wireprotov2: advertise redirect targets in capabilities

This is pretty straightforward.

Redirect targets will require an extension to support. So we've added
a function that can be wrapped to define redirect targets.

To test this, we teach our simple cache test extension to read
redirect targets from a file. It's a bit hacky. But it gets the
job done.

Differential Revision: https://phab.mercurial-scm.org/D4775

tests/test-wireproto-content-redirects.t

@@ -0,0 +1,601 b''
+  $ . $TESTDIR/wireprotohelpers.sh
+
+  $ hg init server
+  $ enablehttpv2 server
+  $ cd server
+  $ cat >> .hg/hgrc << EOF
+  > [extensions]
+  > simplecache = $TESTDIR/wireprotosimplecache.py
+  > EOF
+
+  $ echo a0 > a
+  $ echo b0 > b
+  $ hg -q commit -A -m 'commit 0'
+  $ echo a1 > a
+  $ hg commit -m 'commit 1'
+
+  $ hg --debug debugindex -m
+     rev linkrev nodeid                                   p1                                       p2
+       0       0 992f4779029a3df8d0666d00bb924f69634e2641 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000
+       1       1 a988fb43583e871d1ed5750ee074c6d840bbbfc8 992f4779029a3df8d0666d00bb924f69634e2641 0000000000000000000000000000000000000000
+
+  $ hg --config simplecache.redirectsfile=redirects.py serve -p $HGPORT -d --pid-file hg.pid -E error.log
+  $ cat hg.pid > $DAEMON_PIDS
+
+  $ cat > redirects.py << EOF
+  > [
+  >   {
+  >     b'name': b'target-a',
+  >     b'protocol': b'http',
+  >     b'snirequired': False,
+  >     b'tlsversions': [b'1.2', b'1.3'],
+  >     b'uris': [b'http://example.com/'],
+  >   },
+  > ]
+  > EOF
+
+Redirect targets advertised when configured
+
+  $ sendhttpv2peerhandshake << EOF
+  > command capabilities
+  > EOF
+  creating http peer for wire protocol version 2
+  s>     GET /?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     vary: X-HgProto-1,X-HgUpgrade-1\r\n
+  s>     x-hgproto-1: cbor\r\n
+  s>     x-hgupgrade-1: exp-http-v2-0002\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 OK\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-cbor\r\n
+  s>     Content-Length: 1970\r\n
+  s>     \r\n
+  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0002\xa6Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x84IbookmarksGparentsEphaseHrevisionInoderange\xa3Gdefault\xf6Hrequired\xf4DtypeDlistEnodes\xa3Gdefault\xf6Hrequired\xf4DtypeDlistJnodesdepth\xa3Gdefault\xf6Hrequired\xf4DtypeCintKpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDpath\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xa3Gdefault\xf4Hrequired\xf4DtypeDboolKpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\xa3Gdefault\x80Hrequired\xf4DtypeDlistKpermissions\x81DpullHlistkeys\xa2Dargs\xa1Inamespace\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullFlookup\xa2Dargs\xa1Ckey\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDtree\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullGpushkey\xa2Dargs\xa4Ckey\xa2Hrequired\xf5DtypeEbytesInamespace\xa2Hrequired\xf5DtypeEbytesCnew\xa2Hrequired\xf5DtypeEbytesCold\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpushKcompression\x82\xa1DnameDzstd\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Hredirect\xa2Fhashes\x82Fsha256Dsha1Gtargets\x81\xa5DnameHtarget-aHprotocolDhttpKsnirequired\xf4Ktlsversions\x82C1.2C1.3Duris\x81Shttp://example.com/Nv1capabilitiesY\x01\xd8batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
+  sending capabilities command
+  s>     POST /api/exp-http-v2-0002/ro/capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-exp-framing-0005\r\n
+  s>     content-type: application/mercurial-exp-framing-0005\r\n
+  s>     content-length: 27\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s>     \x13\x00\x00\x01\x00\x01\x01\x11\xa1DnameLcapabilities
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 OK\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-exp-framing-0005\r\n
+  s>     Transfer-Encoding: chunked\r\n
+  s>     \r\n
+  s>     13\r\n
+  s>     \x0b\x00\x00\x01\x00\x02\x011
+  s>     \xa1FstatusBok
+  s>     \r\n
+  received frame(size=11; request=1; stream=2; streamflags=stream-begin; type=command-response; flags=continuation)
+  s>     5ab\r\n
+  s>     \xa3\x05\x00\x01\x00\x02\x001
+  s>     \xa6Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x84IbookmarksGparentsEphaseHrevisionInoderange\xa3Gdefault\xf6Hrequired\xf4DtypeDlistEnodes\xa3Gdefault\xf6Hrequired\xf4DtypeDlistJnodesdepth\xa3Gdefault\xf6Hrequired\xf4DtypeCintKpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDpath\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xa3Gdefault\xf4Hrequired\xf4DtypeDboolKpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\xa3Gdefault\x80Hrequired\xf4DtypeDlistKpermissions\x81DpullHlistkeys\xa2Dargs\xa1Inamespace\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullFlookup\xa2Dargs\xa1Ckey\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDtree\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullGpushkey\xa2Dargs\xa4Ckey\xa2Hrequired\xf5DtypeEbytesInamespace\xa2Hrequired\xf5DtypeEbytesCnew\xa2Hrequired\xf5DtypeEbytesCold\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpushKcompression\x82\xa1DnameDzstd\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Hredirect\xa2Fhashes\x82Fsha256Dsha1Gtargets\x81\xa5DnameHtarget-aHprotocolDhttpKsnirequired\xf4Ktlsversions\x82C1.2C1.3Duris\x81Shttp://example.com/
+  s>     \r\n
+  received frame(size=1443; request=1; stream=2; streamflags=; type=command-response; flags=continuation)
+  s>     8\r\n
+  s>     \x00\x00\x00\x01\x00\x02\x002
+  s>     \r\n
+  s>     0\r\n
+  s>     \r\n
+  received frame(size=0; request=1; stream=2; streamflags=; type=command-response; flags=eos)
+  response: gen[
+    {
+      b'commands': {
+        b'branchmap': {
+          b'args': {},
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'capabilities': {
+          b'args': {},
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'changesetdata': {
+          b'args': {
+            b'fields': {
+              b'default': set([]),
+              b'required': False,
+              b'type': b'set',
+              b'validvalues': set([
+                b'bookmarks',
+                b'parents',
+                b'phase',
+                b'revision'
+              ])
+            },
+            b'noderange': {
+              b'default': None,
+              b'required': False,
+              b'type': b'list'
+            },
+            b'nodes': {
+              b'default': None,
+              b'required': False,
+              b'type': b'list'
+            },
+            b'nodesdepth': {
+              b'default': None,
+              b'required': False,
+              b'type': b'int'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'filedata': {
+          b'args': {
+            b'fields': {
+              b'default': set([]),
+              b'required': False,
+              b'type': b'set',
+              b'validvalues': set([
+                b'parents',
+                b'revision'
+              ])
+            },
+            b'haveparents': {
+              b'default': False,
+              b'required': False,
+              b'type': b'bool'
+            },
+            b'nodes': {
+              b'required': True,
+              b'type': b'list'
+            },
+            b'path': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'heads': {
+          b'args': {
+            b'publiconly': {
+              b'default': False,
+              b'required': False,
+              b'type': b'bool'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'known': {
+          b'args': {
+            b'nodes': {
+              b'default': [],
+              b'required': False,
+              b'type': b'list'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'listkeys': {
+          b'args': {
+            b'namespace': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'lookup': {
+          b'args': {
+            b'key': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'manifestdata': {
+          b'args': {
+            b'fields': {
+              b'default': set([]),
+              b'required': False,
+              b'type': b'set',
+              b'validvalues': set([
+                b'parents',
+                b'revision'
+              ])
+            },
+            b'haveparents': {
+              b'default': False,
+              b'required': False,
+              b'type': b'bool'
+            },
+            b'nodes': {
+              b'required': True,
+              b'type': b'list'
+            },
+            b'tree': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'pushkey': {
+          b'args': {
+            b'key': {
+              b'required': True,
+              b'type': b'bytes'
+            },
+            b'namespace': {
+              b'required': True,
+              b'type': b'bytes'
+            },
+            b'new': {
+              b'required': True,
+              b'type': b'bytes'
+            },
+            b'old': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'push'
+          ]
+        }
+      },
+      b'compression': [
+        {
+          b'name': b'zstd'
+        },
+        {
+          b'name': b'zlib'
+        }
+      ],
+      b'framingmediatypes': [
+        b'application/mercurial-exp-framing-0005'
+      ],
+      b'pathfilterprefixes': set([
+        b'path:',
+        b'rootfilesin:'
+      ]),
+      b'rawrepoformats': [
+        b'generaldelta',
+        b'revlogv1'
+      ],
+      b'redirect': {
+        b'hashes': [
+          b'sha256',
+          b'sha1'
+        ],
+        b'targets': [
+          {
+            b'name': b'target-a',
+            b'protocol': b'http',
+            b'snirequired': False,
+            b'tlsversions': [
+              b'1.2',
+              b'1.3'
+            ],
+            b'uris': [
+              b'http://example.com/'
+            ]
+          }
+        ]
+      }
+    }
+  ]
+
+  $ cat > redirects.py << EOF
+  > [
+  >   {
+  >     b'name': b'target-a',
+  >     b'protocol': b'http',
+  >     b'uris': [b'http://example.com/'],
+  >   },
+  >   {
+  >     b'name': b'target-b',
+  >     b'protocol': b'unknown',
+  >     b'uris': [b'unknown://example.com/'],
+  >   },
+  > ]
+  > EOF
+
+  $ sendhttpv2peerhandshake << EOF
+  > command capabilities
+  > EOF
+  creating http peer for wire protocol version 2
+  s>     GET /?cmd=capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     vary: X-HgProto-1,X-HgUpgrade-1\r\n
+  s>     x-hgproto-1: cbor\r\n
+  s>     x-hgupgrade-1: exp-http-v2-0002\r\n
+  s>     accept: application/mercurial-0.1\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 OK\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-cbor\r\n
+  s>     Content-Length: 1997\r\n
+  s>     \r\n
+  s>     \xa3GapibaseDapi/Dapis\xa1Pexp-http-v2-0002\xa6Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x84IbookmarksGparentsEphaseHrevisionInoderange\xa3Gdefault\xf6Hrequired\xf4DtypeDlistEnodes\xa3Gdefault\xf6Hrequired\xf4DtypeDlistJnodesdepth\xa3Gdefault\xf6Hrequired\xf4DtypeCintKpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDpath\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xa3Gdefault\xf4Hrequired\xf4DtypeDboolKpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\xa3Gdefault\x80Hrequired\xf4DtypeDlistKpermissions\x81DpullHlistkeys\xa2Dargs\xa1Inamespace\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullFlookup\xa2Dargs\xa1Ckey\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDtree\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullGpushkey\xa2Dargs\xa4Ckey\xa2Hrequired\xf5DtypeEbytesInamespace\xa2Hrequired\xf5DtypeEbytesCnew\xa2Hrequired\xf5DtypeEbytesCold\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpushKcompression\x82\xa1DnameDzstd\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Hredirect\xa2Fhashes\x82Fsha256Dsha1Gtargets\x82\xa3DnameHtarget-aHprotocolDhttpDuris\x81Shttp://example.com/\xa3DnameHtarget-bHprotocolGunknownDuris\x81Vunknown://example.com/Nv1capabilitiesY\x01\xd8batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset compression=$BUNDLE2_COMPRESSIONS$ getbundle httpheader=1024 httpmediatype=0.1rx,0.1tx,0.2tx known lookup pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
+  sending capabilities command
+  s>     POST /api/exp-http-v2-0002/ro/capabilities HTTP/1.1\r\n
+  s>     Accept-Encoding: identity\r\n
+  s>     accept: application/mercurial-exp-framing-0005\r\n
+  s>     content-type: application/mercurial-exp-framing-0005\r\n
+  s>     content-length: 27\r\n
+  s>     host: $LOCALIP:$HGPORT\r\n (glob)
+  s>     user-agent: Mercurial debugwireproto\r\n
+  s>     \r\n
+  s>     \x13\x00\x00\x01\x00\x01\x01\x11\xa1DnameLcapabilities
+  s> makefile('rb', None)
+  s>     HTTP/1.1 200 OK\r\n
+  s>     Server: testing stub value\r\n
+  s>     Date: $HTTP_DATE$\r\n
+  s>     Content-Type: application/mercurial-exp-framing-0005\r\n
+  s>     Transfer-Encoding: chunked\r\n
+  s>     \r\n
+  s>     13\r\n
+  s>     \x0b\x00\x00\x01\x00\x02\x011
+  s>     \xa1FstatusBok
+  s>     \r\n
+  received frame(size=11; request=1; stream=2; streamflags=stream-begin; type=command-response; flags=continuation)
+  s>     5c6\r\n
+  s>     \xbe\x05\x00\x01\x00\x02\x001
+  s>     \xa6Hcommands\xaaIbranchmap\xa2Dargs\xa0Kpermissions\x81DpullLcapabilities\xa2Dargs\xa0Kpermissions\x81DpullMchangesetdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x84IbookmarksGparentsEphaseHrevisionInoderange\xa3Gdefault\xf6Hrequired\xf4DtypeDlistEnodes\xa3Gdefault\xf6Hrequired\xf4DtypeDlistJnodesdepth\xa3Gdefault\xf6Hrequired\xf4DtypeCintKpermissions\x81DpullHfiledata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDpath\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullEheads\xa2Dargs\xa1Jpubliconly\xa3Gdefault\xf4Hrequired\xf4DtypeDboolKpermissions\x81DpullEknown\xa2Dargs\xa1Enodes\xa3Gdefault\x80Hrequired\xf4DtypeDlistKpermissions\x81DpullHlistkeys\xa2Dargs\xa1Inamespace\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullFlookup\xa2Dargs\xa1Ckey\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullLmanifestdata\xa2Dargs\xa4Ffields\xa4Gdefault\xd9\x01\x02\x80Hrequired\xf4DtypeCsetKvalidvalues\xd9\x01\x02\x82GparentsHrevisionKhaveparents\xa3Gdefault\xf4Hrequired\xf4DtypeDboolEnodes\xa2Hrequired\xf5DtypeDlistDtree\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpullGpushkey\xa2Dargs\xa4Ckey\xa2Hrequired\xf5DtypeEbytesInamespace\xa2Hrequired\xf5DtypeEbytesCnew\xa2Hrequired\xf5DtypeEbytesCold\xa2Hrequired\xf5DtypeEbytesKpermissions\x81DpushKcompression\x82\xa1DnameDzstd\xa1DnameDzlibQframingmediatypes\x81X&application/mercurial-exp-framing-0005Rpathfilterprefixes\xd9\x01\x02\x82Epath:Lrootfilesin:Nrawrepoformats\x82LgeneraldeltaHrevlogv1Hredirect\xa2Fhashes\x82Fsha256Dsha1Gtargets\x82\xa3DnameHtarget-aHprotocolDhttpDuris\x81Shttp://example.com/\xa3DnameHtarget-bHprotocolGunknownDuris\x81Vunknown://example.com/
+  s>     \r\n
+  received frame(size=1470; request=1; stream=2; streamflags=; type=command-response; flags=continuation)
+  s>     8\r\n
+  s>     \x00\x00\x00\x01\x00\x02\x002
+  s>     \r\n
+  s>     0\r\n
+  s>     \r\n
+  received frame(size=0; request=1; stream=2; streamflags=; type=command-response; flags=eos)
+  response: gen[
+    {
+      b'commands': {
+        b'branchmap': {
+          b'args': {},
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'capabilities': {
+          b'args': {},
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'changesetdata': {
+          b'args': {
+            b'fields': {
+              b'default': set([]),
+              b'required': False,
+              b'type': b'set',
+              b'validvalues': set([
+                b'bookmarks',
+                b'parents',
+                b'phase',
+                b'revision'
+              ])
+            },
+            b'noderange': {
+              b'default': None,
+              b'required': False,
+              b'type': b'list'
+            },
+            b'nodes': {
+              b'default': None,
+              b'required': False,
+              b'type': b'list'
+            },
+            b'nodesdepth': {
+              b'default': None,
+              b'required': False,
+              b'type': b'int'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'filedata': {
+          b'args': {
+            b'fields': {
+              b'default': set([]),
+              b'required': False,
+              b'type': b'set',
+              b'validvalues': set([
+                b'parents',
+                b'revision'
+              ])
+            },
+            b'haveparents': {
+              b'default': False,
+              b'required': False,
+              b'type': b'bool'
+            },
+            b'nodes': {
+              b'required': True,
+              b'type': b'list'
+            },
+            b'path': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'heads': {
+          b'args': {
+            b'publiconly': {
+              b'default': False,
+              b'required': False,
+              b'type': b'bool'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'known': {
+          b'args': {
+            b'nodes': {
+              b'default': [],
+              b'required': False,
+              b'type': b'list'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'listkeys': {
+          b'args': {
+            b'namespace': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'lookup': {
+          b'args': {
+            b'key': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'manifestdata': {
+          b'args': {
+            b'fields': {
+              b'default': set([]),
+              b'required': False,
+              b'type': b'set',
+              b'validvalues': set([
+                b'parents',
+                b'revision'
+              ])
+            },
+            b'haveparents': {
+              b'default': False,
+              b'required': False,
+              b'type': b'bool'
+            },
+            b'nodes': {
+              b'required': True,
+              b'type': b'list'
+            },
+            b'tree': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'pull'
+          ]
+        },
+        b'pushkey': {
+          b'args': {
+            b'key': {
+              b'required': True,
+              b'type': b'bytes'
+            },
+            b'namespace': {
+              b'required': True,
+              b'type': b'bytes'
+            },
+            b'new': {
+              b'required': True,
+              b'type': b'bytes'
+            },
+            b'old': {
+              b'required': True,
+              b'type': b'bytes'
+            }
+          },
+          b'permissions': [
+            b'push'
+          ]
+        }
+      },
+      b'compression': [
+        {
+          b'name': b'zstd'
+        },
+        {
+          b'name': b'zlib'
+        }
+      ],
+      b'framingmediatypes': [
+        b'application/mercurial-exp-framing-0005'
+      ],
+      b'pathfilterprefixes': set([
+        b'path:',
+        b'rootfilesin:'
+      ]),
+      b'rawrepoformats': [
+        b'generaldelta',
+        b'revlogv1'
+      ],
+      b'redirect': {
+        b'hashes': [
+          b'sha256',
+          b'sha1'
+        ],
+        b'targets': [
+          {
+            b'name': b'target-a',
+            b'protocol': b'http',
+            b'uris': [
+              b'http://example.com/'
+            ]
+          },
+          {
+            b'name': b'target-b',
+            b'protocol': b'unknown',
+            b'uris': [
+              b'unknown://example.com/'
+            ]
+          }
+        ]
+      }
+    }
+  ]
+
+  $ cat error.log
+  $ killdaemons.py

mercurial/wireprotov2server.py

@@ -517,8 +517,66 b' def _capabilitiesv2(repo, proto):'
         caps['rawrepoformats'] = sorted(repo.requirements &
                                         repo.supportedformats)
 
+    targets = getadvertisedredirecttargets(repo, proto)
+    if targets:
+        caps[b'redirect'] = {
+            b'targets': [],
+            b'hashes': [b'sha256', b'sha1'],
+        }
+
+        for target in targets:
+            entry = {
+                b'name': target['name'],
+                b'protocol': target['protocol'],
+                b'uris': target['uris'],
+            }
+
+            for key in ('snirequired', 'tlsversions'):
+                if key in target:
+                    entry[key] = target[key]
+
+            caps[b'redirect'][b'targets'].append(entry)
+
     return proto.addcapabilities(repo, caps)
 
+def getadvertisedredirecttargets(repo, proto):
+    """Obtain a list of content redirect targets.
+
+    Returns a list containing potential redirect targets that will be
+    advertised in capabilities data. Each dict MUST have the following
+    keys:
+
+    name
+       The name of this redirect target. This is the identifier clients use
+       to refer to a target. It is transferred as part of every command
+       request.
+
+    protocol
+       Network protocol used by this target. Typically this is the string
+       in front of the ``://`` in a URL. e.g. ``https``.
+
+    uris
+       List of representative URIs for this target. Clients can use the
+       URIs to test parsing for compatibility or for ordering preference
+       for which target to use.
+
+    The following optional keys are recognized:
+
+    snirequired
+       Bool indicating if Server Name Indication (SNI) is required to
+       connect to this target.
+
+    tlsversions
+       List of bytes indicating which TLS versions are supported by this
+       target.
+
+    By default, clients reflect the target order advertised by servers
+    and servers will use the first client-advertised target when picking
+    a redirect target. So targets should be advertised in the order the
+    server prefers they be used.
+    """
+    return []
+
 def wireprotocommand(name, args=None, permission='push', cachekeyfn=None):
     """Decorator to declare a wire protocol command.
 

tests/wireprotosimplecache.py

@@ -17,6 +17,7 b' from mercurial import ('
 )
 from mercurial.utils import (
     interfaceutil,
+    stringutil,
 )
 
 CACHE = None
@@ -26,6 +27,8 b' configitem = registrar.configitem(config'
 
 configitem('simplecache', 'cacheobjects',
            default=False)
+configitem('simplecache', 'redirectsfile',
+           default=None)
 
 @interfaceutil.implementer(repository.iwireprotocolcommandcacher)
 class memorycacher(object):
@@ -91,6 +94,19 b' class memorycacher(object):'
 def makeresponsecacher(orig, repo, proto, command, args, objencoderfn):
     return memorycacher(repo.ui, command, objencoderfn)
 
+def loadredirecttargets(ui):
+    path = ui.config('simplecache', 'redirectsfile')
+    if not path:
+        return []
+
+    with open(path, 'rb') as fh:
+        s = fh.read()
+
+    return stringutil.evalpythonliteral(s)
+
+def getadvertisedredirecttargets(orig, repo, proto):
+    return loadredirecttargets(repo.ui)
+
 def extsetup(ui):
     global CACHE
 
@@ -98,3 +114,5 b' def extsetup(ui):'
 
     extensions.wrapfunction(wireprotov2server, 'makeresponsecacher',
                             makeresponsecacher)
+    extensions.wrapfunction(wireprotov2server, 'getadvertisedredirecttargets',
+                            getadvertisedredirecttargets)