Managing Local Changes with Mercurial Queues

Repository Management Problem

gem5 users typically opt to freeze their repository at a particular changeset when starting a new research project. This approach has several downsides:

• It discourages users from contributing back any useful changes they may develop.
• If a useful change is added upstream, it's a long, tedious process to update.

If a user chooses to keep their local repository up-to-date with the source tree they typically use named branches and merge any upstream changes into their branches. This approach also has its downsides:

• If any local change needs to be updated, it requires a separate commit.
• If you have several small, unrelated changes, separate branches must be maintained.
• Upstream changes must be merged into the local branches.

A powerful tool that overcomes these problems is the mercurial queue extension.

Mercurial Queues

The mercurial queue extension is a powerful tool that allows you to:

• Manage small changes easily as a set of well-defined patches.
• Edit previous patches without having a new commit.
• Keep your local changes cleanly separated from upstream changes.
• Prevent changes from being recorded in the project history until they are ready.

Example Mercurial Queue Use

Enable the MQ extension

To enable the mercurial queue extension, simply add the following to your .hgrc file:

[extensions]
hgext.mq =

Basic MQ commands

Help Command

• hg help mq --- Gives a list of mercurial queue commands and a brief description of each.

Creating and handling new patches

• hg qnew change1.patch -m "commit message" --- Create a new patch named "change1.patch" with a commit message.

Checking the status of patches in the queue

• hg qapplied --- List all applied patches in the queue.
• hg qseries --- List all patches in the current series (this includes even patches that aren't applied).
• hg qdiff --- Display the diff for the applied patch at the top of the queue

Adding patches from other queues

• hg qimport -e pre_existing.patch --- Adds a pre-existing patch called "pre_existing.patch" to the local queue.

Simple workflow with MQs

Here is a simple example outlining basic MQ usage:

# clone a clean copy of gem5
hg clone http://repo.gem5.org/gem5

# initialize a new mercurial queue
cd ./gem5
hg init --mq

# make some local changes and turn them into a patch
hg qnew change1.patch -m "cpu: made some changes to the cpu model"

# we have some more changes that we want to turn into a separate patch
hg qnew change2.patch -m "cache: made some changes to the cache"

# now you want to make some more changes and include them in change1
# make sure change1 is at the top of the queue
hg qtop

>>> change2.patch

# it's not, so we have to pop change2 off the queue
hg qpop
hg qtop

>>> change1.patch

# now it's the top patch. make the necessary changes and update
hg qrefresh

# re-apply change2
hg qpush

# let's check that all of our patches are applied
hg qapplied

>>> change1.patch
>>> change2.patch