exchange: send bundle2 stream clones uncompressed

Stream clones don't compress well. And compression undermines
a point of stream clones which is to trade significant CPU
reductions by increasing size.

Building upon our introduction of metadata to communicate bundle
information back to callers of exchange.getbundlechunks(), we add
an attribute to the bundler that communicates whether the bundle is
best left uncompressed. We return this attribute as part of the bundle
metadata. And the wire protocol honors it when determining whether
to compress the wire protocol response.

The added test demonstrates that the raw result from the wire
protocol is not compressed. It also demonstrates that the server
will serve stream responses when the feature isn't enabled. We'll
address that in another commit.

The effect of this change is that server-side CPU usage for bundle2
stream clones is significantly reduced by removing zstd compression.
For the mozilla-unified repository:

before: 37.69 user 8.01 system
after:  27.38 user 7.34 system

Assuming things are CPU bound, that ~10s reduction would translate to
faster clones on the client. zstd can decompress at >1 GB/s. So the
overhead from decompression on the client is small in the grand scheme
of things. But if zlib compression were being used, the overhead would
be much greater.

Differential Revision: https://phab.mercurial-scm.org/D1926

mercurial/bundle2.py

@@ -599,6 +599,10 b' class bundle20(object):'
         self.capabilities = dict(capabilities)
         self._compengine = util.compengines.forbundletype('UN')
         self._compopts = None
+        # If compression is being handled by a consumer of the raw
+        # data (e.g. the wire protocol), unsetting this flag tells
+        # consumers that the bundle is best left uncompressed.
+        self.prefercompressed = True
 
     def setcompression(self, alg, compopts=None):
         """setup core part compression to <alg>"""

mercurial/exchange.py

@@ -1763,6 +1763,8 b' def getbundlechunks(repo, source, heads='
         func(bundler, repo, source, bundlecaps=bundlecaps, b2caps=b2caps,
              **pycompat.strkwargs(kwargs))
 
+    info['prefercompressed'] = bundler.prefercompressed
+
     return info, bundler.getchunks()
 
 @getbundle2partsgenerator('stream')
@@ -1770,6 +1772,12 b' def _getbundlestream(bundler, repo, sour'
                      b2caps=None, heads=None, common=None, **kwargs):
     if not kwargs.get('stream', False):
         return
+
+    # Stream clones don't compress well. And compression undermines a
+    # goal of stream clones, which is to be fast. Communicate the desire
+    # to avoid compression to consumers of the bundle.
+    bundler.prefercompressed = False
+
     filecount, bytecount, it = streamclone.generatev2(repo)
     requirements = ' '.join(sorted(repo.requirements))
     part = bundler.newpart('stream', data=it)

mercurial/wireproto.py

@@ -862,7 +862,7 b' def getbundle(repo, proto, others):'
             raise error.Abort(bundle2requiredmain,
                               hint=bundle2requiredhint)
 
-    preferuncompressed = False
+    prefercompressed = True
 
     try:
         if repo.ui.configbool('server', 'disablefullbundle'):
@@ -879,6 +879,7 b' def getbundle(repo, proto, others):'
 
         info, chunks = exchange.getbundlechunks(repo, 'serve',
                                                 **pycompat.strkwargs(opts))
+        prefercompressed = info.get('prefercompressed', True)
     except error.Abort as exc:
         # cleanly forward Abort error to the client
         if not exchange.bundle2requested(opts.get('bundlecaps')):
@@ -894,9 +895,9 b' def getbundle(repo, proto, others):'
         bundler.addpart(bundle2.bundlepart('error:abort',
                                            manargs, advargs))
         chunks = bundler.getchunks()
-        preferuncompressed = True
+        prefercompressed = False
 
-    return streamres(gen=chunks, prefer_uncompressed=preferuncompressed)
+    return streamres(gen=chunks, prefer_uncompressed=not prefercompressed)
 
 @wireprotocommand('heads')
 def heads(repo, proto):

tests/test-clone-uncompressed.t

@@ -47,6 +47,32 b' Basic clone'
   rbc-revs-v1
 #endif
 
+getbundle requests with stream=1 are uncompressed
+
+  $ get-with-headers.py $LOCALIP:$HGPORT '?cmd=getbundle' content-type --bodyfile body --hgproto '0.1 0.2 comp=zlib,none' --requestheader "x-hgarg-1=bundlecaps=HG20%2Cbundle2%3DHG20%250Abookmarks%250Achangegroup%253D01%252C02%250Adigests%253Dmd5%252Csha1%252Csha512%250Aerror%253Dabort%252Cunsupportedcontent%252Cpushraced%252Cpushkey%250Ahgtagsfnodes%250Alistkeys%250Aphases%253Dheads%250Apushkey%250Aremote-changegroup%253Dhttp%252Chttps&cg=0&common=0000000000000000000000000000000000000000&heads=c17445101a72edac06facd130d14808dfbd5c7c2&stream=1"
+  200 Script output follows
+  content-type: application/mercurial-0.2
+  
+
+  $ f --size --hex --bytes 256 body
+  body: size=112328
+  0000: 04 6e 6f 6e 65 48 47 32 30 00 00 00 00 00 00 00 |.noneHG20.......|
+  0010: 72 06 53 54 52 45 41 4d 00 00 00 00 04 00 09 05 |r.STREAM........|
+  0020: 09 04 0c 2d 07 02 62 79 74 65 63 6f 75 6e 74 39 |...-..bytecount9|
+  0030: 38 37 35 38 66 69 6c 65 63 6f 75 6e 74 31 30 33 |8758filecount103|
+  0040: 30 72 65 71 75 69 72 65 6d 65 6e 74 73 64 6f 74 |0requirementsdot|
+  0050: 65 6e 63 6f 64 65 20 66 6e 63 61 63 68 65 20 67 |encode fncache g|
+  0060: 65 6e 65 72 61 6c 64 65 6c 74 61 20 72 65 76 6c |eneraldelta revl|
+  0070: 6f 67 76 31 20 73 74 6f 72 65 76 65 72 73 69 6f |ogv1 storeversio|
+  0080: 6e 76 32 00 00 10 00 73 08 42 64 61 74 61 2f 30 |nv2....s.Bdata/0|
+  0090: 2e 69 00 03 00 01 00 00 00 00 00 00 00 02 00 00 |.i..............|
+  00a0: 00 01 00 00 00 00 00 00 00 01 ff ff ff ff ff ff |................|
+  00b0: ff ff 80 29 63 a0 49 d3 23 87 bf ce fe 56 67 92 |...)c.I.#....Vg.|
+  00c0: 67 2c 69 d1 ec 39 00 00 00 00 00 00 00 00 00 00 |g,i..9..........|
+  00d0: 00 00 75 30 73 08 42 64 61 74 61 2f 31 2e 69 00 |..u0s.Bdata/1.i.|
+  00e0: 03 00 01 00 00 00 00 00 00 00 02 00 00 00 01 00 |................|
+  00f0: 00 00 00 00 00 00 01 ff ff ff ff ff ff ff ff f9 |................|
+
 --uncompressed is an alias to --stream
 
 #if stream-legacy