\chapter{Managing change with Mercurial Queues}

\label{chap:mq}

\section{The patch management problem}

\label{sec:mq:patch-mgmt}

Here is a common scenario: you need to install a software package from

source, but you find a bug that you must fix in the source before you

can start using the package.  You make your changes, forget about the

package for a while, and a few months later you need to upgrade to a

newer version of the package.  If the newer version of the package

still has the bug, you must extract your fix from the older source

tree and apply it against the newer version.  This is a tedious task,

and it's easy to make mistakes.

This is a simple case of the ``patch management'' problem.  You have

an ``upstream'' source tree that you can't change; you need to make

some local changes on top of the upstream tree; and you'd like to be

able to keep those changes separate, so that you can apply them to

newer versions of the upstream source.

The patch management problem arises in many situations.  Probably the

most visible is that a user of an open source software project will

contribute a bug fix or new feature to the project's maintainers in the

form of a patch.

Distributors of operating systems that include open source software

often need to make changes to the packages they distribute so that

they will build properly in their environments.

When you have few changes to maintain, it is easy to manage a single

patch using the standard \command{diff} and \command{patch} programs

(see section~\ref{sec:mq:patch} for a discussion of these tools).

Once the number of changes grows, it starts to make sense to maintain

patches as discrete ``chunks of work,'' so that for example a single

patch will contain only one bug fix (the patch might modify several

files, but it's doing ``only one thing''), and you may have a number

of such patches for different bugs you need fixed and local changes

you require.  In this situation, if you submit a bug fix patch to the

upstream maintainers of a package and they include your fix in a

subsequent release, you can simply drop that single patch when you're

updating to the newer release.

Maintaining a single patch against an upstream tree is a little

tedious and error-prone, but not difficult.  However, the complexity

of the problem grows rapidly as the number of patches you have to

maintain increases.  With more than a tiny number of patches in hand,

understanding which ones you have applied and maintaining them moves

from messy to overwhelming.

Fortunately, Mercurial includes a powerful extension, Mercurial Queues

(or simply ``MQ''), that massively simplifies the patch management

problem.

\section{The prehistory of Mercurial Queues}

\label{sec:mq:history}

During the late 1990s, several Linux kernel developers started to

maintain ``patch series'' that modified the behaviour of the Linux

kernel.  Some of these series were focused on stability, some on

feature coverage, and others were more speculative.

The sizes of these patch series grew rapidly.  In 2002, Andrew Morton

published some shell scripts he had been using to automate the task of

managing his patch queues.  Andrew was successfully using these

scripts to manage hundreds (sometimes thousands) of patches on top of

the Linux kernel.

\subsection{A patchwork quilt}

\label{sec:mq:quilt}

In early 2003, Andreas Gruenbacher and Martin Quinson borrowed the

approach of Andrew's scripts and published a tool called ``patchwork

quilt''~\cite{web:quilt}, or simply ``quilt''

(see~\cite{gruenbacher:2005} for a paper describing it).  Because

quilt substantially automated patch management, it rapidly gained a

large following among open source software developers.

Quilt manages a \emph{stack of patches} on top of a directory tree.

To begin, you tell quilt to manage a directory tree, and tell it which

files you want to manage; it stores away the names and contents of

those files.  To fix a bug, you create a new patch (using a single

command), edit the files you need to fix, then ``refresh'' the patch.

The refresh step causes quilt to scan the directory tree; it updates

the patch with all of the changes you have made.  You can create

another patch on top of the first, which will track the changes

required to modify the tree from ``tree with one patch applied'' to

``tree with two patches applied''.

You can \emph{change} which patches are applied to the tree.  If you

``pop'' a patch, the changes made by that patch will vanish from the

directory tree.  Quilt remembers which patches you have popped,

though, so you can ``push'' a popped patch again, and the directory

tree will be restored to contain the modifications in the patch.  Most

importantly, you can run the ``refresh'' command at any time, and the

topmost applied patch will be updated.  This means that you can, at

any time, change both which patches are applied and what

modifications those patches make.

Quilt knows nothing about revision control tools, so it works equally

well on top of an unpacked tarball or a Subversion working copy.

\subsection{From patchwork quilt to Mercurial Queues}

\label{sec:mq:quilt-mq}

In mid-2005, Chris Mason took the features of quilt and wrote an

extension that he called Mercurial Queues, which added quilt-like

behaviour to Mercurial.

The key difference between quilt and MQ is that quilt knows nothing

about revision control systems, while MQ is \emph{integrated} into

Mercurial.  Each patch that you push is represented as a Mercurial

changeset.  Pop a patch, and the changeset goes away.

Because quilt does not care about revision control tools, it is still

a tremendously useful piece of software to know about for situations

where you cannot use Mercurial and MQ.

\section{The huge advantage of MQ}

I cannot overstate the value that MQ offers through the unification of

patches and revision control.

A major reason that patches have persisted in the free software and

open source world---in spite of the availability of increasingly

capable revision control tools over the years---is the \emph{agility}

they offer.  

Traditional revision control tools make a permanent, irreversible

record of everything that you do.  While this has great value, it's

also somewhat stifling.  If you want to perform a wild-eyed

experiment, you have to be careful in how you go about it, or you risk

leaving unneeded---or worse, misleading or destabilising---traces of

your missteps and errors in the permanent revision record.

By contrast, MQ's marriage of distributed revision control with

patches makes it much easier to isolate your work.  Your patches live

on top of normal revision history, and you can make them disappear or

reappear at will.  If you don't like a patch, you can drop it.  If a

patch isn't quite as you want it to be, simply fix it---as many times

as you need to, until you have refined it into the form you desire.

As an example, the integration of patches with revision control makes

understanding patches and debugging their effects---and their

interplay with the code they're based on---\emph{enormously} easier.

Since every applied patch has an associated changeset, you can use

\hgcmdargs{log}{\emph{filename}} to see which changesets and patches

affected a file.  You can use the \hgext{bisect} command to

binary-search through all changesets and applied patches to see where

a bug got introduced or fixed.  You can use the \hgcmd{annotate}

command to see which changeset or patch modified a particular line of

a source file.  And so on.

\section{Understanding patches}

\label{sec:mq:patch}

Because MQ doesn't hide its patch-oriented nature, it is helpful to

understand what patches are, and a little about the tools that work

with them.

The traditional Unix \command{diff} command compares two files, and

prints a list of differences between them. The \command{patch} command

understands these differences as \emph{modifications} to make to a

file.  Take a look at figure~\ref{ex:mq:diff} for a simple example of

these commands in action.

\begin{figure}[ht]

  \interaction{mq.dodiff.diff}

  \caption{Simple uses of the \command{diff} and \command{patch} commands}

  \label{ex:mq:diff}

\end{figure}

The type of file that \command{diff} generates (and \command{patch}

takes as input) is called a ``patch'' or a ``diff''; there is no

difference between a patch and a diff.  (We'll use the term ``patch'',

since it's more commonly used.)

A patch file can start with arbitrary text; the \command{patch}

command ignores this text, but MQ uses it as the commit message when

creating changesets.  To find the beginning of the patch content,

\command{patch} searches for the first line that starts with the

string ``\texttt{diff~-}''.

MQ works with \emph{unified} diffs (\command{patch} can accept several

other diff formats, but MQ doesn't).  A unified diff contains two

kinds of header.  The \emph{file header} describes the file being

modified; it contains the name of the file to modify.  When

\command{patch} sees a new file header, it looks for a file with that

name to start modifying.

After the file header comes a series of \emph{hunks}.  Each hunk

starts with a header; this identifies the range of line numbers within

the file that the hunk should modify.  Following the header, a hunk

starts and ends with a few (usually three) lines of text from the

unmodified file; these are called the \emph{context} for the hunk.  If

there's only a small amount of context between successive hunks,

\command{diff} doesn't print a new hunk header; it just runs the hunks

together, with a few lines of context between modifications.

Each line of context begins with a space character.  Within the hunk,

a line that begins with ``\texttt{-}'' means ``remove this line,''

while a line that begins with ``\texttt{+}'' means ``insert this

line.''  For example, a line that is modified is represented by one

deletion and one insertion.

We will return to some of the more subtle aspects of patches later (in

section~\ref{sec:mq:adv-patch}), but you should have enough information

now to use MQ.

\section{Getting started with Mercurial Queues}

\label{sec:mq:start}

Because MQ is implemented as an extension, you must explicitly enable

before you can use it.  (You don't need to download anything; MQ ships

with the standard Mercurial distribution.)  To enable MQ, edit your

\tildefile{.hgrc} file, and add the lines in figure~\ref{ex:mq:config}.

\begin{figure}[ht]

  \begin{codesample4}

    [extensions]

    hgext.mq =

  \end{codesample4}

  \label{ex:mq:config}

  \caption{Contents to add to \tildefile{.hgrc} to enable the MQ extension}

\end{figure}

Once the extension is enabled, it will make a number of new commands

available.  To verify that the extension is working, you can use

\hgcmd{help} to see if the \hgxcmd{mq}{qinit} command is now available; see

the example in figure~\ref{ex:mq:enabled}.

\begin{figure}[ht]

  \interaction{mq.qinit-help.help}

  \caption{How to verify that MQ is enabled}

  \label{ex:mq:enabled}

\end{figure}

You can use MQ with \emph{any} Mercurial repository, and its commands

only operate within that repository.  To get started, simply prepare

the repository using the \hgxcmd{mq}{qinit} command (see

figure~\ref{ex:mq:qinit}).  This command creates an empty directory

called \sdirname{.hg/patches}, where MQ will keep its metadata.  As

with many Mercurial commands, the \hgxcmd{mq}{qinit} command prints nothing

if it succeeds.

\begin{figure}[ht]

  \interaction{mq.tutorial.qinit}

  \caption{Preparing a repository for use with MQ}

  \label{ex:mq:qinit}

\end{figure}

\begin{figure}[ht]

  \interaction{mq.tutorial.qnew}

  \caption{Creating a new patch}

  \label{ex:mq:qnew}

\end{figure}

\subsection{Creating a new patch}

To begin work on a new patch, use the \hgxcmd{mq}{qnew} command.  This

command takes one argument, the name of the patch to create.  MQ will

use this as the name of an actual file in the \sdirname{.hg/patches}

directory, as you can see in figure~\ref{ex:mq:qnew}.

Also newly present in the \sdirname{.hg/patches} directory are two

other files, \sfilename{series} and \sfilename{status}.  The

\sfilename{series} file lists all of the patches that MQ knows about

for this repository, with one patch per line.  Mercurial uses the

\sfilename{status} file for internal book-keeping; it tracks all of the

patches that MQ has \emph{applied} in this repository.

\begin{note}

  You may sometimes want to edit the \sfilename{series} file by hand;

  for example, to change the sequence in which some patches are

  applied.  However, manually editing the \sfilename{status} file is

  almost always a bad idea, as it's easy to corrupt MQ's idea of what

  is happening.

\end{note}

Once you have created your new patch, you can edit files in the

working directory as you usually would.  All of the normal Mercurial

commands, such as \hgcmd{diff} and \hgcmd{annotate}, work exactly as

they did before.

\subsection{Refreshing a patch}

When you reach a point where you want to save your work, use the

\hgxcmd{mq}{qrefresh} command (figure~\ref{ex:mq:qnew}) to update the patch

you are working on.  This command folds the changes you have made in

the working directory into your patch, and updates its corresponding

changeset to contain those changes.

\begin{figure}[ht]

  \interaction{mq.tutorial.qrefresh}

  \caption{Refreshing a patch}

  \label{ex:mq:qrefresh}

\end{figure}

You can run \hgxcmd{mq}{qrefresh} as often as you like, so it's a good way

to ``checkpoint'' your work.  Refresh your patch at an opportune

time; try an experiment; and if the experiment doesn't work out,

\hgcmd{revert} your modifications back to the last time you refreshed.

\begin{figure}[ht]

  \interaction{mq.tutorial.qrefresh2}

  \caption{Refresh a patch many times to accumulate changes}

  \label{ex:mq:qrefresh2}

\end{figure}

\subsection{Stacking and tracking patches}

Once you have finished working on a patch, or need to work on another,

you can use the \hgxcmd{mq}{qnew} command again to create a new patch.

Mercurial will apply this patch on top of your existing patch.  See

figure~\ref{ex:mq:qnew2} for an example.  Notice that the patch

contains the changes in our prior patch as part of its context (you

can see this more clearly in the output of \hgcmd{annotate}).

\begin{figure}[ht]

  \interaction{mq.tutorial.qnew2}

  \caption{Stacking a second patch on top of the first}

  \label{ex:mq:qnew2}

\end{figure}

So far, with the exception of \hgxcmd{mq}{qnew} and \hgxcmd{mq}{qrefresh}, we've

been careful to only use regular Mercurial commands.  However, MQ

provides many commands that are easier to use when you are thinking

about patches, as illustrated in figure~\ref{ex:mq:qseries}:

\begin{itemize}

\item The \hgxcmd{mq}{qseries} command lists every patch that MQ knows

  about in this repository, from oldest to newest (most recently

  \emph{created}).

\item The \hgxcmd{mq}{qapplied} command lists every patch that MQ has

  \emph{applied} in this repository, again from oldest to newest (most

  recently applied).

\end{itemize}

\begin{figure}[ht]

  \interaction{mq.tutorial.qseries}

  \caption{Understanding the patch stack with \hgxcmd{mq}{qseries} and

    \hgxcmd{mq}{qapplied}}

  \label{ex:mq:qseries}

\end{figure}

\subsection{Manipulating the patch stack}

The previous discussion implied that there must be a difference

between ``known'' and ``applied'' patches, and there is.  MQ can

manage a patch without it being applied in the repository.

An \emph{applied} patch has a corresponding changeset in the

repository, and the effects of the patch and changeset are visible in

the working directory.  You can undo the application of a patch using

the \hgxcmd{mq}{qpop} command.  MQ still \emph{knows about}, or manages, a

popped patch, but the patch no longer has a corresponding changeset in

the repository, and the working directory does not contain the changes

made by the patch.  Figure~\ref{fig:mq:stack} illustrates the

difference between applied and tracked patches.

\begin{figure}[ht]

  \centering

  \grafix{mq-stack}

  \caption{Applied and unapplied patches in the MQ patch stack}

  \label{fig:mq:stack}

\end{figure}

You can reapply an unapplied, or popped, patch using the \hgxcmd{mq}{qpush}

command.  This creates a new changeset to correspond to the patch, and

the patch's changes once again become present in the working

directory.  See figure~\ref{ex:mq:qpop} for examples of \hgxcmd{mq}{qpop}

and \hgxcmd{mq}{qpush} in action.  Notice that once we have popped a patch

or two patches, the output of \hgxcmd{mq}{qseries} remains the same, while

that of \hgxcmd{mq}{qapplied} has changed.

\begin{figure}[ht]

  \interaction{mq.tutorial.qpop}

  \caption{Modifying the stack of applied patches}

  \label{ex:mq:qpop}

\end{figure}

\subsection{Pushing and popping many patches}

While \hgxcmd{mq}{qpush} and \hgxcmd{mq}{qpop} each operate on a single patch at

a time by default, you can push and pop many patches in one go.  The

\hgxopt{mq}{qpush}{-a} option to \hgxcmd{mq}{qpush} causes it to push all

unapplied patches, while the \hgxopt{mq}{qpop}{-a} option to \hgxcmd{mq}{qpop}

causes it to pop all applied patches.  (For some more ways to push and

pop many patches, see section~\ref{sec:mq:perf} below.)

\begin{figure}[ht]

  \interaction{mq.tutorial.qpush-a}

  \caption{Pushing all unapplied patches}

  \label{ex:mq:qpush-a}

\end{figure}

\subsection{Safety checks, and overriding them}

Several MQ commands check the working directory before they do

anything, and fail if they find any modifications.  They do this to

ensure that you won't lose any changes that you have made, but not yet

incorporated into a patch.  Figure~\ref{ex:mq:add} illustrates this;

the \hgxcmd{mq}{qnew} command will not create a new patch if there are

outstanding changes, caused in this case by the \hgcmd{add} of

\filename{file3}.

\begin{figure}[ht]

  \interaction{mq.tutorial.add}

  \caption{Forcibly creating a patch}

  \label{ex:mq:add}

\end{figure}

Commands that check the working directory all take an ``I know what

I'm doing'' option, which is always named \option{-f}.  The exact

meaning of \option{-f} depends on the command.  For example,

\hgcmdargs{qnew}{\hgxopt{mq}{qnew}{-f}} will incorporate any outstanding

changes into the new patch it creates, but

\hgcmdargs{qpop}{\hgxopt{mq}{qpop}{-f}} will revert modifications to any

files affected by the patch that it is popping.  Be sure to read the

documentation for a command's \option{-f} option before you use it!

\subsection{Working on several patches at once}

The \hgxcmd{mq}{qrefresh} command always refreshes the \emph{topmost}

applied patch.  This means that you can suspend work on one patch (by

refreshing it), pop or push to make a different patch the top, and

work on \emph{that} patch for a while.

Here's an example that illustrates how you can use this ability.

Let's say you're developing a new feature as two patches.  The first

is a change to the core of your software, and the second---layered on

top of the first---changes the user interface to use the code you just

added to the core.  If you notice a bug in the core while you're

working on the UI patch, it's easy to fix the core.  Simply

\hgxcmd{mq}{qrefresh} the UI patch to save your in-progress changes, and

\hgxcmd{mq}{qpop} down to the core patch.  Fix the core bug,

\hgxcmd{mq}{qrefresh} the core patch, and \hgxcmd{mq}{qpush} back to the UI

patch to continue where you left off.

\section{More about patches}

\label{sec:mq:adv-patch}

MQ uses the GNU \command{patch} command to apply patches, so it's

helpful to know a few more detailed aspects of how \command{patch}

works, and about patches themselves.

\subsection{The strip count}

If you look at the file headers in a patch, you will notice that the

pathnames usually have an extra component on the front that isn't

present in the actual path name.  This is a holdover from the way that

people used to generate patches (people still do this, but it's

somewhat rare with modern revision control tools).  

Alice would unpack a tarball, edit her files, then decide that she

wanted to create a patch.  So she'd rename her working directory,

unpack the tarball again (hence the need for the rename), and use the

\cmdopt{diff}{-r} and \cmdopt{diff}{-N} options to \command{diff} to

recursively generate a patch between the unmodified directory and the

modified one.  The result would be that the name of the unmodified

directory would be at the front of the left-hand path in every file

header, and the name of the modified directory would be at the front

of the right-hand path.

Since someone receiving a patch from the Alices of the net would be

unlikely to have unmodified and modified directories with exactly the

same names, the \command{patch} command has a \cmdopt{patch}{-p}

option that indicates the number of leading path name components to

strip when trying to apply a patch.  This number is called the

\emph{strip count}.

An option of ``\texttt{-p1}'' means ``use a strip count of one''.  If

\command{patch} sees a file name \filename{foo/bar/baz} in a file

header, it will strip \filename{foo} and try to patch a file named

\filename{bar/baz}.  (Strictly speaking, the strip count refers to the

number of \emph{path separators} (and the components that go with them

) to strip.  A strip count of one will turn \filename{foo/bar} into

\filename{bar}, but \filename{/foo/bar} (notice the extra leading

slash) into \filename{foo/bar}.)

The ``standard'' strip count for patches is one; almost all patches

contain one leading path name component that needs to be stripped.

Mercurial's \hgcmd{diff} command generates path names in this form,

and the \hgcmd{import} command and MQ expect patches to have a strip

count of one.

If you receive a patch from someone that you want to add to your patch

queue, and the patch needs a strip count other than one, you cannot

just \hgxcmd{mq}{qimport} the patch, because \hgxcmd{mq}{qimport} does not yet

have a \texttt{-p} option (see~\bug{311}).  Your best bet is to

\hgxcmd{mq}{qnew} a patch of your own, then use \cmdargs{patch}{-p\emph{N}}

to apply their patch, followed by \hgcmd{addremove} to pick up any

files added or removed by the patch, followed by \hgxcmd{mq}{qrefresh}.

This complexity may become unnecessary; see~\bug{311} for details.

\subsection{Strategies for applying a patch}

When \command{patch} applies a hunk, it tries a handful of

successively less accurate strategies to try to make the hunk apply.

This falling-back technique often makes it possible to take a patch

that was generated against an old version of a file, and apply it

against a newer version of that file.

First, \command{patch} tries an exact match, where the line numbers,

the context, and the text to be modified must apply exactly.  If it

cannot make an exact match, it tries to find an exact match for the

context, without honouring the line numbering information.  If this

succeeds, it prints a line of output saying that the hunk was applied,

but at some \emph{offset} from the original line number.

If a context-only match fails, \command{patch} removes the first and

last lines of the context, and tries a \emph{reduced} context-only

match.  If the hunk with reduced context succeeds, it prints a message

saying that it applied the hunk with a \emph{fuzz factor} (the number

after the fuzz factor indicates how many lines of context

\command{patch} had to trim before the patch applied).

When neither of these techniques works, \command{patch} prints a

message saying that the hunk in question was rejected.  It saves

rejected hunks (also simply called ``rejects'') to a file with the

same name, and an added \sfilename{.rej} extension.  It also saves an

unmodified copy of the file with a \sfilename{.orig} extension; the

copy of the file without any extensions will contain any changes made

by hunks that \emph{did} apply cleanly.  If you have a patch that

modifies \filename{foo} with six hunks, and one of them fails to

apply, you will have: an unmodified \filename{foo.orig}, a

\filename{foo.rej} containing one hunk, and \filename{foo}, containing

the changes made by the five successful five hunks.

\subsection{Some quirks of patch representation}

There are a few useful things to know about how \command{patch} works

with files.

\begin{itemize}

\item This should already be obvious, but \command{patch} cannot

  handle binary files.

\item Neither does it care about the executable bit; it creates new

  files as readable, but not executable.

\item \command{patch} treats the removal of a file as a diff between

  the file to be removed and the empty file.  So your idea of ``I

  deleted this file'' looks like ``every line of this file was

  deleted'' in a patch.

\item It treats the addition of a file as a diff between the empty

  file and the file to be added.  So in a patch, your idea of ``I

  added this file'' looks like ``every line of this file was added''.

\item It treats a renamed file as the removal of the old name, and the

  addition of the new name.  This means that renamed files have a big

  footprint in patches.  (Note also that Mercurial does not currently

  try to infer when files have been renamed or copied in a patch.)

\item \command{patch} cannot represent empty files, so you cannot use

  a patch to represent the notion ``I added this empty file to the

  tree''.

\end{itemize}

\subsection{Beware the fuzz}

While applying a hunk at an offset, or with a fuzz factor, will often

be completely successful, these inexact techniques naturally leave

open the possibility of corrupting the patched file.  The most common

cases typically involve applying a patch twice, or at an incorrect

location in the file.  If \command{patch} or \hgxcmd{mq}{qpush} ever

mentions an offset or fuzz factor, you should make sure that the

modified files are correct afterwards.  

It's often a good idea to refresh a patch that has applied with an

offset or fuzz factor; refreshing the patch generates new context

information that will make it apply cleanly.  I say ``often,'' not

``always,'' because sometimes refreshing a patch will make it fail to

apply against a different revision of the underlying files.  In some

cases, such as when you're maintaining a patch that must sit on top of

multiple versions of a source tree, it's acceptable to have a patch

apply with some fuzz, provided you've verified the results of the

patching process in such cases.

\subsection{Handling rejection}

If \hgxcmd{mq}{qpush} fails to apply a patch, it will print an error

message and exit.  If it has left \sfilename{.rej} files behind, it is

usually best to fix up the rejected hunks before you push more patches

or do any further work.

If your patch \emph{used to} apply cleanly, and no longer does because

you've changed the underlying code that your patches are based on,

Mercurial Queues can help; see section~\ref{sec:mq:merge} for details.

Unfortunately, there aren't any great techniques for dealing with

rejected hunks.  Most often, you'll need to view the \sfilename{.rej}

file and edit the target file, applying the rejected hunks by hand.

If you're feeling adventurous, Neil Brown, a Linux kernel hacker,

wrote a tool called \command{wiggle}~\cite{web:wiggle}, which is more

vigorous than \command{patch} in its attempts to make a patch apply.

Another Linux kernel hacker, Chris Mason (the author of Mercurial

Queues), wrote a similar tool called

\command{mpatch}~\cite{web:mpatch}, which takes a simple approach to

automating the application of hunks rejected by \command{patch}.  The

\command{mpatch} command can help with four common reasons that a hunk

may be rejected:

\begin{itemize}

\item The context in the middle of a hunk has changed.

\item A hunk is missing some context at the beginning or end.

\item A large hunk might apply better---either entirely or in

  part---if it was broken up into smaller hunks.

\item A hunk removes lines with slightly different content than those

  currently present in the file.

\end{itemize}

If you use \command{wiggle} or \command{mpatch}, you should be doubly

careful to check your results when you're done.  In fact,

\command{mpatch} enforces this method of double-checking the tool's

output, by automatically dropping you into a merge program when it has

done its job, so that you can verify its work and finish off any

remaining merges.

\section{Getting the best performance out of MQ}

\label{sec:mq:perf}

MQ is very efficient at handling a large number of patches.  I ran

some performance experiments in mid-2006 for a talk that I gave at the

2006 EuroPython conference~\cite{web:europython}.  I used as my data

set the Linux 2.6.17-mm1 patch series, which consists of 1,738

patches.  I applied these on top of a Linux kernel repository

containing all 27,472 revisions between Linux 2.6.12-rc2 and Linux

2.6.17.

On my old, slow laptop, I was able to

\hgcmdargs{qpush}{\hgxopt{mq}{qpush}{-a}} all 1,738 patches in 3.5 minutes,

and \hgcmdargs{qpop}{\hgxopt{mq}{qpop}{-a}} them all in 30 seconds.  (On a

newer laptop, the time to push all patches dropped to two minutes.)  I

could \hgxcmd{mq}{qrefresh} one of the biggest patches (which made 22,779

lines of changes to 287 files) in 6.6 seconds.

Clearly, MQ is well suited to working in large trees, but there are a

few tricks you can use to get the best performance of it.

First of all, try to ``batch'' operations together.  Every time you

run \hgxcmd{mq}{qpush} or \hgxcmd{mq}{qpop}, these commands scan the working

directory once to make sure you haven't made some changes and then

forgotten to run \hgxcmd{mq}{qrefresh}.  On a small tree, the time that

this scan takes is unnoticeable.  However, on a medium-sized tree

(containing tens of thousands of files), it can take a second or more.

The \hgxcmd{mq}{qpush} and \hgxcmd{mq}{qpop} commands allow you to push and pop

multiple patches at a time.  You can identify the ``destination

patch'' that you want to end up at.  When you \hgxcmd{mq}{qpush} with a

destination specified, it will push patches until that patch is at the

top of the applied stack.  When you \hgxcmd{mq}{qpop} to a destination, MQ

will pop patches until the destination patch is at the top.

You can identify a destination patch using either the name of the

patch, or by number.  If you use numeric addressing, patches are

counted from zero; this means that the first patch is zero, the second

is one, and so on.

\section{Updating your patches when the underlying code changes}

\label{sec:mq:merge}

It's common to have a stack of patches on top of an underlying

repository that you don't modify directly.  If you're working on

changes to third-party code, or on a feature that is taking longer to

develop than the rate of change of the code beneath, you will often

need to sync up with the underlying code, and fix up any hunks in your

patches that no longer apply.  This is called \emph{rebasing} your

patch series.

The simplest way to do this is to \hgcmdargs{qpop}{\hgxopt{mq}{qpop}{-a}}

your patches, then \hgcmd{pull} changes into the underlying

repository, and finally \hgcmdargs{qpush}{\hgxopt{mq}{qpop}{-a}} your

patches again.  MQ will stop pushing any time it runs across a patch

that fails to apply during conflicts, allowing you to fix your

conflicts, \hgxcmd{mq}{qrefresh} the affected patch, and continue pushing

until you have fixed your entire stack.

This approach is easy to use and works well if you don't expect

changes to the underlying code to affect how well your patches apply.

If your patch stack touches code that is modified frequently or

invasively in the underlying repository, however, fixing up rejected

hunks by hand quickly becomes tiresome.

It's possible to partially automate the rebasing process.  If your

patches apply cleanly against some revision of the underlying repo, MQ

can use this information to help you to resolve conflicts between your

patches and a different revision.

The process is a little involved.

\begin{enumerate}

\item To begin, \hgcmdargs{qpush}{-a} all of your patches on top of

  the revision where you know that they apply cleanly.

\item Save a backup copy of your patch directory using

  \hgcmdargs{qsave}{\hgxopt{mq}{qsave}{-e} \hgxopt{mq}{qsave}{-c}}.  This prints

  the name of the directory that it has saved the patches in.  It will

  save the patches to a directory called

  \sdirname{.hg/patches.\emph{N}}, where \texttt{\emph{N}} is a small

  integer.  It also commits a ``save changeset'' on top of your

  applied patches; this is for internal book-keeping, and records the

  states of the \sfilename{series} and \sfilename{status} files.

\item Use \hgcmd{pull} to bring new changes into the underlying

  repository.  (Don't run \hgcmdargs{pull}{-u}; see below for why.)

\item Update to the new tip revision, using

  \hgcmdargs{update}{\hgopt{update}{-C}} to override the patches you

  have pushed.

\item Merge all patches using \hgcmdargs{qpush}{\hgxopt{mq}{qpush}{-m}

    \hgxopt{mq}{qpush}{-a}}.  The \hgxopt{mq}{qpush}{-m} option to \hgxcmd{mq}{qpush}

  tells MQ to perform a three-way merge if the patch fails to apply.

\end{enumerate}

During the \hgcmdargs{qpush}{\hgxopt{mq}{qpush}{-m}}, each patch in the

\sfilename{series} file is applied normally.  If a patch applies with

fuzz or rejects, MQ looks at the queue you \hgxcmd{mq}{qsave}d, and

performs a three-way merge with the corresponding changeset.  This

merge uses Mercurial's normal merge machinery, so it may pop up a GUI

merge tool to help you to resolve problems.

When you finish resolving the effects of a patch, MQ refreshes your

patch based on the result of the merge.

At the end of this process, your repository will have one extra head

from the old patch queue, and a copy of the old patch queue will be in

\sdirname{.hg/patches.\emph{N}}. You can remove the extra head using

\hgcmdargs{qpop}{\hgxopt{mq}{qpop}{-a} \hgxopt{mq}{qpop}{-n} patches.\emph{N}}

or \hgcmd{strip}.  You can delete \sdirname{.hg/patches.\emph{N}} once

you are sure that you no longer need it as a backup.

\section{Identifying patches}

MQ commands that work with patches let you refer to a patch either by

using its name or by a number.  By name is obvious enough; pass the

name \filename{foo.patch} to \hgxcmd{mq}{qpush}, for example, and it will

push patches until \filename{foo.patch} is applied.  

As a shortcut, you can refer to a patch using both a name and a

numeric offset; \texttt{foo.patch-2} means ``two patches before

\texttt{foo.patch}'', while \texttt{bar.patch+4} means ``four patches

after \texttt{bar.patch}''.

Referring to a patch by index isn't much different.  The first patch

printed in the output of \hgxcmd{mq}{qseries} is patch zero (yes, it's one

of those start-at-zero counting systems); the second is patch one; and

so on.

MQ also makes it easy to work with patches when you are using normal

Mercurial commands.  Every command that accepts a changeset ID will

also accept the name of an applied patch.  MQ augments the tags

normally in the repository with an eponymous one for each applied

patch.  In addition, the special tags \index{tags!special tag

  names!\texttt{qbase}}\texttt{qbase} and \index{tags!special tag

  names!\texttt{qtip}}\texttt{qtip} identify the ``bottom-most'' and

topmost applied patches, respectively.

These additions to Mercurial's normal tagging capabilities make

dealing with patches even more of a breeze.

\begin{itemize}

\item Want to patchbomb a mailing list with your latest series of

  changes?

  \begin{codesample4}

    hg email qbase:qtip

  \end{codesample4}

  (Don't know what ``patchbombing'' is?  See

  section~\ref{sec:hgext:patchbomb}.)

\item Need to see all of the patches since \texttt{foo.patch} that

  have touched files in a subdirectory of your tree?

  \begin{codesample4}

    hg log -r foo.patch:qtip \emph{subdir}

  \end{codesample4}

\end{itemize}

Because MQ makes the names of patches available to the rest of

Mercurial through its normal internal tag machinery, you don't need to

type in the entire name of a patch when you want to identify it by

name.

\begin{figure}[ht]

  \interaction{mq.id.output}

  \caption{Using MQ's tag features to work with patches}

  \label{ex:mq:id}

\end{figure}

Another nice consequence of representing patch names as tags is that

when you run the \hgcmd{log} command, it will display a patch's name

as a tag, simply as part of its normal output.  This makes it easy to

visually distinguish applied patches from underlying ``normal''

revisions.  Figure~\ref{ex:mq:id} shows a few normal Mercurial

commands in use with applied patches.

\section{Useful things to know about}

There are a number of aspects of MQ usage that don't fit tidily into

sections of their own, but that are good to know.  Here they are, in

one place.

\begin{itemize}

\item Normally, when you \hgxcmd{mq}{qpop} a patch and \hgxcmd{mq}{qpush} it

  again, the changeset that represents the patch after the pop/push

  will have a \emph{different identity} than the changeset that

  represented the hash beforehand.  See

  section~\ref{sec:mqref:cmd:qpush} for information as to why this is.

\item It's not a good idea to \hgcmd{merge} changes from another

  branch with a patch changeset, at least if you want to maintain the

  ``patchiness'' of that changeset and changesets below it on the

  patch stack.  If you try to do this, it will appear to succeed, but

  MQ will become confused.

\end{itemize}

\section{Managing patches in a repository}

\label{sec:mq:repo}

Because MQ's \sdirname{.hg/patches} directory resides outside a

Mercurial repository's working directory, the ``underlying'' Mercurial

repository knows nothing about the management or presence of patches.

This presents the interesting possibility of managing the contents of

the patch directory as a Mercurial repository in its own right.  This

can be a useful way to work.  For example, you can work on a patch for

a while, \hgxcmd{mq}{qrefresh} it, then \hgcmd{commit} the current state of

the patch.  This lets you ``roll back'' to that version of the patch

later on.

You can then share different versions of the same patch stack among

multiple underlying repositories.  I use this when I am developing a

Linux kernel feature.  I have a pristine copy of my kernel sources for

each of several CPU architectures, and a cloned repository under each

that contains the patches I am working on.  When I want to test a

change on a different architecture, I push my current patches to the

patch repository associated with that kernel tree, pop and push all of

my patches, and build and test that kernel.

Managing patches in a repository makes it possible for multiple

developers to work on the same patch series without colliding with

each other, all on top of an underlying source base that they may or

may not control.

\subsection{MQ support for patch repositories}

MQ helps you to work with the \sdirname{.hg/patches} directory as a

repository; when you prepare a repository for working with patches

using \hgxcmd{mq}{qinit}, you can pass the \hgxopt{mq}{qinit}{-c} option to

create the \sdirname{.hg/patches} directory as a Mercurial repository.

\begin{note}

  If you forget to use the \hgxopt{mq}{qinit}{-c} option, you can simply go

  into the \sdirname{.hg/patches} directory at any time and run

  \hgcmd{init}.  Don't forget to add an entry for the

  \sfilename{status} file to the \sfilename{.hgignore} file, though

  (\hgcmdargs{qinit}{\hgxopt{mq}{qinit}{-c}} does this for you

  automatically); you \emph{really} don't want to manage the

  \sfilename{status} file.

\end{note}

As a convenience, if MQ notices that the \dirname{.hg/patches}

directory is a repository, it will automatically \hgcmd{add} every

patch that you create and import.

MQ provides a shortcut command, \hgxcmd{mq}{qcommit}, that runs

\hgcmd{commit} in the \sdirname{.hg/patches} directory.  This saves

some bothersome typing.

Finally, as a convenience to manage the patch directory, you can

define the alias \command{mq} on Unix systems. For example, on Linux

systems using the \command{bash} shell, you can include the following

snippet in your \tildefile{.bashrc}.

\begin{codesample2}

  alias mq=`hg -R \$(hg root)/.hg/patches'

\end{codesample2}

You can then issue commands of the form \cmdargs{mq}{pull} from

the main repository.

\subsection{A few things to watch out for}

MQ's support for working with a repository full of patches is limited

in a few small respects.

MQ cannot automatically detect changes that you make to the patch

directory.  If you \hgcmd{pull}, manually edit, or \hgcmd{update}

changes to patches or the \sfilename{series} file, you will have to

\hgcmdargs{qpop}{\hgxopt{mq}{qpop}{-a}} and then

\hgcmdargs{qpush}{\hgxopt{mq}{qpush}{-a}} in the underlying repository to

see those changes show up there.  If you forget to do this, you can

confuse MQ's idea of which patches are applied.

\section{Third party tools for working with patches}

\label{sec:mq:tools}

Once you've been working with patches for a while, you'll find

yourself hungry for tools that will help you to understand and

manipulate the patches you're dealing with.

The \command{diffstat} command~\cite{web:diffstat} generates a

histogram of the modifications made to each file in a patch.  It

provides a good way to ``get a sense of'' a patch---which files it

affects, and how much change it introduces to each file and as a

whole.  (I find that it's a good idea to use \command{diffstat}'s

\cmdopt{diffstat}{-p} option as a matter of course, as otherwise it

will try to do clever things with prefixes of file names that

inevitably confuse at least me.)

\begin{figure}[ht]

  \interaction{mq.tools.tools}

  \caption{The \command{diffstat}, \command{filterdiff}, and \command{lsdiff} commands}

  \label{ex:mq:tools}

\end{figure}

The \package{patchutils} package~\cite{web:patchutils} is invaluable.

It provides a set of small utilities that follow the ``Unix

philosophy;'' each does one useful thing with a patch.  The

\package{patchutils} command I use most is \command{filterdiff}, which

extracts subsets from a patch file.  For example, given a patch that

modifies hundreds of files across dozens of directories, a single

invocation of \command{filterdiff} can generate a smaller patch that

only touches files whose names match a particular glob pattern.  See

section~\ref{mq-collab:tips:interdiff} for another example.

\section{Good ways to work with patches}

Whether you are working on a patch series to submit to a free software

or open source project, or a series that you intend to treat as a

sequence of regular changesets when you're done, you can use some

simple techniques to keep your work well organised.

Give your patches descriptive names.  A good name for a patch might be

\filename{rework-device-alloc.patch}, because it will immediately give

you a hint what the purpose of the patch is.  Long names shouldn't be

a problem; you won't be typing the names often, but you \emph{will} be

running commands like \hgxcmd{mq}{qapplied} and \hgxcmd{mq}{qtop} over and over.

Good naming becomes especially important when you have a number of

patches to work with, or if you are juggling a number of different

tasks and your patches only get a fraction of your attention.

Be aware of what patch you're working on.  Use the \hgxcmd{mq}{qtop}

command and skim over the text of your patches frequently---for

example, using \hgcmdargs{tip}{\hgopt{tip}{-p}})---to be sure of where

you stand.  I have several times worked on and \hgxcmd{mq}{qrefresh}ed a

patch other than the one I intended, and it's often tricky to migrate

changes into the right patch after making them in the wrong one.

For this reason, it is very much worth investing a little time to

learn how to use some of the third-party tools I described in

section~\ref{sec:mq:tools}, particularly \command{diffstat} and

\command{filterdiff}.  The former will give you a quick idea of what

changes your patch is making, while the latter makes it easy to splice

hunks selectively out of one patch and into another.

\section{MQ cookbook}

\subsection{Manage ``trivial'' patches}

Because the overhead of dropping files into a new Mercurial repository

is so low, it makes a lot of sense to manage patches this way even if

you simply want to make a few changes to a source tarball that you

downloaded.

Begin by downloading and unpacking the source tarball,

and turning it into a Mercurial repository.

\interaction{mq.tarball.download}

Continue by creating a patch stack and making your changes.

\interaction{mq.tarball.qinit}

Let's say a few weeks or months pass, and your package author releases

a new version.  First, bring their changes into the repository.

\interaction{mq.tarball.newsource}

The pipeline starting with \hgcmd{locate} above deletes all files in

the working directory, so that \hgcmd{commit}'s

\hgopt{commit}{--addremove} option can actually tell which files have

really been removed in the newer version of the source.

Finally, you can apply your patches on top of the new tree.

\interaction{mq.tarball.repush}

\subsection{Combining entire patches}

\label{sec:mq:combine}

MQ provides a command, \hgxcmd{mq}{qfold} that lets you combine entire

patches.  This ``folds'' the patches you name, in the order you name

them, into the topmost applied patch, and concatenates their

descriptions onto the end of its description.  The patches that you

fold must be unapplied before you fold them.

The order in which you fold patches matters.  If your topmost applied

patch is \texttt{foo}, and you \hgxcmd{mq}{qfold} \texttt{bar} and

\texttt{quux} into it, you will end up with a patch that has the same

effect as if you applied first \texttt{foo}, then \texttt{bar},

followed by \texttt{quux}.

\subsection{Merging part of one patch into another}

Merging \emph{part} of one patch into another is more difficult than

combining entire patches.

If you want to move changes to entire files, you can use

\command{filterdiff}'s \cmdopt{filterdiff}{-i} and