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view en/hook.tex @ 34:c0979ed1eabd
Get started on hook chapter.
| author | Bryan O'Sullivan <bos@serpentine.com> |
|---|---|
| date | Sun Jul 16 00:01:43 2006 -0700 (2006-07-16) |
| parents | |
| children | 9fd0c59b009a |
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1 \chapter{Handling repository events with hooks}
2 \label{chap:hook}
4 Mercurial offers a powerful mechanism to let you perform automated
5 actions in response to events that occur in a repository. In some
6 cases, you can even control Mercurial's response to those events.
8 The name Mercurial uses for one of these actions is a \emph{hook}.
9 Hooks are called ``triggers'' in some revision control systems, but
10 the two names refer to the same idea.
12 \section{A short tutorial on using hooks}
13 \label{sec:hook:simple}
15 It is easy to write a Mercurial hook. Let's start with a hook that
16 runs when you finish a \hgcmd{commit}, and simply prints the hash of
17 the changeset you just created. The hook is called \hook{commit}.
19 \begin{figure}[ht]
20 \interaction{hook.simple.init}
21 \caption{A simple hook that runs when a changeset is committed}
22 \label{ex:hook:init}
23 \end{figure}
25 All hooks follow the pattern in example~\ref{ex:hook:init}. You add
26 an entry to the \rcsection{hooks} section of your \hgrc\. On the left
27 is the name of the event to trigger on; on the right is the action to
28 take. As you can see, you can run an arbitrary shell command in a
29 hook. Mercurial passes extra information to the hook using
30 environment variables (look for \envar{HG\_NODE} in the example).
32 \subsection{Performing multiple actions per event}
34 Quite often, you will want to define more than one hook for a
35 particular kind of event, as shown in example~\ref{ex:hook:ext}.
36 Mercurial lets you do this by adding an \emph{extension} to the end of
37 a hook's name. You extend a hook's name by giving the name of the
38 hook, followed by a full stop (the ``\texttt{.}'' character), followed
39 by some more text of your choosing. For example, Mercurial will run
40 both \texttt{commit.foo} and \texttt{commit.bar} when the
41 \texttt{commit} event occurs.
43 \begin{figure}[ht]
44 \interaction{hook.simple.ext}
45 \caption{Defining a second \hook{commit} hook}
46 \label{ex:hook:ext}
47 \end{figure}
49 To give a well-defined order of execution when there are multiple
50 hooks defined for an event, Mercurial sorts hooks by extension, and
51 executes the hook commands in this sorted order. In the above
52 example, it will execute \texttt{commit.bar} before
53 \texttt{commit.foo}, and \texttt{commit} before both.
55 It is a good idea to use a somewhat descriptive extension when you
56 define a new hook. This will help you to remember what the hook was
57 for. If the hook fails, you'll get an error message that contains the
58 hook name and extension, so using a descriptive extension could give
59 you an immediate hint as to why the hook failed (see
60 section~\ref{sec:hook:perm} for an example).
62 \subsection{Controlling whether an activity can proceed}
63 \label{sec:hook:perm}
65 In our earlier examples, we used the \hook{commit} hook, which is
66 run after a commit has completed. This is one of several Mercurial
67 hooks that run after an activity finishes. Such hooks have no way of
68 influencing the activity itself.
70 Mercurial defines a number of events that occur before an activity
71 starts; or after it starts, but before it finishes. Hooks that
72 trigger on these events have the added ability to choose whether the
73 activity can continue, or will abort.
75 The \hook{pretxncommit} hook runs after a commit has all but
76 completed. In other words, the metadata representing the changeset
77 has been written out to disk, but the transaction has not yet been
78 allowed to complete. The \hook{pretxncommit} hook has the ability to
79 decide whether the transaction can complete, or must be rolled back.
81 If the \hook{pretxncommit} hook exits with a status code of zero, the
82 transaction is allowed to complete; the commit finishes; and the
83 \hook{commit} hook is run. If the \hook{pretxncommit} hook exits with
84 a non-zero status code, the transaction is rolled back; the metadata
85 representing the changeset is erased; and the \hook{commit} hook is
86 not run.
88 \begin{figure}[ht]
89 \interaction{hook.simple.pretxncommit}
90 \caption{Using the \hook{pretxncommit} hook to control commits}
91 \label{ex:hook:pretxncommit}
92 \end{figure}
94 The hook in example~\ref{ex:hook:pretxncommit} checks that a commit
95 comment contains a bug ID. If it does, the commit can complete. If
96 not, the commit is rolled back.
98 \section{Choosing how to write a hook}
99 \label{sec:hook:impl}
101 You can write a hook either as a normal program---typically a shell
102 script---or as a Python function that is called within the Mercurial
103 process.
105 Writing a hook as an external program has the advantage that it
106 requires no knowledge of Mercurial's internals. You can call normal
107 Mercurial commands to get any added information you need. The
108 trade-off is that external hooks are slower than in-process hooks.
110 An in-process Python hook has complete access to the Mercurial API,
111 and does not ``shell out'' to another process, so it is inherently
112 faster than an external hook. It is also easier to obtain much of the
113 information that a hook requires by using the Mercurial API than by
114 running Mercurial commands.
116 If you are comfortable with Python, or require high performance,
117 writing your hooks in Python may be a good choice. However, when you
118 have a straightforward hook to write and you don't need to care about
119 performance (probably the majority of hooks), a shell script is
120 perfectly fine.
122 \section{Hook parameters}
123 \label{sec:hook:param}
125 Mercurial calls each hook with a set of well-defined parameters. In
126 Python, a parameter is passed as a keyword argument to your hook
127 function. For an external program, a parameter is passed as an
128 environment variable.
130 Whether your hook is written in Python or as a shell script, the
131 parameter names and values will be the same. A boolean parameter will
132 be represented as a boolean value in Python, but as the number 1 (for
133 ``true'') or 0 (for ``false'')
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