Adding functionality with extensions

While the core of Mercurial is quite complete from a functionality standpoint, it's deliberately shorn of fancy features. This approach of preserving simplicity keeps the software easy to deal with for both maintainers and users.

However, Mercurial doesn't box you in with an inflexible command set: you can add features to it as extensions (sometimes known as plugins). We've already discussed a few of these extensions in earlier chapters.

• sec:tour-merge:fetch covers the fetch extension; this combines pulling new changes and merging them with local changes into a single command, fetch.
• In chap:hook, we covered several extensions that are useful for hook-related functionality: acl adds access control lists; bugzilla adds integration with the Bugzilla bug tracking system; and notify sends notification emails on new changes.

In this chapter, we'll cover some of the other extensions that are available for Mercurial, and briefly touch on some of the machinery you'll need to know about if you want to write an extension of your own.

• In sec:hgext:inotify, we'll discuss the possibility of huge performance improvements using the inotify extension.

Improve performance with the inotify extension

Are you interested in having some of the most common Mercurial operations run as much as a hundred times faster? Read on!

Mercurial has great performance under normal circumstances. For example, when you run the hg status command, Mercurial has to scan almost every directory and file in your repository so that it can display file status. Many other Mercurial commands need to do the same work behind the scenes; for example, the hg diff command uses the status machinery to avoid doing an expensive comparison operation on files that obviously haven't changed.

Because obtaining file status is crucial to good performance, the authors of Mercurial have optimised this code to within an inch of its life. However, there's no avoiding the fact that when you run hg status, Mercurial is going to have to perform at least one expensive system call for each managed file to determine whether it's changed since the last time Mercurial checked. For a sufficiently large repository, this can take a long time.

To put a number on the magnitude of this effect, I created a repository containing 150,000 managed files. I timed hg status as taking ten seconds to run, even when none of those files had been modified.

Many modern operating systems contain a file notification facility. If a program signs up to an appropriate service, the operating system will notify it every time a file of interest is created, modified, or deleted. On Linux systems, the kernel component that does this is called inotify.

Mercurial's inotify extension talks to the kernel's inotify component to optimise hg status commands. The extension has two components. A daemon sits in the background and receives notifications from the inotify subsystem. It also listens for connections from a regular Mercurial command. The extension modifies Mercurial's behavior so that instead of scanning the filesystem, it queries the daemon. Since the daemon has perfect information about the state of the repository, it can respond with a result instantaneously, avoiding the need to scan every directory and file in the repository.

Recall the ten seconds that I measured plain Mercurial as taking to run hg status on a 150,000 file repository. With the inotify extension enabled, the time dropped to 0.1 seconds, a factor of one hundred faster.

Before we continue, please pay attention to some caveats.

• The inotify extension is Linux-specific. Because it interfaces directly to the Linux kernel's inotify subsystem, it does not work on other operating systems.
• It should work on any Linux distribution that was released after early 2005. Older distributions are likely to have a kernel that lacks inotify, or a version of glibc that does not have the necessary interfacing support.
• Not all filesystems are suitable for use with the inotify extension. Network filesystems such as NFS are a non-starter, for example, particularly if you're running Mercurial on several systems, all mounting the same network filesystem. The kernel's inotify system has no way of knowing about changes made on another system. Most local filesystems (e.g. ext3, XFS, ReiserFS) should work fine.

The inotify extension is shipped with Mercurial since 1.0. All you need to do to enable the inotify extension is add an entry to your ~/.hgrc.

[extensions] inotify =

When the inotify extension is enabled, Mercurial will automatically and transparently start the status daemon the first time you run a command that needs status in a repository. It runs one status daemon per repository.

The status daemon is started silently, and runs in the background. If you look at a list of running processes after you've enabled the inotify extension and run a few commands in different repositories, you'll thus see a few hg processes sitting around, waiting for updates from the kernel and queries from Mercurial.

The first time you run a Mercurial command in a repository when you have the inotify extension enabled, it will run with about the same performance as a normal Mercurial command. This is because the status daemon needs to perform a normal status scan so that it has a baseline against which to apply later updates from the kernel. However, every subsequent command that does any kind of status check should be noticeably faster on repositories of even fairly modest size. Better yet, the bigger your repository is, the greater a performance advantage you'll see. The inotify daemon makes status operations almost instantaneous on repositories of all sizes!

If you like, you can manually start a status daemon using the inserve command. This gives you slightly finer control over how the daemon ought to run. This command will of course only be available when the inotify extension is enabled.

When you're using the inotify extension, you should notice no difference at all in Mercurial's behavior, with the sole exception of status-related commands running a whole lot faster than they used to. You should specifically expect that commands will not print different output; neither should they give different results. If either of these situations occurs, please report a bug.

Flexible diff support with the extdiff extension

Mercurial's built-in hg diff command outputs plaintext unified diffs.

$ hg diff
diff -r c8386671e2ae myfile
--- a/myfile  Thu Jan 01 00:00:00 1970 +0000
+++ b/myfile  Wed May 17 04:01:43 2017 +0000
@@ -1,1 +1,2 @@
 The first line.
+The second line.

If you would like to use an external tool to display modifications, you'll want to use the extdiff extension. This will let you use, for example, a graphical diff tool.

The extdiff extension is bundled with Mercurial, so it's easy to set up. In the extensions section of your ~/.hgrc, simply add a one-line entry to enable the extension.

[extensions]
extdiff =

This introduces a command named extdiff, which by default uses your system's diff command to generate a unified diff in the same form as the built-in hg diff command.

$ hg extdiff
--- /tmp/extdiff.Dyl5td/a.c8386671e2ae/myfile 2017-05-17 04:01:43.844015497 +0000
+++ $TESTTMP/a/myfile 2017-05-17 04:01:43.680681761 +0000
@@ -1 +1,2 @@
 The first line.
+The second line.

The result won't be exactly the same as with the built-in hg diff variations, because the output of diff varies from one system to another, even when passed the same options.

As the “making snapshot” lines of output above imply, the extdiff command works by creating two snapshots of your source tree. The first snapshot is of the source revision; the second, of the target revision or working directory. The extdiff command generates these snapshots in a temporary directory, passes the name of each directory to an external diff viewer, then deletes the temporary directory. For efficiency, it only snapshots the directories and files that have changed between the two revisions.

Snapshot directory names have the same base name as your repository. If your repository path is /quux/bar/foo, then foo will be the name of each snapshot directory. Each snapshot directory name has its changeset ID appended, if appropriate. If a snapshot is of revision a631aca1083f, the directory will be named foo.a631aca1083f. A snapshot of the working directory won't have a changeset ID appended, so it would just be foo in this example. To see what this looks like in practice, look again at the extdiff example above. Notice that the diff has the snapshot directory names embedded in its header.

The extdiff command accepts two important options. The hg -p option lets you choose a program to view differences with, instead of diff. With the hg -o option, you can change the options that extdiff passes to the program (by default, these options are “-Npru”, which only make sense if you're running diff). In other respects, the extdiff command acts similarly to the built-in hg diff command: you use the same option names, syntax, and arguments to specify the revisions you want, the files you want, and so on.

As an example, here's how to run the normal system diff command, getting it to generate context diffs (using the -c option) instead of unified diffs, and five lines of context instead of the default three (passing 5 as the argument to the -C option).

$ hg extdiff -o -NprcC5
*** /tmp/extdiff.lyhyYC/a.c8386671e2ae/myfile Wed May 17 04:01:44 2017
--- $TESTTMP/a/myfile Wed May 17 04:01:43 2017
***************
*** 1 ****
--- 1,2 ----
  The first line.
+ The second line.

Launching a visual diff tool is just as easy. Here's how to launch the kdiff3 viewer.

hg extdiff -p kdiff3 -o

Defining command aliases

It can be cumbersome to remember the options to both the extdiff command and the diff viewer you want to use, so the extdiff extension lets you define new commands that will invoke your diff viewer with exactly the right options.

All you need to do is edit your ~/.hgrc, and add a section named extdiff. Inside this section, you can define multiple commands. Here's how to add a kdiff3 command. Once you've defined this, you can type “hg kdiff3” and the extdiff extension will run kdiff3 for you.

[extdiff]
cmd.kdiff3 =

If you leave the right hand side of the definition empty, as above, the extdiff extension uses the name of the command you defined as the name of the external program to run. But these names don't have to be the same. Here, we define a command named “hg wibble”, which runs kdiff3.

[extdiff]
 cmd.wibble = kdiff3

You can also specify the default options that you want to invoke your diff viewing program with. The prefix to use is “opts.”, followed by the name of the command to which the options apply. This example defines a “hg vimdiff” command that runs the vim editor's DirDiff extension.

[extdiff]
 cmd.vimdiff = vim
opts.vimdiff = -f '+next' '+execute "DirDiff" argv(0) argv(1)'

Cherrypicking changes with the transplant extension

Need to have a long chat with Brendan about this.

Send changes via email with the patchbomb extension

Many projects have a culture of “change review”, in which people send their modifications to a mailing list for others to read and comment on before they commit the final version to a shared repository. Some projects have people who act as gatekeepers; they apply changes from other people to a repository to which those others don't have access.

Mercurial makes it easy to send changes over email for review or application, via its patchbomb extension. The extension is so named because changes are formatted as patches, and it's usual to send one changeset per email message. Sending a long series of changes by email is thus much like “bombing” the recipient's inbox, hence “patchbomb”.

As usual, the basic configuration of the patchbomb extension takes just one or two lines in your /.hgrc.

[extensions]
patchbomb =

Once you've enabled the extension, you will have a new command available, named email.

The safest and best way to invoke the email command is to always run it first with the hg -n option. This will show you what the command would send, without actually sending anything. Once you've had a quick glance over the changes and verified that you are sending the right ones, you can rerun the same command, with the hg -n option removed.

The email command accepts the same kind of revision syntax as every other Mercurial command. For example, this command will send every revision between 7 and tip, inclusive.

hg email -n 7:tip

You can also specify a repository to compare with. If you provide a repository but no revisions, the email command will send all revisions in the local repository that are not present in the remote repository. If you additionally specify revisions or a branch name (the latter using the hg -b option), this will constrain the revisions sent.

It's perfectly safe to run the email command without the names of the people you want to send to: if you do this, it will just prompt you for those values interactively. (If you're using a Linux or Unix-like system, you should have enhanced readline-style editing capabilities when entering those headers, too, which is useful.)

When you are sending just one revision, the email command will by default use the first line of the changeset description as the subject of the single email message it sends.

If you send multiple revisions, the email command will usually send one message per changeset. It will preface the series with an introductory message, in which you should describe the purpose of the series of changes you're sending.

Changing the behavior of patchbombs

Not every project has exactly the same conventions for sending changes in email; the patchbomb extension tries to accommodate a number of variations through command line options.

• You can write a subject for the introductory message on the command line using the hg -s option. This takes one argument, the text of the subject to use.
• To change the email address from which the messages originate, use the hg -f option. This takes one argument, the email address to use.
• The default behavior is to send unified diffs, one per message. You can send a binary bundle instead with the hg -b option.
• Unified diffs are normally prefaced with a metadata header. You can omit this, and send unadorned diffs, with the hg --plain option.
• Diffs are normally sent “inline”, in the same body part as the description of a patch. This makes it easiest for the largest number of readers to quote and respond to parts of a diff, as some mail clients will only quote the first MIME body part in a message. If you'd prefer to send the description and the diff in separate body parts, use the hg -a option.
• Instead of sending mail messages, you can write them to an mbox-format mail folder using the hg -m option. That option takes one argument, the name of the file to write to.
• If you would like to add a diffstat-format summary to each patch, and one to the introductory message, use the hg -d option. The diffstat command displays a table containing the name of each file patched, the number of lines affected, and a histogram showing how much each file is modified. This gives readers a qualitative glance at how complex a patch is.