script output follows
  content-type: application/mercurial-0.2
  date: * (glob)
  server: * (glob)
  transfer-encoding: chunked

  $ get-with-headers.py --hgproto '0.1 0.2' --headeronly 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' -
  200 Script output follows
  content-type: application/mercurial-0.2
  date: * (glob)
  server: * (glob)
  transfer-encoding: chunked

Requesting a compression format that server doesn't support results will fall back to 0.1

  $ get-with-headers.py --hgproto '0.2 comp=aa' --headeronly 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' -
  200 Script output follows
  content-type: application/mercurial-0.1
  date: * (glob)
  server: * (glob)
  transfer-encoding: chunked

#if zstd
zstd is used if available

  $ get-with-headers.py --hgproto '0.2 comp=zstd' 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' > resp
  $ f --size --hexdump --bytes 36 --sha1 resp
  resp: size=248, sha1=4d8d8f87fb82bd542ce52881fdc94f850748
  0000: 32 30 30 20 53 63 72 69 70 74 20 6f 75 74 70 75 |200 Script outpu|
  0010: 74 20 66 6f 6c 6c 6f 77 73 0a 0a 04 7a 73 74 64 |t follows...zstd|
  0020: 28 b5 2f fd                                     |(./.|

#endif

application/mercurial-0.2 is not yet used on non-streaming responses

  $ get-with-headers.py --hgproto '0.2' 127.0.0.1:$HGPORT '?cmd=heads' -
  200 Script output follows
  content-length: 41
  content-type: application/mercurial-0.1
  date: * (glob)
  server: * (glob)
  
  e93700bd72895c5addab234c56d4024b487a362f

Now test protocol preference usage

  $ killdaemons.py
  $ hg --config server.compressionengines=none,zlib -R server serve -p $HGPORT -d --pid-file hg.pid
  $ cat hg.pid > $DAEMON_PIDS

No Accept will send 0.1+zlib, even though "none" is preferred b/c "none" isn't supported on 0.1

  $ get-with-headers.py --headeronly 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000' Content-Type
  200 Script output follows
  content-type: application/mercurial-0.1

  $ get-with-headers.py 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000'  > resp
  $ f --size --hexdump --bytes 28 --sha1 resp
  resp: size=227, sha1=35a4c074da74f32f5440da3cbf04
  0000: 32 30 30 20 53 63 72 69 70 74 20 6f 75 74 70 75 |200 Script outpu|
  0010: 74 20 66 6f 6c 6c 6f 77 73 0a 0a 78             |t follows..x|

Explicit 0.1 will send zlib because "none" isn't supported on 0.1

  $ get-with-headers.py --hgproto '0.1' 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000'  > resp
  $ f --size --hexdump --bytes 28 --sha1 resp
  resp: size=227, sha1=35a4c074da74f32f5440da3cbf04
  0000: 32 30 30 20 53 63 72 69 70 74 20 6f 75 74 70 75 |200 Script outpu|
  0010: 74 20 66 6f 6c 6c 6f 77 73 0a 0a 78             |t follows..x|

0.2 with no compression will get "none" because that is server's preference
(spec says ZL and UN are implicitly supported)

  $ get-with-headers.py --hgproto '0.2' 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000'  > resp
  $ f --size --hexdump --bytes 32 --sha1 resp
  resp: size=432, sha1=ac931b412ec185a02e0e5bcff98dac83
  0000: 32 30 30 20 53 63 72 69 70 74 20 6f 75 74 70 75 |200 Script outpu|
  0010: 74 20 66 6f 6c 6c 6f 77 73 0a 0a 04 6e 6f 6e 65 |t follows...none|

Client receives server preference even if local order doesn't match

  $ get-with-headers.py --hgproto '0.2 comp=zlib,none' 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000'  > resp
  $ f --size --hexdump --bytes 32 --sha1 resp
  resp: size=432, sha1=ac931b412ec185a02e0e5bcff98dac83
  0000: 32 30 30 20 53 63 72 69 70 74 20 6f 75 74 70 75 |200 Script outpu|
  0010: 74 20 66 6f 6c 6c 6f 77 73 0a 0a 04 6e 6f 6e 65 |t follows...none|

Client receives only supported format even if not server preferred format

  $ get-with-headers.py --hgproto '0.2 comp=zlib' 127.0.0.1:$HGPORT '?cmd=getbundle&heads=e93700bd72895c5addab234c56d4024b487a362f&common=0000000000000000000000000000000000000000'  > resp
  $ f --size --hexdump --bytes 33 --sha1 resp
  resp: size=232, sha1=a1c727f0c9693ca15742a75c30419bc36
  0000: 32 30 30 20 53 63 72 69 70 74 20 6f 75 74 70 75 |200 Script outpu|
  0010: 74 20 66 6f 6c 6c 6f 77 73 0a 0a 04 7a 6c 69 62 |t follows...zlib|
  0020: 78                                              |x|