view tests/test-hgweb-json.t @ 26623:5a95fe44121d

clonebundles: support for seeding clones from pre-generated bundles

Cloning can be an expensive operation for servers because the server
generates a bundle from existing repository data at request time. For
a large repository like mozilla-central, this consumes 4+ minutes
of CPU time on the server. It also results in significant network
utilization. Multiplied by hundreds or even thousands of clients and
the ensuing load can result in difficulties scaling the Mercurial server.

Despite generation of bundles being deterministic until the next
changeset is added, the generation of bundles to service a clone request
is not cached. Each clone thus performs redundant work. This is
wasteful.

This patch introduces the "clonebundles" extension and related
client-side functionality to help alleviate this deficiency. The
client-side feature is behind an experimental flag and is not enabled by
default.

It works as follows:

1) Server operator generates a bundle and makes it available on a
   server (likely HTTP).
2) Server operator defines the URL of a bundle file in a
   .hg/clonebundles.manifest file.
3) Client `hg clone`ing sees the server is advertising bundle URLs.
4) Client fetches and applies the advertised bundle.
5) Client performs equivalent of `hg pull` to fetch changes made since
   the bundle was created.

Essentially, the server performs the expensive work of generating a
bundle once and all subsequent clones fetch a static file from
somewhere. Scaling static file serving is a much more manageable
problem than scaling a Python application like Mercurial. Assuming your
repository grows less than 1% per day, the end result is 99+% of CPU
and network load from clones is eliminated, allowing Mercurial servers
to scale more easily. Serving static files also means data can be
transferred to clients as fast as they can consume it, rather than as
fast as servers can generate it. This makes clones faster.

Mozilla has implemented similar functionality of this patch on
hg.mozilla.org using a custom extension. We are hosting bundle files in
Amazon S3 and CloudFront (a CDN) and have successfully offloaded
>1 TB/day in data transfer from hg.mozilla.org, freeing up significant
bandwidth and CPU resources. The positive impact has been stellar and
I believe it has proved its value to be included in Mercurial core. I
feel it is important for the client-side support to be enabled in core
by default because it means that clients will get faster, more reliable
clones and will enable server operators to reduce load without
requiring any client-side configuration changes (assuming clients are
up to date, of course).

The scope of this feature is narrowly and specifically tailored to
cloning, despite "serve pulls from pre-generated bundles" being a valid
and useful feature. I would eventually like for Mercurial servers to
support transferring *all* repository data via statically hosted files.
You could imagine a server that siphons all pushed data to bundle files
and instructs clients to apply a stream of bundles to reconstruct all
repository data. This feature, while useful and powerful, is
significantly more work to implement because it requires the server
component have awareness of discovery and a mapping of which changesets
are in which files. Full, clone bundles, by contrast, are much simpler.

The wire protocol command is named "clonebundles" instead of something
more generic like "staticbundles" to leave the door open for a new, more
powerful and more generic server-side component with minimal backwards
compatibility implications. The name "bundleclone" is used by Mozilla's
extension and would cause problems since there are subtle differences
in Mozilla's extension.

Mozilla's experience with this idea has taught us that some form of
"content negotiation" is required. Not all clients will support all
bundle formats or even URLs (advanced TLS requirements, etc). To ensure
the highest uptake possible, a server needs to advertise multiple
versions of bundles and clients need to be able to choose the most
appropriate from that list one. The "attributes" in each
server-advertised entry facilitate this filtering and sorting. Their
use will become apparent in subsequent patches.

Initial inspiration and credit for the idea of cloning from static files
belongs to Augie Fackler and his "lookaside clone" extension proof of
concept.

#require json
#require serve

  $ request() {
  >   get-with-headers.py --json localhost:$HGPORT "$1"
  > }

  $ hg init test
  $ cd test
  $ mkdir da
  $ echo foo > da/foo
  $ echo foo > foo
  $ hg -q ci -A -m initial
  $ echo bar > foo
  $ hg ci -m 'modify foo'
  $ echo bar > da/foo
  $ hg ci -m 'modify da/foo'
  $ hg bookmark bookmark1
  $ hg up default
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (leaving bookmark bookmark1)
  $ hg mv foo foo-new
  $ hg commit -m 'move foo'
  $ hg tag -m 'create tag' tag1
  $ hg phase --public -r .
  $ echo baz > da/foo
  $ hg commit -m 'another commit to da/foo'
  $ hg tag -m 'create tag2' tag2
  $ hg bookmark bookmark2
  $ hg -q up -r 0
  $ hg -q branch test-branch
  $ echo branch > foo
  $ hg commit -m 'create test branch'
  $ echo branch_commit_2 > foo
  $ hg commit -m 'another commit in test-branch'
  $ hg -q up default
  $ hg merge --tool :local test-branch
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m 'merge test-branch into default'

  $ hg log -G
  @    changeset:   9:cc725e08502a
  |\   tag:         tip
  | |  parent:      6:ceed296fe500
  | |  parent:      8:ed66c30e87eb
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     merge test-branch into default
  | |
  | o  changeset:   8:ed66c30e87eb
  | |  branch:      test-branch
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     another commit in test-branch
  | |
  | o  changeset:   7:6ab967a8ab34
  | |  branch:      test-branch
  | |  parent:      0:06e557f3edf6
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     create test branch
  | |
  o |  changeset:   6:ceed296fe500
  | |  bookmark:    bookmark2
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     create tag2
  | |
  o |  changeset:   5:f2890a05fea4
  | |  tag:         tag2
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     another commit to da/foo
  | |
  o |  changeset:   4:93a8ce14f891
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     create tag
  | |
  o |  changeset:   3:78896eb0e102
  | |  tag:         tag1
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     move foo
  | |
  o |  changeset:   2:8d7c456572ac
  | |  bookmark:    bookmark1
  | |  user:        test
  | |  date:        Thu Jan 01 00:00:00 1970 +0000
  | |  summary:     modify da/foo
  | |
  o |  changeset:   1:f8bbb9024b10
  |/   user:        test
  |    date:        Thu Jan 01 00:00:00 1970 +0000
  |    summary:     modify foo
  |
  o  changeset:   0:06e557f3edf6
     user:        test
     date:        Thu Jan 01 00:00:00 1970 +0000
     summary:     initial
  

  $ hg serve -p $HGPORT -d --pid-file=hg.pid -A access.log -E error.log
  $ cat hg.pid >> $DAEMON_PIDS

(Try to keep these in roughly the order they are defined in webcommands.py)

(log is handled by filelog/ and changelog/ - ignore it)

(rawfile/ doesn't use templating - nothing to test)

file/{revision}/{path} shows file revision

  $ request json-file/06e557f3edf6/foo
  200 Script output follows
  
  "not yet implemented"

file/{revision} shows root directory info

  $ request json-file/cc725e08502a
  200 Script output follows
  
  {
    "abspath": "/",
    "bookmarks": [],
    "directories": [
      {
        "abspath": "/da",
        "basename": "da",
        "emptydirs": ""
      }
    ],
    "files": [
      {
        "abspath": ".hgtags",
        "basename": ".hgtags",
        "date": [
          0.0,
          0
        ],
        "flags": "",
        "size": 92
      },
      {
        "abspath": "foo-new",
        "basename": "foo-new",
        "date": [
          0.0,
          0
        ],
        "flags": "",
        "size": 4
      }
    ],
    "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7",
    "tags": [
      "tip"
    ]
  }

changelog/ shows information about several changesets

  $ request json-changelog
  200 Script output follows
  
  {
    "changeset_count": 10,
    "changesets": [
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "merge test-branch into default",
        "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7",
        "tags": [
          "tip"
        ],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "another commit in test-branch",
        "node": "ed66c30e87eb65337c05a4229efaa5f1d5285a90",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "create test branch",
        "node": "6ab967a8ab3489227a83f80e920faa039a71819f",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [
          "bookmark2"
        ],
        "date": [
          0.0,
          0
        ],
        "desc": "create tag2",
        "node": "ceed296fe500c3fac9541e31dad860cb49c89e45",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "another commit to da/foo",
        "node": "f2890a05fea49bfaf9fb27ed5490894eba32da78",
        "tags": [
          "tag2"
        ],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "create tag",
        "node": "93a8ce14f89156426b7fa981af8042da53f03aa0",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "move foo",
        "node": "78896eb0e102174ce9278438a95e12543e4367a7",
        "tags": [
          "tag1"
        ],
        "user": "test"
      },
      {
        "bookmarks": [
          "bookmark1"
        ],
        "date": [
          0.0,
          0
        ],
        "desc": "modify da/foo",
        "node": "8d7c456572acf3557e8ed8a07286b10c408bcec5",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "modify foo",
        "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "initial",
        "node": "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e",
        "tags": [],
        "user": "test"
      }
    ],
    "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7"
  }

changelog/{revision} shows information starting at a specific changeset

  $ request json-changelog/f8bbb9024b10
  200 Script output follows
  
  {
    "changeset_count": 10,
    "changesets": [
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "modify foo",
        "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "initial",
        "node": "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e",
        "tags": [],
        "user": "test"
      }
    ],
    "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8"
  }

shortlog/ shows information about a set of changesets

  $ request json-shortlog
  200 Script output follows
  
  {
    "changeset_count": 10,
    "changesets": [
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "merge test-branch into default",
        "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7",
        "tags": [
          "tip"
        ],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "another commit in test-branch",
        "node": "ed66c30e87eb65337c05a4229efaa5f1d5285a90",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "create test branch",
        "node": "6ab967a8ab3489227a83f80e920faa039a71819f",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [
          "bookmark2"
        ],
        "date": [
          0.0,
          0
        ],
        "desc": "create tag2",
        "node": "ceed296fe500c3fac9541e31dad860cb49c89e45",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "another commit to da/foo",
        "node": "f2890a05fea49bfaf9fb27ed5490894eba32da78",
        "tags": [
          "tag2"
        ],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "create tag",
        "node": "93a8ce14f89156426b7fa981af8042da53f03aa0",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "move foo",
        "node": "78896eb0e102174ce9278438a95e12543e4367a7",
        "tags": [
          "tag1"
        ],
        "user": "test"
      },
      {
        "bookmarks": [
          "bookmark1"
        ],
        "date": [
          0.0,
          0
        ],
        "desc": "modify da/foo",
        "node": "8d7c456572acf3557e8ed8a07286b10c408bcec5",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "modify foo",
        "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
        "tags": [],
        "user": "test"
      },
      {
        "bookmarks": [],
        "date": [
          0.0,
          0
        ],
        "desc": "initial",
        "node": "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e",
        "tags": [],
        "user": "test"
      }
    ],
    "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7"
  }

changeset/ renders the tip changeset

  $ request json-rev
  200 Script output follows
  
  {
    "bookmarks": [],
    "branch": "default",
    "date": [
      0.0,
      0
    ],
    "desc": "merge test-branch into default",
    "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7",
    "parents": [
      "ceed296fe500c3fac9541e31dad860cb49c89e45",
      "ed66c30e87eb65337c05a4229efaa5f1d5285a90"
    ],
    "phase": "draft",
    "tags": [
      "tip"
    ],
    "user": "test"
  }

changeset/{revision} shows tags

  $ request json-rev/78896eb0e102
  200 Script output follows
  
  {
    "bookmarks": [],
    "branch": "default",
    "date": [
      0.0,
      0
    ],
    "desc": "move foo",
    "node": "78896eb0e102174ce9278438a95e12543e4367a7",
    "parents": [
      "8d7c456572acf3557e8ed8a07286b10c408bcec5"
    ],
    "phase": "public",
    "tags": [
      "tag1"
    ],
    "user": "test"
  }

changeset/{revision} shows bookmarks

  $ request json-rev/8d7c456572ac
  200 Script output follows
  
  {
    "bookmarks": [
      "bookmark1"
    ],
    "branch": "default",
    "date": [
      0.0,
      0
    ],
    "desc": "modify da/foo",
    "node": "8d7c456572acf3557e8ed8a07286b10c408bcec5",
    "parents": [
      "f8bbb9024b10f93cdbb8d940337398291d40dea8"
    ],
    "phase": "public",
    "tags": [],
    "user": "test"
  }

changeset/{revision} shows branches

  $ request json-rev/6ab967a8ab34
  200 Script output follows
  
  {
    "bookmarks": [],
    "branch": "test-branch",
    "date": [
      0.0,
      0
    ],
    "desc": "create test branch",
    "node": "6ab967a8ab3489227a83f80e920faa039a71819f",
    "parents": [
      "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e"
    ],
    "phase": "draft",
    "tags": [],
    "user": "test"
  }

manifest/{revision}/{path} shows info about a directory at a revision

  $ request json-manifest/06e557f3edf6/
  200 Script output follows
  
  {
    "abspath": "/",
    "bookmarks": [],
    "directories": [
      {
        "abspath": "/da",
        "basename": "da",
        "emptydirs": ""
      }
    ],
    "files": [
      {
        "abspath": "foo",
        "basename": "foo",
        "date": [
          0.0,
          0
        ],
        "flags": "",
        "size": 4
      }
    ],
    "node": "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e",
    "tags": []
  }

tags/ shows tags info

  $ request json-tags
  200 Script output follows
  
  {
    "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7",
    "tags": [
      {
        "date": [
          0.0,
          0
        ],
        "node": "f2890a05fea49bfaf9fb27ed5490894eba32da78",
        "tag": "tag2"
      },
      {
        "date": [
          0.0,
          0
        ],
        "node": "78896eb0e102174ce9278438a95e12543e4367a7",
        "tag": "tag1"
      }
    ]
  }

bookmarks/ shows bookmarks info

  $ request json-bookmarks
  200 Script output follows
  
  {
    "bookmarks": [
      {
        "bookmark": "bookmark1",
        "date": [
          0.0,
          0
        ],
        "node": "8d7c456572acf3557e8ed8a07286b10c408bcec5"
      },
      {
        "bookmark": "bookmark2",
        "date": [
          0.0,
          0
        ],
        "node": "ceed296fe500c3fac9541e31dad860cb49c89e45"
      }
    ],
    "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7"
  }

branches/ shows branches info

  $ request json-branches
  200 Script output follows
  
  {
    "branches": [
      {
        "branch": "default",
        "date": [
          0.0,
          0
        ],
        "node": "cc725e08502a79dd1eda913760fbe06ed7a9abc7",
        "status": "open"
      },
      {
        "branch": "test-branch",
        "date": [
          0.0,
          0
        ],
        "node": "ed66c30e87eb65337c05a4229efaa5f1d5285a90",
        "status": "inactive"
      }
    ]
  }

summary/ shows a summary of repository state

  $ request json-summary
  200 Script output follows
  
  "not yet implemented"

filediff/{revision}/{path} shows changes to a file in a revision

  $ request json-diff/f8bbb9024b10/foo
  200 Script output follows
  
  {
    "author": "test",
    "children": [],
    "date": [
      0.0,
      0
    ],
    "desc": "modify foo",
    "diff": [
      {
        "blockno": 1,
        "lines": [
          {
            "l": "--- a/foo\tThu Jan 01 00:00:00 1970 +0000\n",
            "n": 1,
            "t": "-"
          },
          {
            "l": "+++ b/foo\tThu Jan 01 00:00:00 1970 +0000\n",
            "n": 2,
            "t": "+"
          },
          {
            "l": "@@ -1,1 +1,1 @@\n",
            "n": 3,
            "t": "@"
          },
          {
            "l": "-foo\n",
            "n": 4,
            "t": "-"
          },
          {
            "l": "+bar\n",
            "n": 5,
            "t": "+"
          }
        ]
      }
    ],
    "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
    "parents": [
      "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e"
    ],
    "path": "foo"
  }

comparison/{revision}/{path} shows information about before and after for a file

  $ request json-comparison/f8bbb9024b10/foo
  200 Script output follows
  
  {
    "author": "test",
    "children": [],
    "comparison": [
      {
        "lines": [
          {
            "ll": "foo",
            "ln": 1,
            "rl": "bar",
            "rn": 1,
            "t": "replace"
          }
        ]
      }
    ],
    "date": [
      0.0,
      0
    ],
    "desc": "modify foo",
    "leftnode": "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e",
    "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
    "parents": [
      "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e"
    ],
    "path": "foo",
    "rightnode": "f8bbb9024b10f93cdbb8d940337398291d40dea8"
  }

annotate/{revision}/{path} shows annotations for each line

  $ request json-annotate/f8bbb9024b10/foo
  200 Script output follows
  
  {
    "abspath": "foo",
    "annotate": [
      {
        "abspath": "foo",
        "author": "test",
        "desc": "modify foo",
        "line": "bar\n",
        "lineno": 1,
        "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
        "revdate": [
          0.0,
          0
        ],
        "targetline": 1
      }
    ],
    "author": "test",
    "children": [],
    "date": [
      0.0,
      0
    ],
    "desc": "modify foo",
    "node": "f8bbb9024b10f93cdbb8d940337398291d40dea8",
    "parents": [
      "06e557f3edf66faa1ccaba5dd8c203c21cc79f1e"
    ],
    "permissions": ""
  }

filelog/{revision}/{path} shows history of a single file

  $ request json-filelog/f8bbb9024b10/foo
  200 Script output follows
  
  "not yet implemented"

(archive/ doesn't use templating, so ignore it)

(static/ doesn't use templating, so ignore it)

graph/ shows information that can be used to render a graph of the DAG

  $ request json-graph
  200 Script output follows
  
  "not yet implemented"

help/ shows help topics

  $ request json-help
  200 Script output follows
  
  {
    "earlycommands": [
      {
        "summary": "add the specified files on the next commit",
        "topic": "add"
      },
      {
        "summary": "show changeset information by line for each file",
        "topic": "annotate"
      },
      {
        "summary": "make a copy of an existing repository",
        "topic": "clone"
      },
      {
        "summary": "commit the specified files or all outstanding changes",
        "topic": "commit"
      },
      {
        "summary": "diff repository (or selected files)",
        "topic": "diff"
      },
      {
        "summary": "dump the header and diffs for one or more changesets",
        "topic": "export"
      },
      {
        "summary": "forget the specified files on the next commit",
        "topic": "forget"
      },
      {
        "summary": "create a new repository in the given directory",
        "topic": "init"
      },
      {
        "summary": "show revision history of entire repository or files",
        "topic": "log"
      },
      {
        "summary": "merge another revision into working directory",
        "topic": "merge"
      },
      {
        "summary": "pull changes from the specified source",
        "topic": "pull"
      },
      {
        "summary": "push changes to the specified destination",
        "topic": "push"
      },
      {
        "summary": "remove the specified files on the next commit",
        "topic": "remove"
      },
      {
        "summary": "start stand-alone webserver",
        "topic": "serve"
      },
      {
        "summary": "show changed files in the working directory",
        "topic": "status"
      },
      {
        "summary": "summarize working directory state",
        "topic": "summary"
      },
      {
        "summary": "update working directory (or switch revisions)",
        "topic": "update"
      }
    ],
    "othercommands": [
      {
        "summary": "add all new files, delete all missing files",
        "topic": "addremove"
      },
      {
        "summary": "create an unversioned archive of a repository revision",
        "topic": "archive"
      },
      {
        "summary": "reverse effect of earlier changeset",
        "topic": "backout"
      },
      {
        "summary": "subdivision search of changesets",
        "topic": "bisect"
      },
      {
        "summary": "create a new bookmark or list existing bookmarks",
        "topic": "bookmarks"
      },
      {
        "summary": "set or show the current branch name",
        "topic": "branch"
      },
      {
        "summary": "list repository named branches",
        "topic": "branches"
      },
      {
        "summary": "create a changegroup file",
        "topic": "bundle"
      },
      {
        "summary": "output the current or given revision of files",
        "topic": "cat"
      },
      {
        "summary": "show combined config settings from all hgrc files",
        "topic": "config"
      },
      {
        "summary": "mark files as copied for the next commit",
        "topic": "copy"
      },
      {
        "summary": "list tracked files",
        "topic": "files"
      },
      {
        "summary": "copy changes from other branches onto the current branch",
        "topic": "graft"
      },
      {
        "summary": "search for a pattern in specified files and revisions",
        "topic": "grep"
      },
      {
        "summary": "show branch heads",
        "topic": "heads"
      },
      {
        "summary": "show help for a given topic or a help overview",
        "topic": "help"
      },
      {
        "summary": "identify the working directory or specified revision",
        "topic": "identify"
      },
      {
        "summary": "import an ordered set of patches",
        "topic": "import"
      },
      {
        "summary": "show new changesets found in source",
        "topic": "incoming"
      },
      {
        "summary": "output the current or given revision of the project manifest",
        "topic": "manifest"
      },
      {
        "summary": "show changesets not found in the destination",
        "topic": "outgoing"
      },
      {
        "summary": "show aliases for remote repositories",
        "topic": "paths"
      },
      {
        "summary": "set or show the current phase name",
        "topic": "phase"
      },
      {
        "summary": "roll back an interrupted transaction",
        "topic": "recover"
      },
      {
        "summary": "rename files; equivalent of copy + remove",
        "topic": "rename"
      },
      {
        "summary": "redo merges or set/view the merge status of files",
        "topic": "resolve"
      },
      {
        "summary": "restore files to their checkout state",
        "topic": "revert"
      },
      {
        "summary": "print the root (top) of the current working directory",
        "topic": "root"
      },
      {
        "summary": "add one or more tags for the current or given revision",
        "topic": "tag"
      },
      {
        "summary": "list repository tags",
        "topic": "tags"
      },
      {
        "summary": "apply one or more changegroup files",
        "topic": "unbundle"
      },
      {
        "summary": "verify the integrity of the repository",
        "topic": "verify"
      },
      {
        "summary": "output version and copyright information",
        "topic": "version"
      }
    ],
    "topics": [
      {
        "summary": "Configuration Files",
        "topic": "config"
      },
      {
        "summary": "Date Formats",
        "topic": "dates"
      },
      {
        "summary": "Diff Formats",
        "topic": "diffs"
      },
      {
        "summary": "Environment Variables",
        "topic": "environment"
      },
      {
        "summary": "Using Additional Features",
        "topic": "extensions"
      },
      {
        "summary": "Specifying File Sets",
        "topic": "filesets"
      },
      {
        "summary": "Glossary",
        "topic": "glossary"
      },
      {
        "summary": "Syntax for Mercurial Ignore Files",
        "topic": "hgignore"
      },
      {
        "summary": "Configuring hgweb",
        "topic": "hgweb"
      },
      {
        "summary": "Merge Tools",
        "topic": "merge-tools"
      },
      {
        "summary": "Specifying Multiple Revisions",
        "topic": "multirevs"
      },
      {
        "summary": "File Name Patterns",
        "topic": "patterns"
      },
      {
        "summary": "Working with Phases",
        "topic": "phases"
      },
      {
        "summary": "Specifying Single Revisions",
        "topic": "revisions"
      },
      {
        "summary": "Specifying Revision Sets",
        "topic": "revsets"
      },
      {
        "summary": "Using Mercurial from scripts and automation",
        "topic": "scripting"
      },
      {
        "summary": "Subrepositories",
        "topic": "subrepos"
      },
      {
        "summary": "Template Usage",
        "topic": "templating"
      },
      {
        "summary": "URL Paths",
        "topic": "urls"
      }
    ]
  }

help/{topic} shows an individual help topic

  $ request json-help/phases
  200 Script output follows
  
  {
    "rawdoc": "Working with Phases\n*", (glob)
    "topic": "phases"
  }