Subrepositories let you nest external repositories or projects into a
parent Mercurial repository, and make commands operate on them as a
group. External Mercurial and Subversion projects are currently
supported.

Subrepositories are made of three components:

1. Nested repository checkouts. They can appear anywhere in the
   parent working directory, and are Mercurial clones or Subversion
   checkouts.

2. Nested repository references. They are defined in ``.hgsub`` and
   tell where the subrepository checkouts come from. Mercurial
   subrepositories are referenced like:

     path/to/nested = https://example.com/nested/repo/path

   where ``path/to/nested`` is the checkout location relatively to the
   parent Mercurial root, and ``https://example.com/nested/repo/path``
   is the source repository path. The source can also reference a
   filesystem path. Subversion repositories are defined with:

     path/to/nested = [svn]https://example.com/nested/trunk/path

   Note that ``.hgsub`` does not exist by default in Mercurial
   repositories, you have to create and add it to the parent
   repository before using subrepositories.

3. Nested repository states. They are defined in ``.hgsubstate`` and
   capture whatever information is required to restore the
   subrepositories to the state they were committed in a parent
   repository changeset. Mercurial automatically record the nested
   repositories states when committing in the parent repository.

   .. note::
      The ``.hgsubstate`` file should not be edited manually.


Adding a Subrepository
----------------------

If ``.hgsub`` does not exist, create it and add it to the parent
repository. Clone or checkout the external projects where you want it
to live in the parent repository. Edit ``.hgsub`` and add the
subrepository entry as described above. At this point, the
subrepository is tracked and the next commit will record its state in
``.hgsubstate`` and bind it to the committed changeset.

Synchronizing a Subrepository
-----------------------------

Subrepos do not automatically track the latest changeset of their
sources. Instead, they are updated to the changeset that corresponds
with the changeset checked out in the top-level changeset. This is so
developers always get a consistent set of compatible code and
libraries when they update.

Thus, updating subrepos is a manual process. Simply check out target
subrepo at the desired revision, test in the top-level repo, then
commit in the parent repository to record the new combination.

Deleting a Subrepository
------------------------

To remove a subrepository from the parent repository, delete its
reference from ``.hgsub``, then remove its files.

Interaction with Mercurial Commands
-----------------------------------

:add: add does not recurse in subrepos unless -S/--subrepos is
    specified. Subversion subrepositories are currently silently
    ignored.

:archive: archive does not recurse in subrepositories unless
    -S/--subrepos is specified.

:commit: commit creates a consistent snapshot of the state of the
    entire project and its subrepositories. It does this by first
    attempting to commit all modified subrepositories, then recording
    their state and finally committing it in the parent
    repository. Mercurial can be made to abort if any subrepository
    content is modified by setting "ui.commitsubrepos=no" in a
    configuration file (see :hg:`help config`).

:diff: diff does not recurse in subrepos unless -S/--subrepos is
    specified. Changes are displayed as usual, on the subrepositories
    elements. Subversion subrepositories are currently silently
    ignored.

:incoming: incoming does not recurse in subrepos unless -S/--subrepos
    is specified. Subversion subrepositories are currently silently
    ignored.

:outgoing: outgoing does not recurse in subrepos unless -S/--subrepos
    is specified. Subversion subrepositories are currently silently
    ignored.

:pull: pull is not recursive since it is not clear what to pull prior
    to running :hg:`update`. Listing and retrieving all
    subrepositories changes referenced by the parent repository pulled
    changesets is expensive at best, impossible in the Subversion
    case.

:push: Mercurial will automatically push all subrepositories first
    when the parent repository is being pushed. This ensures new
    subrepository changes are available when referenced by top-level
    repositories.

:status: status does not recurse into subrepositories unless
    -S/--subrepos is specified. Subrepository changes are displayed as
    regular Mercurial changes on the subrepository
    elements. Subversion subrepositories are currently silently
    ignored.

:update: update restores the subrepos in the state they were
    originally committed in target changeset. If the recorded
    changeset is not available in the current subrepository, Mercurial
    will pull it in first before updating.  This means that updating
    can require network access when using subrepositories.

Remapping Subrepositories Sources
---------------------------------

A subrepository source location may change during a project life,
invalidating references stored in the parent repository history. To
fix this, rewriting rules can be defined in parent repository ``hgrc``
file or in Mercurial configuration. See the ``[subpaths]`` section in
hgrc(5) for more details.