bos@34: \chapter{Handling repository events with hooks}
bos@34: \label{chap:hook}
bos@34: 
bos@34: Mercurial offers a powerful mechanism to let you perform automated
bos@34: actions in response to events that occur in a repository.  In some
bos@34: cases, you can even control Mercurial's response to those events.
bos@34: 
bos@34: The name Mercurial uses for one of these actions is a \emph{hook}.
bos@34: Hooks are called ``triggers'' in some revision control systems, but
bos@34: the two names refer to the same idea.
bos@34: 
bos@38: \section{An overview of hooks in Mercurial}
bos@38: 
bos@41: Here is a brief list of the hooks that Mercurial supports.  We will
bos@41: revisit each of these hooks in more detail later, in
bos@41: section~\ref{sec:hook:ref}.
bos@41: 
bos@38: \begin{itemize}
bos@38: \item[\small\hook{changegroup}] This is run after a group of
bos@41:   changesets has been brought into the repository from elsewhere.
bos@38: \item[\small\hook{commit}] This is run after a new changeset has been
bos@41:   created in the local repository.
bos@38: \item[\small\hook{incoming}] This is run once for each new changeset
bos@38:   that is brought into the repository from elsewhere.  Notice the
bos@38:   difference from \hook{changegroup}, which is run once per
bos@41:   \emph{group} of changesets brought in.
bos@38: \item[\small\hook{outgoing}] This is run after a group of changesets
bos@41:   has been transmitted from this repository.
bos@38: \item[\small\hook{prechangegroup}] This is run before starting to
bos@41:   bring a group of changesets into the repository.
bos@41: \item[\small\hook{precommit}] Controlling. This is run before starting
bos@41:   a commit.
bos@41: \item[\small\hook{preoutgoing}] Controlling. This is run before
bos@41:   starting to transmit a group of changesets from this repository.
bos@41: \item[\small\hook{pretag}] Controlling. This is run before creating a tag.
bos@41: \item[\small\hook{pretxnchangegroup}] Controlling. This is run after a
bos@41:   group of changesets has been brought into the local repository from
bos@41:   another, but before the transaction completes that will make the
bos@41:   changes permanent in the repository.
bos@41: \item[\small\hook{pretxncommit}] Controlling. This is run after a new
bos@41:   changeset has been created in the local repository, but before the
bos@41:   transaction completes that will make it permanent.
bos@41: \item[\small\hook{preupdate}] Controlling. This is run before starting
bos@41:   an update or merge of the working directory.
bos@38: \item[\small\hook{tag}] This is run after a tag is created.
bos@38: \item[\small\hook{update}] This is run after an update or merge of the
bos@38:   working directory has finished.
bos@38: \end{itemize}
bos@41: Each of the hooks whose description begins with the word
bos@41: ``Controlling'' has the ability to determine whether an activity can
bos@41: proceed.  If the hook succeeds, the activity may proceed; if it fails,
bos@41: the activity is either not permitted or undone, depending on the hook.
bos@38: 
bos@38: \section{Hooks and security}
bos@38: 
bos@38: \subsection{Hooks are run with your privileges}
bos@38: 
bos@38: When you run a Mercurial command in a repository, and the command
bos@41: causes a hook to run, that hook runs on \emph{your} system, under
bos@41: \emph{your} user account, with \emph{your} privilege level.  Since
bos@41: hooks are arbitrary pieces of executable code, you should treat them
bos@41: with an appropriate level of suspicion.  Do not install a hook unless
bos@41: you are confident that you know who created it and what it does.
bos@38: 
bos@38: In some cases, you may be exposed to hooks that you did not install
bos@38: yourself.  If you work with Mercurial on an unfamiliar system,
bos@38: Mercurial will run hooks defined in that system's global \hgrc\ file.
bos@38: 
bos@38: If you are working with a repository owned by another user, Mercurial
bos@41: can run hooks defined in that user's repository, but it will still run
bos@41: them as ``you''.  For example, if you \hgcmd{pull} from that
bos@41: repository, and its \sfilename{.hg/hgrc} defines a local
bos@41: \hook{outgoing} hook, that hook will run under your user account, even
bos@41: though you don't own that repository.
bos@38: 
bos@38: \begin{note}
bos@38:   This only applies if you are pulling from a repository on a local or
bos@38:   network filesystem.  If you're pulling over http or ssh, any
bos@41:   \hook{outgoing} hook will run under whatever account is executing
bos@41:   the server process, on the server.
bos@38: \end{note}
bos@38: 
bos@38: XXX To see what hooks are defined in a repository, use the
bos@38: \hgcmdargs{config}{hooks} command.  If you are working in one
bos@38: repository, but talking to another that you do not own (e.g.~using
bos@38: \hgcmd{pull} or \hgcmd{incoming}), remember that it is the other
bos@38: repository's hooks you should be checking, not your own.
bos@38: 
bos@38: \subsection{Hooks do not propagate}
bos@38: 
bos@38: In Mercurial, hooks are not revision controlled, and do not propagate
bos@38: when you clone, or pull from, a repository.  The reason for this is
bos@38: simple: a hook is a completely arbitrary piece of executable code.  It
bos@38: runs under your user identity, with your privilege level, on your
bos@38: machine.
bos@38: 
bos@38: It would be extremely reckless for any distributed revision control
bos@38: system to implement revision-controlled hooks, as this would offer an
bos@38: easily exploitable way to subvert the accounts of users of the
bos@38: revision control system.
bos@38: 
bos@38: Since Mercurial does not propagate hooks, if you are collaborating
bos@38: with other people on a common project, you should not assume that they
bos@38: are using the same Mercurial hooks as you are, or that theirs are
bos@38: correctly configured.  You should document the hooks you expect people
bos@38: to use.
bos@38: 
bos@38: In a corporate intranet, this is somewhat easier to control, as you
bos@38: can for example provide a ``standard'' installation of Mercurial on an
bos@38: NFS filesystem, and use a site-wide \hgrc\ file to define hooks that
bos@38: all users will see.  However, this too has its limits; see below.
bos@38: 
bos@38: \subsection{Hooks can be overridden}
bos@38: 
bos@38: Mercurial allows you to override a hook definition by redefining the
bos@38: hook.  You can disable it by setting its value to the empty string, or
bos@38: change its behaviour as you wish.
bos@38: 
bos@38: If you deploy a system-~or site-wide \hgrc\ file that defines some
bos@38: hooks, you should thus understand that your users can disable or
bos@38: override those hooks.
bos@38: 
bos@38: \subsection{Ensuring that critical hooks are run}
bos@38: 
bos@38: Sometimes you may want to enforce a policy that you do not want others
bos@38: to be able to work around.  For example, you may have a requirement
bos@38: that every changeset must pass a rigorous set of tests.  Defining this
bos@38: requirement via a hook in a site-wide \hgrc\ won't work for remote
bos@38: users on laptops, and of course local users can subvert it at will by
bos@38: overriding the hook.
bos@38: 
bos@38: Instead, you can set up your policies for use of Mercurial so that
bos@38: people are expected to propagate changes through a well-known
bos@38: ``canonical'' server that you have locked down and configured
bos@38: appropriately.
bos@38: 
bos@38: One way to do this is via a combination of social engineering and
bos@38: technology.  Set up a restricted-access account; users can push
bos@38: changes over the network to repositories managed by this account, but
bos@38: they cannot log into the account and run normal shell commands.  In
bos@38: this scenario, a user can commit a changeset that contains any old
bos@38: garbage they want.
bos@38: 
bos@38: When someone pushes a changeset to the server that everyone pulls
bos@38: from, the server will test the changeset before it accepts it as
bos@38: permanent, and reject it if it fails to pass the test suite.  If
bos@38: people only pull changes from this filtering server, it will serve to
bos@38: ensure that all changes that people pull have been automatically
bos@38: vetted.
bos@38: 
bos@41: \section{Using hooks with shared access to a repository}
bos@41: 
bos@41: If you want to use hooks to so some automated work in a repository
bos@54: that a number of people have shared access to, you need to be careful
bos@41: in how you do this.
bos@41: 
bos@41: Mercurial only locks a repository when it is writing to the
bos@41: repository, and only the parts of Mercurial that write to the
bos@41: repository pay attention to locks.  Write locks are necessary to
bos@41: prevent multiple simultaneous writers from scribbling on each other's
bos@41: work, corrupting the repository.
bos@41: 
bos@41: Because Mercurial is careful with the order in which it reads and
bos@41: writes data, it does not need to acquire a lock when it wants to read
bos@41: data from the repository.  The parts of Mercurial that read from the
bos@41: repository never pay attention to locks.  This lockless reading scheme
bos@41: greatly increases performance and concurrency.
bos@41: 
bos@41: With great performance comes a trade-off, though, one which has the
bos@41: potential to cause you trouble unless you're aware of it.  To describe
bos@41: this requires a little detail about how Mercurial adds changesets to a
bos@41: repository and reads those changes.
bos@41: 
bos@41: When Mercurial \emph{writes} metadata, it writes it straight into the
bos@41: destination file.  It writes file data first, then manifest data
bos@41: (which contains pointers to the new file data), then changelog data
bos@41: (which contains pointers to the new manifest data).  Before the first
bos@41: write to each file, it stores a record of where the end of the file
bos@41: was in its transaction log.  If the transaction must be rolled back,
bos@54: Mercurial simply truncates each file back to the size it was before the
bos@41: transaction began.
bos@41: 
bos@41: When Mercurial \emph{reads} metadata, it reads the changelog first,
bos@41: then everything else.  Since a reader will only access parts of the
bos@41: manifest or file metadata that it can see in the changelog, it can
bos@41: never see partially written data.
bos@41: 
bos@41: Some controlling hooks (\hook{pretxncommit} and
bos@41: \hook{pretxnchangegroup}) run when a transaction is almost complete.
bos@41: All of the metadata has been written, but Mercurial can still roll the
bos@41: transaction back and cause the newly-written data to disappear.
bos@41: 
bos@41: If one of these hooks runs for long, it opens a window in which a
bos@41: reader can see the metadata for changesets that are, strictly
bos@41: speaking, not yet permanent.  The longer the hook runs, the bigger the
bos@41: window.
bos@41: 
bos@41: A good use for the \hook{pretxnchangegroup} hook would be to
bos@41: automatically build and test incoming changes before they are accepted
bos@41: into the repository, so that you can guarantee that nobody can push
bos@41: changes to this repository that ``break the build''.  But if a client
bos@41: can pull changes while they're being tested, the usefulness of the
bos@41: test is zero; someone can pull untested changes.
bos@41: 
bos@41: The safest answer to this challenge is to set up such a ``gatekeeper''
bos@41: repository as \emph{unidirectional}.  It can take changes pushed in
bos@41: from the outside, but nobody can pull changes from it.  Use the
bos@41: \hook{preoutgoing} hook to lock it down.  Configure a
bos@41: \hook{changegroup} hook so that if a build or test succeeds, the hook
bos@41: will push the new changes out to another repository that people
bos@41: \emph{can} pull from.
bos@41: 
bos@34: \section{A short tutorial on using hooks}
bos@34: \label{sec:hook:simple}
bos@34: 
bos@34: It is easy to write a Mercurial hook.  Let's start with a hook that
bos@34: runs when you finish a \hgcmd{commit}, and simply prints the hash of
bos@34: the changeset you just created.  The hook is called \hook{commit}.
bos@34: 
bos@34: \begin{figure}[ht]
bos@34:   \interaction{hook.simple.init}
bos@34:   \caption{A simple hook that runs when a changeset is committed}
bos@34:   \label{ex:hook:init}
bos@34: \end{figure}
bos@34: 
bos@34: All hooks follow the pattern in example~\ref{ex:hook:init}.  You add
bos@34: an entry to the \rcsection{hooks} section of your \hgrc\.  On the left
bos@34: is the name of the event to trigger on; on the right is the action to
bos@34: take.  As you can see, you can run an arbitrary shell command in a
bos@34: hook.  Mercurial passes extra information to the hook using
bos@34: environment variables (look for \envar{HG\_NODE} in the example).
bos@34: 
bos@34: \subsection{Performing multiple actions per event}
bos@34: 
bos@34: Quite often, you will want to define more than one hook for a
bos@34: particular kind of event, as shown in example~\ref{ex:hook:ext}.
bos@34: Mercurial lets you do this by adding an \emph{extension} to the end of
bos@34: a hook's name.  You extend a hook's name by giving the name of the
bos@34: hook, followed by a full stop (the ``\texttt{.}'' character), followed
bos@34: by some more text of your choosing.  For example, Mercurial will run
bos@34: both \texttt{commit.foo} and \texttt{commit.bar} when the
bos@34: \texttt{commit} event occurs.
bos@34: 
bos@34: \begin{figure}[ht]
bos@34:   \interaction{hook.simple.ext}
bos@34:   \caption{Defining a second \hook{commit} hook}
bos@34:   \label{ex:hook:ext}
bos@34: \end{figure}
bos@34: 
bos@34: To give a well-defined order of execution when there are multiple
bos@34: hooks defined for an event, Mercurial sorts hooks by extension, and
bos@34: executes the hook commands in this sorted order.  In the above
bos@34: example, it will execute \texttt{commit.bar} before
bos@34: \texttt{commit.foo}, and \texttt{commit} before both.
bos@34: 
bos@34: It is a good idea to use a somewhat descriptive extension when you
bos@34: define a new hook.  This will help you to remember what the hook was
bos@34: for.  If the hook fails, you'll get an error message that contains the
bos@34: hook name and extension, so using a descriptive extension could give
bos@34: you an immediate hint as to why the hook failed (see
bos@34: section~\ref{sec:hook:perm} for an example).
bos@34: 
bos@34: \subsection{Controlling whether an activity can proceed}
bos@34: \label{sec:hook:perm}
bos@34: 
bos@34: In our earlier examples, we used the \hook{commit} hook, which is
bos@34: run after a commit has completed.  This is one of several Mercurial
bos@34: hooks that run after an activity finishes.  Such hooks have no way of
bos@34: influencing the activity itself.
bos@34: 
bos@34: Mercurial defines a number of events that occur before an activity
bos@34: starts; or after it starts, but before it finishes.  Hooks that
bos@34: trigger on these events have the added ability to choose whether the
bos@34: activity can continue, or will abort.  
bos@34: 
bos@34: The \hook{pretxncommit} hook runs after a commit has all but
bos@34: completed.  In other words, the metadata representing the changeset
bos@34: has been written out to disk, but the transaction has not yet been
bos@34: allowed to complete.  The \hook{pretxncommit} hook has the ability to
bos@34: decide whether the transaction can complete, or must be rolled back.
bos@34: 
bos@34: If the \hook{pretxncommit} hook exits with a status code of zero, the
bos@34: transaction is allowed to complete; the commit finishes; and the
bos@34: \hook{commit} hook is run.  If the \hook{pretxncommit} hook exits with
bos@34: a non-zero status code, the transaction is rolled back; the metadata
bos@34: representing the changeset is erased; and the \hook{commit} hook is
bos@34: not run.
bos@34: 
bos@34: \begin{figure}[ht]
bos@34:   \interaction{hook.simple.pretxncommit}
bos@34:   \caption{Using the \hook{pretxncommit} hook to control commits}
bos@34:   \label{ex:hook:pretxncommit}
bos@34: \end{figure}
bos@34: 
bos@34: The hook in example~\ref{ex:hook:pretxncommit} checks that a commit
bos@34: comment contains a bug ID.  If it does, the commit can complete.  If
bos@34: not, the commit is rolled back.
bos@34: 
bos@37: \section{Writing your own hooks}
bos@37: 
bos@37: When you are writing a hook, you might find it useful to run Mercurial
bos@37: either with the \hggopt{-v} option, or the \rcitem{ui}{verbose} config
bos@37: item set to ``true''.  When you do so, Mercurial will print a message
bos@37: before it calls each hook.
bos@37: 
bos@37: \subsection{Choosing how your hook should run}
bos@37: \label{sec:hook:lang}
bos@34: 
bos@34: You can write a hook either as a normal program---typically a shell
bos@37: script---or as a Python function that is executed within the Mercurial
bos@34: process.
bos@34: 
bos@34: Writing a hook as an external program has the advantage that it
bos@34: requires no knowledge of Mercurial's internals.  You can call normal
bos@34: Mercurial commands to get any added information you need.  The
bos@34: trade-off is that external hooks are slower than in-process hooks.
bos@34: 
bos@34: An in-process Python hook has complete access to the Mercurial API,
bos@34: and does not ``shell out'' to another process, so it is inherently
bos@34: faster than an external hook.  It is also easier to obtain much of the
bos@34: information that a hook requires by using the Mercurial API than by
bos@34: running Mercurial commands.
bos@34: 
bos@34: If you are comfortable with Python, or require high performance,
bos@34: writing your hooks in Python may be a good choice.  However, when you
bos@34: have a straightforward hook to write and you don't need to care about
bos@34: performance (probably the majority of hooks), a shell script is
bos@34: perfectly fine.
bos@34: 
bos@37: \subsection{Hook parameters}
bos@34: \label{sec:hook:param}
bos@34: 
bos@34: Mercurial calls each hook with a set of well-defined parameters.  In
bos@34: Python, a parameter is passed as a keyword argument to your hook
bos@34: function.  For an external program, a parameter is passed as an
bos@34: environment variable.
bos@34: 
bos@34: Whether your hook is written in Python or as a shell script, the
bos@37: hook-specific parameter names and values will be the same.  A boolean
bos@37: parameter will be represented as a boolean value in Python, but as the
bos@37: number 1 (for ``true'') or 0 (for ``false'') as an environment
bos@37: variable for an external hook.  If a hook parameter is named
bos@37: \texttt{foo}, the keyword argument for a Python hook will also be
bos@51: named \texttt{foo}, while the environment variable for an external
bos@51: hook will be named \texttt{HG\_FOO}.
bos@37: 
bos@37: \subsection{Hook return values and activity control}
bos@37: 
bos@37: A hook that executes successfully must exit with a status of zero if
bos@37: external, or return boolean ``false'' if in-process.  Failure is
bos@37: indicated with a non-zero exit status from an external hook, or an
bos@37: in-process hook returning boolean ``true''.  If an in-process hook
bos@37: raises an exception, the hook is considered to have failed.
bos@37: 
bos@37: For a hook that controls whether an activity can proceed, zero/false
bos@37: means ``allow'', while non-zero/true/exception means ``deny''.
bos@37: 
bos@37: \subsection{Writing an external hook}
bos@37: 
bos@37: When you define an external hook in your \hgrc\ and the hook is run,
bos@37: its value is passed to your shell, which interprets it.  This means
bos@37: that you can use normal shell constructs in the body of the hook.
bos@37: 
bos@37: An executable hook is always run with its current directory set to a
bos@37: repository's root directory.
bos@37: 
bos@37: Each hook parameter is passed in as an environment variable; the name
bos@37: is upper-cased, and prefixed with the string ``\texttt{HG\_}''.
bos@37: 
bos@37: With the exception of hook parameters, Mercurial does not set or
bos@37: modify any environment variables when running a hook.  This is useful
bos@37: to remember if you are writing a site-wide hook that may be run by a
bos@37: number of different users with differing environment variables set.
bos@37: In multi-user situations, you should not rely on environment variables
bos@37: being set to the values you have in your environment when testing the
bos@37: hook.
bos@37: 
bos@37: \subsection{Telling Mercurial to use an in-process hook}
bos@37: 
bos@37: The \hgrc\ syntax for defining an in-process hook is slightly
bos@37: different than for an executable hook.  The value of the hook must
bos@37: start with the text ``\texttt{python:}'', and continue with the
bos@37: fully-qualified name of a callable object to use as the hook's value.
bos@37: 
bos@37: The module in which a hook lives is automatically imported when a hook
bos@37: is run.  So long as you have the module name and \envar{PYTHONPATH}
bos@37: right, it should ``just work''.
bos@37: 
bos@37: The following \hgrc\ example snippet illustrates the syntax and
bos@37: meaning of the notions we just described.
bos@37: \begin{codesample2}
bos@37:   [hooks]
bos@37:   commit.example = python:mymodule.submodule.myhook
bos@37: \end{codesample2}
bos@37: When Mercurial runs the \texttt{commit.example} hook, it imports
bos@37: \texttt{mymodule.submodule}, looks for the callable object named
bos@37: \texttt{myhook}, and calls it.
bos@37: 
bos@37: \subsection{Writing an in-process hook}
bos@37: 
bos@37: The simplest in-process hook does nothing, but illustrates the basic
bos@37: shape of the hook API:
bos@37: \begin{codesample2}
bos@37:   def myhook(ui, repo, **kwargs):
bos@37:       pass
bos@37: \end{codesample2}
bos@37: The first argument to a Python hook is always a
bos@37: \pymodclass{mercurial.ui}{ui} object.  The second is a repository object;
bos@37: at the moment, it is always an instance of
bos@37: \pymodclass{mercurial.localrepo}{localrepository}.  Following these two
bos@37: arguments are other keyword arguments.  Which ones are passed in
bos@37: depends on the hook being called, but a hook can ignore arguments it
bos@37: doesn't care about by dropping them into a keyword argument dict, as
bos@37: with \texttt{**kwargs} above.
bos@34: 
bos@44: \section{Some hook examples}
bos@44: 
bos@49: \subsection{Writing meaningful commit messages}
bos@49: 
bos@49: It's hard to imagine a useful commit message being very short.  The
bos@49: simple \hook{pretxncommit} hook of figure~\ref{ex:hook:msglen.run}
bos@49: will prevent you from committing a changeset with a message that is
bos@49: less than ten bytes long.
bos@49: 
bos@49: \begin{figure}[ht]
bos@49:   \interaction{hook.msglen.run}
bos@49:   \caption{A hook that forbids overly short commit messages}
bos@49:   \label{ex:hook:msglen.run}
bos@49: \end{figure}
bos@49: 
bos@49: \subsection{Checking for trailing whitespace}
bos@44: 
bos@44: An interesting use of a commit-related hook is to help you to write
bos@44: cleaner code.  A simple example of ``cleaner code'' is the dictum that
bos@44: a change should not add any new lines of text that contain ``trailing
bos@44: whitespace''.  Trailing whitespace is a series of space and tab
bos@44: characters at the end of a line of text.  In most cases, trailing
bos@44: whitespace is unnecessary, invisible noise, but it is occasionally
bos@49: problematic, and people often prefer to get rid of it.
bos@44: 
bos@44: You can use either the \hook{precommit} or \hook{pretxncommit} hook to
bos@44: tell whether you have a trailing whitespace problem.  If you use the
bos@44: \hook{precommit} hook, the hook will not know which files you are
bos@44: committing, so it will have to check every modified file in the
bos@44: repository for trailing white space.  If you want to commit a change
bos@44: to just the file \filename{foo}, but the file \filename{bar} contains
bos@44: trailing whitespace, doing a check in the \hook{precommit} hook will
bos@44: prevent you from committing \filename{foo} due to the problem with
bos@44: \filename{bar}.  This doesn't seem right.
bos@44: 
bos@44: Should you choose the \hook{pretxncommit} hook, the check won't occur
bos@44: until just before the transaction for the commit completes.  This will
bos@44: allow you to check for problems only the exact files that are being
bos@44: committed.  However, if you entered the commit message interactively
bos@44: and the hook fails, the transaction will roll back; you'll have to
bos@44: re-enter the commit message after you fix the trailing whitespace and
bos@44: run \hgcmd{commit} again.
bos@44: 
bos@44: \begin{figure}[ht]
bos@44:   \interaction{hook.ws.simple}
bos@44:   \caption{A simple hook that checks for trailing whitespace}
bos@44:   \label{ex:hook:ws.simple}
bos@44: \end{figure}
bos@44: 
bos@44: Figure~\ref{ex:hook:ws.simple} introduces a simple \hook{pretxncommit}
bos@44: hook that checks for trailing whitespace.  This hook is short, but not
bos@44: very helpful.  It exits with an error status if a change adds a line
bos@44: with trailing whitespace to any file, but does not print any
bos@49: information that might help us to identify the offending file or
bos@49: line.  It also has the nice property of not paying attention to
bos@49: unmodified lines; only lines that introduce new trailing whitespace
bos@49: cause problems.
bos@49: 
bos@49: \begin{figure}[ht]
bos@49:   \interaction{hook.ws.better}
bos@49:   \caption{A better trailing whitespace hook}
bos@49:   \label{ex:hook:ws.better}
bos@49: \end{figure}
bos@49: 
bos@49: The example of figure~\ref{ex:hook:ws.better} is much more complex,
bos@49: but also more useful.  It parses a unified diff to see if any lines
bos@49: add trailing whitespace, and prints the name of the file and the line
bos@49: number of each such occurrence.  Even better, if the change adds
bos@49: trailing whitespace, this hook saves the commit comment and prints the
bos@49: name of the save file before exiting and telling Mercurial to roll the
bos@49: transaction back, so you can use
bos@49: \hgcmdargs{commit}{\hgopt{commit}{-l}~\emph{filename}} to reuse the
bos@49: saved commit message once you've corrected the problem.
bos@49: 
bos@49: As a final aside, note in figure~\ref{ex:hook:ws.better} the use of
bos@49: \command{perl}'s in-place editing feature to get rid of trailing
bos@49: whitespace from a file.  This is concise and useful enough that I will
bos@49: reproduce it here.
bos@49: \begin{codesample2}
bos@54:   perl -pi -e 's,\\s+\$,,' filename
bos@49: \end{codesample2}
bos@49: 
bos@49: \section{Bundled hooks}
bos@49: 
bos@49: Mercurial ships with several bundled hooks.  You can find them in the
bos@49: \dirname{hgext} directory of a Mercurial source tree.  If you are
bos@49: using a Mercurial binary package, the hooks will be located in the
bos@49: \dirname{hgext} directory of wherever your package installer put
bos@49: Mercurial.
bos@49: 
bos@49: \subsection{\hgext{acl}---access control for parts of a repository}
bos@49: 
bos@49: The \hgext{acl} extension lets you control which remote users are
bos@49: allowed to push changesets to a networked server.  You can protect any
bos@49: portion of a repository (including the entire repo), so that a
bos@49: specific remote user can push changes that do not affect the protected
bos@49: portion.
bos@49: 
bos@49: This extension implements access control based on the identity of the
bos@49: user performing a push, \emph{not} on who committed the changesets
bos@49: they're pushing.  (If access control based on committer was to work
bos@49: properly, it would require commits to be cryptographically signed,
bos@49: which is an onerous and hence unusual policy to enforce.)
bos@49: 
bos@49: XXX More help.
bos@44: 
bos@39: \section{Hook reference}
bos@41: \label{sec:hook:ref}
bos@39: 
bos@39: \subsection{In-process hook execution}
bos@39: 
bos@39: An in-process hook is called with arguments of the following form:
bos@39: \begin{codesample2}
bos@39:   def myhook(ui, repo, **kwargs):
bos@39:       pass
bos@39: \end{codesample2}
bos@39: The \texttt{ui} parameter is a \pymodclass{mercurial.ui}{ui} object.
bos@39: The \texttt{repo} parameter is a
bos@39: \pymodclass{mercurial.localrepo}{localrepository} object.  The
bos@39: names and values of the \texttt{**kwargs} parameters depend on the
bos@39: hook being invoked, with the following common features:
bos@39: \begin{itemize}
bos@39: \item If a parameter is named \texttt{node} or
bos@39:   \texttt{parent\emph{N}}, it will contain a hexadecimal changeset ID.
bos@39:   The empty string is used to represent ``null changeset ID'' instead
bos@39:   of a string of zeroes.
bos@39: \item Boolean-valued parameters are represented as Python
bos@39:   \texttt{bool} objects.
bos@39: \end{itemize}
bos@39: 
bos@39: An in-process hook is called without a change to the process's working
bos@39: directory (unlike external hooks, which are run in the root of the
bos@39: repository).  It must not change the process's working directory.  If
bos@39: it were to do so, it would probably cause calls to the Mercurial API,
bos@39: or operations after the hook finishes, to fail.
bos@39: 
bos@39: If a hook returns a boolean ``false'' value, it is considered to
bos@39: have succeeded.  If it returns a boolean ``true'' value or raises an
bos@39: exception, it is considered to have failed.
bos@39: 
bos@39: \subsection{External hook execution}
bos@39: 
bos@39: An external hook is passed to the user's shell for execution, so
bos@39: features of that shell, such as variable substitution and command
bos@39: redirection, are available.  The hook is run in the root directory of
bos@39: the repository.
bos@39: 
bos@39: Hook parameters are passed to the hook as environment variables.  Each
bos@39: environment variable's name is converted in upper case and prefixed
bos@39: with the string ``\texttt{HG\_}''.  For example, if the name of a
bos@39: parameter is ``\texttt{node}'', the name of the environment variable
bos@39: representing that parameter will be ``\texttt{HG\_NODE}''.
bos@39: 
bos@39: A boolean parameter is represented as the string ``\texttt{1}'' for
bos@39: ``true'', ``\texttt{0}'' for ``false''.  If an environment variable is
bos@39: named \envar{HG\_NODE}, \envar{HG\_PARENT1} or \envar{HG\_PARENT2}, it
bos@39: contains a changeset ID represented as a hexadecimal string.  The
bos@39: empty string is used to represent ``null changeset ID'' instead of a
bos@39: string of zeroes.
bos@39: 
bos@39: If a hook exits with a status of zero, it is considered to have
bos@39: succeeded.  If it exits with a non-zero status, it is considered to
bos@39: have failed.
bos@39: 
bos@39: \subsection{The \hook{changegroup} hook}
bos@39: \label{sec:hook:changegroup}
bos@39: 
bos@40: This hook is run after a group of pre-existing changesets has been
bos@40: added to the repository, for example via a \hgcmd{pull} or
bos@40: \hgcmd{unbundle}.  This hook is run once per operation that added one
bos@41: or more changesets.  This is in contrast to the \hook{incoming} hook,
bos@41: which is run once per changeset, regardless of whether the changesets
bos@41: arrive in a group.
bos@41: 
bos@41: Some possible uses for this hook include kicking off an automated
bos@41: build or test of the added changesets, updating a bug database, or
bos@41: notifying subscribers that a repository contains new changes.
bos@40: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{node}] A changeset ID.  The changeset ID of the first
bos@40:   changeset in the group that was added.  All changesets between this
bos@40:   and \index{tags!\texttt{tip}}\texttt{tip}, inclusive, were added by
bos@40:   a single \hgcmd{pull}, \hgcmd{push} or \hgcmd{unbundle}.
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{incoming} (section~\ref{sec:hook:incoming}),
bos@40: \hook{prechangegroup} (section~\ref{sec:hook:prechangegroup}),
bos@40: \hook{pretxnchangegroup} (section~\ref{sec:hook:pretxnchangegroup})
bos@39: 
bos@39: \subsection{The \hook{commit} hook}
bos@39: \label{sec:hook:commit}
bos@39: 
bos@40: This hook is run after a new changeset has been created.
bos@40: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{node}] A changeset ID.  The changeset ID of the newly
bos@40:   committed changeset.
bos@40: \item[\texttt{parent1}] A changeset ID.  The changeset ID of the first
bos@40:   parent of the newly committed changeset.
bos@40: \item[\texttt{parent2}] A changeset ID.  The changeset ID of the second
bos@40:   parent of the newly committed changeset.
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{precommit} (section~\ref{sec:hook:precommit}),
bos@40: \hook{pretxncommit} (section~\ref{sec:hook:pretxncommit})
bos@40: 
bos@40: \subsection{The \hook{incoming} hook}
bos@40: \label{sec:hook:incoming}
bos@40: 
bos@40: This hook is run after a pre-existing changeset has been added to the
bos@40: repository, for example via a \hgcmd{push}.  If a group of changesets
bos@40: was added in a single operation, this hook is called once for each
bos@40: added changeset.
bos@40: 
bos@41: You can use this hook for the same purposes as the \hook{changegroup}
bos@41: hook (section~\ref{sec:hook:changegroup}); it's simply more convenient
bos@54: sometimes to run a hook once per group of changesets, while other
bos@41: times it's handier once per changeset.
bos@41: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{node}] A changeset ID.  The ID of the newly added
bos@39:   changeset.
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{changegroup} (section~\ref{sec:hook:changegroup}) \hook{prechangegroup} (section~\ref{sec:hook:prechangegroup}), \hook{pretxnchangegroup} (section~\ref{sec:hook:pretxnchangegroup})
bos@40: 
bos@40: \subsection{The \hook{outgoing} hook}
bos@40: \label{sec:hook:outgoing}
bos@40: 
bos@40: This hook is run after a group of changesets has been propagated out
bos@40: of this repository, for example by a \hgcmd{push} or \hgcmd{bundle}
bos@40: command.
bos@40: 
bos@41: One possible use for this hook is to notify administrators that
bos@41: changes have been pulled.
bos@41: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{node}] A changeset ID.  The changeset ID of the first
bos@40:   changeset of the group that was sent.
bos@40: \item[\texttt{source}] A string.  The source of the of the operation.
bos@40:   If a remote client pulled changes from this repository,
bos@40:   \texttt{source} will be \texttt{serve}.  If the client that obtained
bos@40:   changes from this repository was local, \texttt{source} will be
bos@40:   \texttt{bundle}, \texttt{pull}, or \texttt{push}, depending on the
bos@40:   operation the client performed.
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{preoutgoing} (section~\ref{sec:hook:preoutgoing})
bos@40: 
bos@40: \subsection{The \hook{prechangegroup} hook}
bos@40: \label{sec:hook:prechangegroup}
bos@40: 
bos@41: This controlling hook is run before Mercurial begins to add a group of
bos@41: changesets from another repository.
bos@41: 
bos@41: This hook does not have any information about the changesets to be
bos@41: added, because it is run before transmission of those changesets is
bos@41: allowed to begin.  If this hook fails, the changesets will not be
bos@41: transmitted.
bos@41: 
bos@41: One use for this hook is to prevent external changes from being added
bos@41: to a repository, for example to ``freeze'' a server-hosted branch
bos@41: temporarily or permanently.
bos@41: 
bos@40: This hook is not passed any parameters.
bos@40: 
bos@40: See also: \hook{changegroup} (section~\ref{sec:hook:changegroup}),
bos@40: \hook{incoming} (section~\ref{sec:hook:incoming}), ,
bos@40: \hook{pretxnchangegroup} (section~\ref{sec:hook:pretxnchangegroup})
bos@40: 
bos@40: \subsection{The \hook{precommit} hook}
bos@40: \label{sec:hook:precommit}
bos@40: 
bos@41: This hook is run before Mercurial begins to commit a new changeset.
bos@41: It is run before Mercurial has any of the metadata for the commit,
bos@41: such as the files to be committed, the commit message, or the commit
bos@41: date.
bos@41: 
bos@41: One use for this hook is to disable the ability to commit new
bos@41: changesets, while still allowing incoming changesets.  Another is to
bos@41: run a build or test, and only allow the commit to begin if the build
bos@41: or test succeeds.
bos@40: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{parent1}] A changeset ID.  The changeset ID of the first
bos@40:   parent of the working directory.
bos@40: \item[\texttt{parent2}] A changeset ID.  The changeset ID of the second
bos@40:   parent of the working directory.
bos@40: \end{itemize}
bos@40: If the commit proceeds, the parents of the working directory will
bos@40: become the parents of the new changeset.
bos@40: 
bos@40: See also: \hook{commit} (section~\ref{sec:hook:commit}),
bos@40: \hook{pretxncommit} (section~\ref{sec:hook:pretxncommit})
bos@40: 
bos@40: \subsection{The \hook{preoutgoing} hook}
bos@40: \label{sec:hook:preoutgoing}
bos@40: 
bos@40: This hook is invoked before Mercurial knows the identities of the
bos@40: changesets to be transmitted.
bos@40: 
bos@41: One use for this hook is to prevent changes from being transmitted to
bos@41: another repository.
bos@41: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{source}] A string.  The source of the operation that is
bos@40:   attempting to obtain changes from this repository.  See the
bos@40:   documentation for the \texttt{source} parameter to the
bos@40:   \hook{outgoing} hook, in section~\ref{sec:hook:outgoing}, for
bos@40:   possible values of this parameter..
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{outgoing} (section~\ref{sec:hook:outgoing})
bos@40: 
bos@40: \subsection{The \hook{pretag} hook}
bos@40: \label{sec:hook:pretag}
bos@40: 
bos@41: This controlling hook is run before a tag is created.  If the hook
bos@41: succeeds, creation of the tag proceeds.  If the hook fails, the tag is
bos@41: not created.
bos@41: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{local}] A boolean.  Whether the tag is local to this
bos@40:   repository instance (i.e.~stored in \sfilename{.hg/tags}) or managed
bos@40:   by Mercurial (stored in \sfilename{.hgtags}).
bos@40: \item[\texttt{node}] A changeset ID.  The ID of the changeset to be tagged.
bos@40: \item[\texttt{tag}] A string.  The name of the tag to be created.
bos@40: \end{itemize}
bos@40: 
bos@40: If the tag to be created is revision-controlled, the \hook{precommit}
bos@40: and \hook{pretxncommit} hooks (sections~\ref{sec:hook:commit}
bos@40: and~\ref{sec:hook:pretxncommit}) will also be run.
bos@40: 
bos@40: See also: \hook{tag} (section~\ref{sec:hook:tag})
bos@40: 
bos@40: \subsection{The \hook{pretxnchangegroup} hook}
bos@40: \label{sec:hook:pretxnchangegroup}
bos@40: 
bos@41: This controlling hook is run before a transaction---that manages the
bos@41: addition of a group of new changesets from outside the
bos@41: repository---completes.  If the hook succeeds, the transaction
bos@41: completes, and all of the changesets become permanent within this
bos@41: repository.  If the hook fails, the transaction is rolled back, and
bos@41: the data for the changesets is erased.
bos@41: 
bos@41: This hook can access the metadata associated with the almost-added
bos@41: changesets, but it should not do anything permanent with this data.
bos@41: It must also not modify the working directory.
bos@41: 
bos@41: While this hook is running, if other Mercurial processes access this
bos@41: repository, they will be able to see the almost-added changesets as if
bos@41: they are permanent.  This may lead to race conditions if you do not
bos@41: take steps to avoid them.
bos@41: 
bos@41: This hook can be used to automatically vet a group of changesets.  If
bos@41: the hook fails, all of the changesets are ``rejected'' when the
bos@41: transaction rolls back.
bos@41: 
bos@40: Parameters to this hook are the same as for the \hook{changegroup}
bos@40: hook; see section~\ref{sec:hook:changegroup} for details.
bos@40: 
bos@40: See also: \hook{changegroup} (section~\ref{sec:hook:changegroup}),
bos@40: \hook{incoming} (section~\ref{sec:hook:incoming}),
bos@40: \hook{prechangegroup} (section~\ref{sec:hook:prechangegroup})
bos@40: 
bos@40: \subsection{The \hook{pretxncommit} hook}
bos@40: \label{sec:hook:pretxncommit}
bos@40: 
bos@41: This controlling hook is run before a transaction---that manages a new
bos@41: commit---completes.  If the hook succeeds, the transaction completes
bos@41: and the changeset becomes permanent within this repository.  If the
bos@41: hook fails, the transaction is rolled back, and the commit data is
bos@41: erased.
bos@41: 
bos@41: This hook can access the metadata associated with the almost-new
bos@41: changeset, but it should not do anything permanent with this data.  It
bos@41: must also not modify the working directory.
bos@41: 
bos@41: While this hook is running, if other Mercurial processes access this
bos@41: repository, they will be able to see the almost-new changeset as if it
bos@41: is permanent.  This may lead to race conditions if you do not take
bos@41: steps to avoid them.
bos@41: 
bos@40: Parameters to this hook are the same as for the \hook{commit} hook;
bos@40: see section~\ref{sec:hook:commit} for details.
bos@40: 
bos@40: See also: \hook{precommit} (section~\ref{sec:hook:precommit})
bos@40: 
bos@40: \subsection{The \hook{preupdate} hook}
bos@40: \label{sec:hook:preupdate}
bos@40: 
bos@41: This controlling hook is run before an update or merge of the working
bos@41: directory begins.  It is run only if Mercurial's normal pre-update
bos@41: checks determine that the update or merge can proceed.  If the hook
bos@41: succeeds, the update or merge may proceed; if it fails, the update or
bos@41: merge does not start.
bos@41: 
bos@40: Parameters to this hook:
bos@40: \begin{itemize}
bos@40: \item[\texttt{parent1}] A changeset ID.  The ID of the parent that the
bos@40:   working directory is to be updated to.  If the working directory is
bos@40:   being merged, it will not change this parent.
bos@40: \item[\texttt{parent2}] A changeset ID.  Only set if the working
bos@40:   directory is being merged.  The ID of the revision that the working
bos@40:   directory is being merged with.
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{update} (section~\ref{sec:hook:update})
bos@40: 
bos@40: \subsection{The \hook{tag} hook}
bos@40: \label{sec:hook:tag}
bos@40: 
bos@41: This hook is run after a tag has been created.
bos@41: 
bos@40: Parameters to this hook are the same as for the \hook{pretag} hook;
bos@40: see section~\ref{sec:hook:pretag} for details.
bos@40: 
bos@40: If the created tag is revision-controlled, the \hook{commit} hook
bos@41: (section~\ref{sec:hook:commit}) is run before this hook.
bos@40: 
bos@40: See also: \hook{pretag} (section~\ref{sec:hook:pretag})
bos@40: 
bos@40: \subsection{The \hook{update} hook}
bos@40: \label{sec:hook:update}
bos@40: 
bos@41: This hook is run after an update or merge of the working directory
bos@41: completes.  Since a merge can fail (if the external \command{hgmerge}
bos@41: command fails to resolve conflicts in a file), this hook communicates
bos@41: whether the update or merge completed cleanly.
bos@41: 
bos@40: \begin{itemize}
bos@40: \item[\texttt{error}] A boolean.  Indicates whether the update or
bos@40:   merge completed successfully.
bos@40: \item[\texttt{parent1}] A changeset ID.  The ID of the parent that the
bos@40:   working directory was updated to.  If the working directory was
bos@40:   merged, it will not have changed this parent.
bos@40: \item[\texttt{parent2}] A changeset ID.  Only set if the working
bos@40:   directory was merged.  The ID of the revision that the working
bos@40:   directory was merged with.
bos@40: \end{itemize}
bos@40: 
bos@40: See also: \hook{preupdate} (section~\ref{sec:hook:preupdate})
bos@34: 
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bos@34: %%% End: