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\chapter{Handling repository events with hooks}
\label{chap:hook}

Mercurial offers a powerful mechanism to let you perform automated
actions in response to events that occur in a repository.  In some
cases, you can even control Mercurial's response to those events.

The name Mercurial uses for one of these actions is a \emph{hook}.
Hooks are called ``triggers'' in some revision control systems, but
the two names refer to the same idea.

\section{An overview of hooks in Mercurial}

Here is a brief list of the hooks that Mercurial supports.  We will
revisit each of these hooks in more detail later, in
section~\ref{sec:hook:ref}.

\begin{itemize}
\item[\small\hook{changegroup}] This is run after a group of
  changesets has been brought into the repository from elsewhere.
\item[\small\hook{commit}] This is run after a new changeset has been
  created in the local repository.
\item[\small\hook{incoming}] This is run once for each new changeset
  that is brought into the repository from elsewhere.  Notice the
  difference from \hook{changegroup}, which is run once per
  \emph{group} of changesets brought in.
\item[\small\hook{outgoing}] This is run after a group of changesets
  has been transmitted from this repository.
\item[\small\hook{prechangegroup}] This is run before starting to
  bring a group of changesets into the repository.
\item[\small\hook{precommit}] Controlling. This is run before starting
  a commit.
\item[\small\hook{preoutgoing}] Controlling. This is run before
  starting to transmit a group of changesets from this repository.
\item[\small\hook{pretag}] Controlling. This is run before creating a tag.
\item[\small\hook{pretxnchangegroup}] Controlling. This is run after a
  group of changesets has been brought into the local repository from
  another, but before the transaction completes that will make the
  changes permanent in the repository.
\item[\small\hook{pretxncommit}] Controlling. This is run after a new
  changeset has been created in the local repository, but before the
  transaction completes that will make it permanent.
\item[\small\hook{preupdate}] Controlling. This is run before starting
  an update or merge of the working directory.
\item[\small\hook{tag}] This is run after a tag is created.
\item[\small\hook{update}] This is run after an update or merge of the
  working directory has finished.
\end{itemize}
Each of the hooks whose description begins with the word
``Controlling'' has the ability to determine whether an activity can
proceed.  If the hook succeeds, the activity may proceed; if it fails,
the activity is either not permitted or undone, depending on the hook.

\section{Hooks and security}

\subsection{Hooks are run with your privileges}

When you run a Mercurial command in a repository, and the command
causes a hook to run, that hook runs on \emph{your} system, under
\emph{your} user account, with \emph{your} privilege level.  Since
hooks are arbitrary pieces of executable code, you should treat them
with an appropriate level of suspicion.  Do not install a hook unless
you are confident that you know who created it and what it does.

In some cases, you may be exposed to hooks that you did not install
yourself.  If you work with Mercurial on an unfamiliar system,
Mercurial will run hooks defined in that system's global \hgrc\ file.

If you are working with a repository owned by another user, Mercurial
can run hooks defined in that user's repository, but it will still run
them as ``you''.  For example, if you \hgcmd{pull} from that
repository, and its \sfilename{.hg/hgrc} defines a local
\hook{outgoing} hook, that hook will run under your user account, even
though you don't own that repository.

\begin{note}
  This only applies if you are pulling from a repository on a local or
  network filesystem.  If you're pulling over http or ssh, any
  \hook{outgoing} hook will run under whatever account is executing
  the server process, on the server.
\end{note}

XXX To see what hooks are defined in a repository, use the
\hgcmdargs{config}{hooks} command.  If you are working in one
repository, but talking to another that you do not own (e.g.~using
\hgcmd{pull} or \hgcmd{incoming}), remember that it is the other
repository's hooks you should be checking, not your own.

\subsection{Hooks do not propagate}

In Mercurial, hooks are not revision controlled, and do not propagate
when you clone, or pull from, a repository.  The reason for this is
simple: a hook is a completely arbitrary piece of executable code.  It
runs under your user identity, with your privilege level, on your
machine.

It would be extremely reckless for any distributed revision control
system to implement revision-controlled hooks, as this would offer an
easily exploitable way to subvert the accounts of users of the
revision control system.

Since Mercurial does not propagate hooks, if you are collaborating
with other people on a common project, you should not assume that they
are using the same Mercurial hooks as you are, or that theirs are
correctly configured.  You should document the hooks you expect people
to use.

In a corporate intranet, this is somewhat easier to control, as you
can for example provide a ``standard'' installation of Mercurial on an
NFS filesystem, and use a site-wide \hgrc\ file to define hooks that
all users will see.  However, this too has its limits; see below.

\subsection{Hooks can be overridden}

Mercurial allows you to override a hook definition by redefining the
hook.  You can disable it by setting its value to the empty string, or
change its behaviour as you wish.

If you deploy a system-~or site-wide \hgrc\ file that defines some
hooks, you should thus understand that your users can disable or
override those hooks.

\subsection{Ensuring that critical hooks are run}

Sometimes you may want to enforce a policy that you do not want others
to be able to work around.  For example, you may have a requirement
that every changeset must pass a rigorous set of tests.  Defining this
requirement via a hook in a site-wide \hgrc\ won't work for remote
users on laptops, and of course local users can subvert it at will by
overriding the hook.

Instead, you can set up your policies for use of Mercurial so that
people are expected to propagate changes through a well-known
``canonical'' server that you have locked down and configured
appropriately.

One way to do this is via a combination of social engineering and
technology.  Set up a restricted-access account; users can push
changes over the network to repositories managed by this account, but
they cannot log into the account and run normal shell commands.  In
this scenario, a user can commit a changeset that contains any old
garbage they want.

When someone pushes a changeset to the server that everyone pulls
from, the server will test the changeset before it accepts it as
permanent, and reject it if it fails to pass the test suite.  If
people only pull changes from this filtering server, it will serve to
ensure that all changes that people pull have been automatically
vetted.

\section{Using hooks with shared access to a repository}

If you want to use hooks to so some automated work in a repository
that a number of people have ahred access to, you need to be careful
in how you do this.

Mercurial only locks a repository when it is writing to the
repository, and only the parts of Mercurial that write to the
repository pay attention to locks.  Write locks are necessary to
prevent multiple simultaneous writers from scribbling on each other's
work, corrupting the repository.

Because Mercurial is careful with the order in which it reads and
writes data, it does not need to acquire a lock when it wants to read
data from the repository.  The parts of Mercurial that read from the
repository never pay attention to locks.  This lockless reading scheme
greatly increases performance and concurrency.

With great performance comes a trade-off, though, one which has the
potential to cause you trouble unless you're aware of it.  To describe
this requires a little detail about how Mercurial adds changesets to a
repository and reads those changes.

When Mercurial \emph{writes} metadata, it writes it straight into the
destination file.  It writes file data first, then manifest data
(which contains pointers to the new file data), then changelog data
(which contains pointers to the new manifest data).  Before the first
write to each file, it stores a record of where the end of the file
was in its transaction log.  If the transaction must be rolled back,
Mercurial simply truncates each file back to te size it was before the
transaction began.

When Mercurial \emph{reads} metadata, it reads the changelog first,
then everything else.  Since a reader will only access parts of the
manifest or file metadata that it can see in the changelog, it can
never see partially written data.

Some controlling hooks (\hook{pretxncommit} and
\hook{pretxnchangegroup}) run when a transaction is almost complete.
All of the metadata has been written, but Mercurial can still roll the
transaction back and cause the newly-written data to disappear.

If one of these hooks runs for long, it opens a window in which a
reader can see the metadata for changesets that are, strictly
speaking, not yet permanent.  The longer the hook runs, the bigger the
window.

A good use for the \hook{pretxnchangegroup} hook would be to
automatically build and test incoming changes before they are accepted
into the repository, so that you can guarantee that nobody can push
changes to this repository that ``break the build''.  But if a client
can pull changes while they're being tested, the usefulness of the
test is zero; someone can pull untested changes.

The safest answer to this challenge is to set up such a ``gatekeeper''
repository as \emph{unidirectional}.  It can take changes pushed in
from the outside, but nobody can pull changes from it.  Use the
\hook{preoutgoing} hook to lock it down.  Configure a
\hook{changegroup} hook so that if a build or test succeeds, the hook
will push the new changes out to another repository that people
\emph{can} pull from.

\section{A short tutorial on using hooks}
\label{sec:hook:simple}

It is easy to write a Mercurial hook.  Let's start with a hook that
runs when you finish a \hgcmd{commit}, and simply prints the hash of
the changeset you just created.  The hook is called \hook{commit}.

\begin{figure}[ht]
  \interaction{hook.simple.init}
  \caption{A simple hook that runs when a changeset is committed}
  \label{ex:hook:init}
\end{figure}

All hooks follow the pattern in example~\ref{ex:hook:init}.  You add
an entry to the \rcsection{hooks} section of your \hgrc\.  On the left
is the name of the event to trigger on; on the right is the action to
take.  As you can see, you can run an arbitrary shell command in a
hook.  Mercurial passes extra information to the hook using
environment variables (look for \envar{HG\_NODE} in the example).

\subsection{Performing multiple actions per event}

Quite often, you will want to define more than one hook for a
particular kind of event, as shown in example~\ref{ex:hook:ext}.
Mercurial lets you do this by adding an \emph{extension} to the end of
a hook's name.  You extend a hook's name by giving the name of the
hook, followed by a full stop (the ``\texttt{.}'' character), followed
by some more text of your choosing.  For example, Mercurial will run
both \texttt{commit.foo} and \texttt{commit.bar} when the
\texttt{commit} event occurs.

\begin{figure}[ht]
  \interaction{hook.simple.ext}
  \caption{Defining a second \hook{commit} hook}
  \label{ex:hook:ext}
\end{figure}

To give a well-defined order of execution when there are multiple
hooks defined for an event, Mercurial sorts hooks by extension, and
executes the hook commands in this sorted order.  In the above
example, it will execute \texttt{commit.bar} before
\texttt{commit.foo}, and \texttt{commit} before both.

It is a good idea to use a somewhat descriptive extension when you
define a new hook.  This will help you to remember what the hook was
for.  If the hook fails, you'll get an error message that contains the
hook name and extension, so using a descriptive extension could give
you an immediate hint as to why the hook failed (see
section~\ref{sec:hook:perm} for an example).

\subsection{Controlling whether an activity can proceed}
\label{sec:hook:perm}

In our earlier examples, we used the \hook{commit} hook, which is
run after a commit has completed.  This is one of several Mercurial
hooks that run after an activity finishes.  Such hooks have no way of
influencing the activity itself.

Mercurial defines a number of events that occur before an activity
starts; or after it starts, but before it finishes.  Hooks that
trigger on these events have the added ability to choose whether the
activity can continue, or will abort.  

The \hook{pretxncommit} hook runs after a commit has all but
completed.  In other words, the metadata representing the changeset
has been written out to disk, but the transaction has not yet been
allowed to complete.  The \hook{pretxncommit} hook has the ability to
decide whether the transaction can complete, or must be rolled back.

If the \hook{pretxncommit} hook exits with a status code of zero, the
transaction is allowed to complete; the commit finishes; and the
\hook{commit} hook is run.  If the \hook{pretxncommit} hook exits with
a non-zero status code, the transaction is rolled back; the metadata
representing the changeset is erased; and the \hook{commit} hook is
not run.

\begin{figure}[ht]
  \interaction{hook.simple.pretxncommit}
  \caption{Using the \hook{pretxncommit} hook to control commits}
  \label{ex:hook:pretxncommit}
\end{figure}

The hook in example~\ref{ex:hook:pretxncommit} checks that a commit
comment contains a bug ID.  If it does, the commit can complete.  If
not, the commit is rolled back.

\section{Writing your own hooks}

When you are writing a hook, you might find it useful to run Mercurial
either with the \hggopt{-v} option, or the \rcitem{ui}{verbose} config
item set to ``true''.  When you do so, Mercurial will print a message
before it calls each hook.

\subsection{Choosing how your hook should run}
\label{sec:hook:lang}

You can write a hook either as a normal program---typically a shell
script---or as a Python function that is executed within the Mercurial
process.

Writing a hook as an external program has the advantage that it
requires no knowledge of Mercurial's internals.  You can call normal
Mercurial commands to get any added information you need.  The
trade-off is that external hooks are slower than in-process hooks.

An in-process Python hook has complete access to the Mercurial API,
and does not ``shell out'' to another process, so it is inherently
faster than an external hook.  It is also easier to obtain much of the
information that a hook requires by using the Mercurial API than by
running Mercurial commands.

If you are comfortable with Python, or require high performance,
writing your hooks in Python may be a good choice.  However, when you
have a straightforward hook to write and you don't need to care about
performance (probably the majority of hooks), a shell script is
perfectly fine.

\subsection{Hook parameters}
\label{sec:hook:param}

Mercurial calls each hook with a set of well-defined parameters.  In
Python, a parameter is passed as a keyword argument to your hook
function.  For an external program, a parameter is passed as an
environment variable.

Whether your hook is written in Python or as a shell script, the
hook-specific parameter names and values will be the same.  A boolean
parameter will be represented as a boolean value in Python, but as the
number 1 (for ``true'') or 0 (for ``false'') as an environment
variable for an external hook.  If a hook parameter is named
\texttt{foo}, the keyword argument for a Python hook will also be
named \texttt{foo} Python, while the environment variable for an
external hook will be named \texttt{HG\_FOO}.

\subsection{Hook return values and activity control}

A hook that executes successfully must exit with a status of zero if
external, or return boolean ``false'' if in-process.  Failure is
indicated with a non-zero exit status from an external hook, or an
in-process hook returning boolean ``true''.  If an in-process hook
raises an exception, the hook is considered to have failed.

For a hook that controls whether an activity can proceed, zero/false
means ``allow'', while non-zero/true/exception means ``deny''.

\subsection{Writing an external hook}

When you define an external hook in your \hgrc\ and the hook is run,
its value is passed to your shell, which interprets it.  This means
that you can use normal shell constructs in the body of the hook.

An executable hook is always run with its current directory set to a
repository's root directory.

Each hook parameter is passed in as an environment variable; the name
is upper-cased, and prefixed with the string ``\texttt{HG\_}''.

With the exception of hook parameters, Mercurial does not set or
modify any environment variables when running a hook.  This is useful
to remember if you are writing a site-wide hook that may be run by a
number of different users with differing environment variables set.
In multi-user situations, you should not rely on environment variables
being set to the values you have in your environment when testing the
hook.

\subsection{Telling Mercurial to use an in-process hook}

The \hgrc\ syntax for defining an in-process hook is slightly
different than for an executable hook.  The value of the hook must
start with the text ``\texttt{python:}'', and continue with the
fully-qualified name of a callable object to use as the hook's value.

The module in which a hook lives is automatically imported when a hook
is run.  So long as you have the module name and \envar{PYTHONPATH}
right, it should ``just work''.

The following \hgrc\ example snippet illustrates the syntax and
meaning of the notions we just described.
\begin{codesample2}
  [hooks]
  commit.example = python:mymodule.submodule.myhook
\end{codesample2}
When Mercurial runs the \texttt{commit.example} hook, it imports
\texttt{mymodule.submodule}, looks for the callable object named
\texttt{myhook}, and calls it.

\subsection{Writing an in-process hook}

The simplest in-process hook does nothing, but illustrates the basic
shape of the hook API:
\begin{codesample2}
  def myhook(ui, repo, **kwargs):
      pass
\end{codesample2}
The first argument to a Python hook is always a
\pymodclass{mercurial.ui}{ui} object.  The second is a repository object;
at the moment, it is always an instance of
\pymodclass{mercurial.localrepo}{localrepository}.  Following these two
arguments are other keyword arguments.  Which ones are passed in
depends on the hook being called, but a hook can ignore arguments it
doesn't care about by dropping them into a keyword argument dict, as
with \texttt{**kwargs} above.

\section{Some hook examples}

\subsection{Enforcing coding guidelines in your own repository}

An interesting use of a commit-related hook is to help you to write
cleaner code.  A simple example of ``cleaner code'' is the dictum that
a change should not add any new lines of text that contain ``trailing
whitespace''.  Trailing whitespace is a series of space and tab
characters at the end of a line of text.  In most cases, trailing
whitespace is unnecessary, invisible noise, but it is occasionally
problematic, and people tend to prefer to get rid of it.

You can use either the \hook{precommit} or \hook{pretxncommit} hook to
tell whether you have a trailing whitespace problem.  If you use the
\hook{precommit} hook, the hook will not know which files you are
committing, so it will have to check every modified file in the
repository for trailing white space.  If you want to commit a change
to just the file \filename{foo}, but the file \filename{bar} contains
trailing whitespace, doing a check in the \hook{precommit} hook will
prevent you from committing \filename{foo} due to the problem with
\filename{bar}.  This doesn't seem right.

Should you choose the \hook{pretxncommit} hook, the check won't occur
until just before the transaction for the commit completes.  This will
allow you to check for problems only the exact files that are being
committed.  However, if you entered the commit message interactively
and the hook fails, the transaction will roll back; you'll have to
re-enter the commit message after you fix the trailing whitespace and
run \hgcmd{commit} again.

\begin{figure}[ht]
  \interaction{hook.ws.simple}
  \caption{A simple hook that checks for trailing whitespace}
  \label{ex:hook:ws.simple}
\end{figure}

Figure~\ref{ex:hook:ws.simple} introduces a simple \hook{pretxncommit}
hook that checks for trailing whitespace.  This hook is short, but not
very helpful.  It exits with an error status if a change adds a line
with trailing whitespace to any file, but does not print any
information that might help us to identify the offending file or line.

\section{Hook reference}
\label{sec:hook:ref}

\subsection{In-process hook execution}

An in-process hook is called with arguments of the following form:
\begin{codesample2}
  def myhook(ui, repo, **kwargs):
      pass
\end{codesample2}
The \texttt{ui} parameter is a \pymodclass{mercurial.ui}{ui} object.
The \texttt{repo} parameter is a
\pymodclass{mercurial.localrepo}{localrepository} object.  The
names and values of the \texttt{**kwargs} parameters depend on the
hook being invoked, with the following common features:
\begin{itemize}
\item If a parameter is named \texttt{node} or
  \texttt{parent\emph{N}}, it will contain a hexadecimal changeset ID.
  The empty string is used to represent ``null changeset ID'' instead
  of a string of zeroes.
\item Boolean-valued parameters are represented as Python
  \texttt{bool} objects.
\end{itemize}

An in-process hook is called without a change to the process's working
directory (unlike external hooks, which are run in the root of the
repository).  It must not change the process's working directory.  If
it were to do so, it would probably cause calls to the Mercurial API,
or operations after the hook finishes, to fail.

If a hook returns a boolean ``false'' value, it is considered to
have succeeded.  If it returns a boolean ``true'' value or raises an
exception, it is considered to have failed.

\subsection{External hook execution}

An external hook is passed to the user's shell for execution, so
features of that shell, such as variable substitution and command
redirection, are available.  The hook is run in the root directory of
the repository.

Hook parameters are passed to the hook as environment variables.  Each
environment variable's name is converted in upper case and prefixed
with the string ``\texttt{HG\_}''.  For example, if the name of a
parameter is ``\texttt{node}'', the name of the environment variable
representing that parameter will be ``\texttt{HG\_NODE}''.

A boolean parameter is represented as the string ``\texttt{1}'' for
``true'', ``\texttt{0}'' for ``false''.  If an environment variable is
named \envar{HG\_NODE}, \envar{HG\_PARENT1} or \envar{HG\_PARENT2}, it
contains a changeset ID represented as a hexadecimal string.  The
empty string is used to represent ``null changeset ID'' instead of a
string of zeroes.

If a hook exits with a status of zero, it is considered to have
succeeded.  If it exits with a non-zero status, it is considered to
have failed.

\subsection{The \hook{changegroup} hook}
\label{sec:hook:changegroup}

This hook is run after a group of pre-existing changesets has been
added to the repository, for example via a \hgcmd{pull} or
\hgcmd{unbundle}.  This hook is run once per operation that added one
or more changesets.  This is in contrast to the \hook{incoming} hook,
which is run once per changeset, regardless of whether the changesets
arrive in a group.

Some possible uses for this hook include kicking off an automated
build or test of the added changesets, updating a bug database, or
notifying subscribers that a repository contains new changes.

Parameters to this hook:
\begin{itemize}
\item[\texttt{node}] A changeset ID.  The changeset ID of the first
  changeset in the group that was added.  All changesets between this
  and \index{tags!\texttt{tip}}\texttt{tip}, inclusive, were added by
  a single \hgcmd{pull}, \hgcmd{push} or \hgcmd{unbundle}.
\end{itemize}

See also: \hook{incoming} (section~\ref{sec:hook:incoming}),
\hook{prechangegroup} (section~\ref{sec:hook:prechangegroup}),
\hook{pretxnchangegroup} (section~\ref{sec:hook:pretxnchangegroup})

\subsection{The \hook{commit} hook}
\label{sec:hook:commit}

This hook is run after a new changeset has been created.

Parameters to this hook:
\begin{itemize}
\item[\texttt{node}] A changeset ID.  The changeset ID of the newly
  committed changeset.
\item[\texttt{parent1}] A changeset ID.  The changeset ID of the first
  parent of the newly committed changeset.
\item[\texttt{parent2}] A changeset ID.  The changeset ID of the second
  parent of the newly committed changeset.
\end{itemize}

See also: \hook{precommit} (section~\ref{sec:hook:precommit}),
\hook{pretxncommit} (section~\ref{sec:hook:pretxncommit})

\subsection{The \hook{incoming} hook}
\label{sec:hook:incoming}

This hook is run after a pre-existing changeset has been added to the
repository, for example via a \hgcmd{push}.  If a group of changesets
was added in a single operation, this hook is called once for each
added changeset.

You can use this hook for the same purposes as the \hook{changegroup}
hook (section~\ref{sec:hook:changegroup}); it's simply more convenient
sometimes to run a hook once per group of changesets, while othher
times it's handier once per changeset.

Parameters to this hook:
\begin{itemize}
\item[\texttt{node}] A changeset ID.  The ID of the newly added
  changeset.
\end{itemize}

See also: \hook{changegroup} (section~\ref{sec:hook:changegroup}) \hook{prechangegroup} (section~\ref{sec:hook:prechangegroup}), \hook{pretxnchangegroup} (section~\ref{sec:hook:pretxnchangegroup})

\subsection{The \hook{outgoing} hook}
\label{sec:hook:outgoing}

This hook is run after a group of changesets has been propagated out
of this repository, for example by a \hgcmd{push} or \hgcmd{bundle}
command.

One possible use for this hook is to notify administrators that
changes have been pulled.

Parameters to this hook:
\begin{itemize}
\item[\texttt{node}] A changeset ID.  The changeset ID of the first
  changeset of the group that was sent.
\item[\texttt{source}] A string.  The source of the of the operation.
  If a remote client pulled changes from this repository,
  \texttt{source} will be \texttt{serve}.  If the client that obtained
  changes from this repository was local, \texttt{source} will be
  \texttt{bundle}, \texttt{pull}, or \texttt{push}, depending on the
  operation the client performed.
\end{itemize}

See also: \hook{preoutgoing} (section~\ref{sec:hook:preoutgoing})

\subsection{The \hook{prechangegroup} hook}
\label{sec:hook:prechangegroup}

This controlling hook is run before Mercurial begins to add a group of
changesets from another repository.

This hook does not have any information about the changesets to be
added, because it is run before transmission of those changesets is
allowed to begin.  If this hook fails, the changesets will not be
transmitted.

One use for this hook is to prevent external changes from being added
to a repository, for example to ``freeze'' a server-hosted branch
temporarily or permanently.

This hook is not passed any parameters.

See also: \hook{changegroup} (section~\ref{sec:hook:changegroup}),
\hook{incoming} (section~\ref{sec:hook:incoming}), ,
\hook{pretxnchangegroup} (section~\ref{sec:hook:pretxnchangegroup})

\subsection{The \hook{precommit} hook}
\label{sec:hook:precommit}

This hook is run before Mercurial begins to commit a new changeset.
It is run before Mercurial has any of the metadata for the commit,
such as the files to be committed, the commit message, or the commit
date.

One use for this hook is to disable the ability to commit new
changesets, while still allowing incoming changesets.  Another is to
run a build or test, and only allow the commit to begin if the build
or test succeeds.

Parameters to this hook:
\begin{itemize}
\item[\texttt{parent1}] A changeset ID.  The changeset ID of the first
  parent of the working directory.
\item[\texttt{parent2}] A changeset ID.  The changeset ID of the second
  parent of the working directory.
\end{itemize}
If the commit proceeds, the parents of the working directory will
become the parents of the new changeset.

See also: \hook{commit} (section~\ref{sec:hook:commit}),
\hook{pretxncommit} (section~\ref{sec:hook:pretxncommit})

\subsection{The \hook{preoutgoing} hook}
\label{sec:hook:preoutgoing}

This hook is invoked before Mercurial knows the identities of the
changesets to be transmitted.

One use for this hook is to prevent changes from being transmitted to
another repository.

Parameters to this hook:
\begin{itemize}
\item[\texttt{source}] A string.  The source of the operation that is
  attempting to obtain changes from this repository.  See the
  documentation for the \texttt{source} parameter to the
  \hook{outgoing} hook, in section~\ref{sec:hook:outgoing}, for
  possible values of this parameter..
\end{itemize}

See also: \hook{outgoing} (section~\ref{sec:hook:outgoing})

\subsection{The \hook{pretag} hook}
\label{sec:hook:pretag}

This controlling hook is run before a tag is created.  If the hook
succeeds, creation of the tag proceeds.  If the hook fails, the tag is
not created.

Parameters to this hook:
\begin{itemize}
\item[\texttt{local}] A boolean.  Whether the tag is local to this
  repository instance (i.e.~stored in \sfilename{.hg/tags}) or managed
  by Mercurial (stored in \sfilename{.hgtags}).
\item[\texttt{node}] A changeset ID.  The ID of the changeset to be tagged.
\item[\texttt{tag}] A string.  The name of the tag to be created.
\end{itemize}

If the tag to be created is revision-controlled, the \hook{precommit}
and \hook{pretxncommit} hooks (sections~\ref{sec:hook:commit}
and~\ref{sec:hook:pretxncommit}) will also be run.

See also: \hook{tag} (section~\ref{sec:hook:tag})

\subsection{The \hook{pretxnchangegroup} hook}
\label{sec:hook:pretxnchangegroup}

This controlling hook is run before a transaction---that manages the
addition of a group of new changesets from outside the
repository---completes.  If the hook succeeds, the transaction
completes, and all of the changesets become permanent within this
repository.  If the hook fails, the transaction is rolled back, and
the data for the changesets is erased.

This hook can access the metadata associated with the almost-added
changesets, but it should not do anything permanent with this data.
It must also not modify the working directory.

While this hook is running, if other Mercurial processes access this
repository, they will be able to see the almost-added changesets as if
they are permanent.  This may lead to race conditions if you do not
take steps to avoid them.

This hook can be used to automatically vet a group of changesets.  If
the hook fails, all of the changesets are ``rejected'' when the
transaction rolls back.

Parameters to this hook are the same as for the \hook{changegroup}
hook; see section~\ref{sec:hook:changegroup} for details.

See also: \hook{changegroup} (section~\ref{sec:hook:changegroup}),
\hook{incoming} (section~\ref{sec:hook:incoming}),
\hook{prechangegroup} (section~\ref{sec:hook:prechangegroup})

\subsection{The \hook{pretxncommit} hook}
\label{sec:hook:pretxncommit}

This controlling hook is run before a transaction---that manages a new
commit---completes.  If the hook succeeds, the transaction completes
and the changeset becomes permanent within this repository.  If the
hook fails, the transaction is rolled back, and the commit data is
erased.

This hook can access the metadata associated with the almost-new
changeset, but it should not do anything permanent with this data.  It
must also not modify the working directory.

While this hook is running, if other Mercurial processes access this
repository, they will be able to see the almost-new changeset as if it
is permanent.  This may lead to race conditions if you do not take
steps to avoid them.

Parameters to this hook are the same as for the \hook{commit} hook;
see section~\ref{sec:hook:commit} for details.

See also: \hook{precommit} (section~\ref{sec:hook:precommit})

\subsection{The \hook{preupdate} hook}
\label{sec:hook:preupdate}

This controlling hook is run before an update or merge of the working
directory begins.  It is run only if Mercurial's normal pre-update
checks determine that the update or merge can proceed.  If the hook
succeeds, the update or merge may proceed; if it fails, the update or
merge does not start.

Parameters to this hook:
\begin{itemize}
\item[\texttt{parent1}] A changeset ID.  The ID of the parent that the
  working directory is to be updated to.  If the working directory is
  being merged, it will not change this parent.
\item[\texttt{parent2}] A changeset ID.  Only set if the working
  directory is being merged.  The ID of the revision that the working
  directory is being merged with.
\end{itemize}

See also: \hook{update} (section~\ref{sec:hook:update})

\subsection{The \hook{tag} hook}
\label{sec:hook:tag}

This hook is run after a tag has been created.

Parameters to this hook are the same as for the \hook{pretag} hook;
see section~\ref{sec:hook:pretag} for details.

If the created tag is revision-controlled, the \hook{commit} hook
(section~\ref{sec:hook:commit}) is run before this hook.

See also: \hook{pretag} (section~\ref{sec:hook:pretag})

\subsection{The \hook{update} hook}
\label{sec:hook:update}

This hook is run after an update or merge of the working directory
completes.  Since a merge can fail (if the external \command{hgmerge}
command fails to resolve conflicts in a file), this hook communicates
whether the update or merge completed cleanly.

\begin{itemize}
\item[\texttt{error}] A boolean.  Indicates whether the update or
  merge completed successfully.
\item[\texttt{parent1}] A changeset ID.  The ID of the parent that the
  working directory was updated to.  If the working directory was
  merged, it will not have changed this parent.
\item[\texttt{parent2}] A changeset ID.  Only set if the working
  directory was merged.  The ID of the revision that the working
  directory was merged with.
\end{itemize}

See also: \hook{preupdate} (section~\ref{sec:hook:preupdate})

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