Evolve: User Guide

Life without evolve

Before we dive into learning about evolve, let’s look into some features of core Mercurial that interact with evolve. commit affects evolve, and evolve modifies how commit --amend works.

Example 1: Commit a new changeset

To create a new changeset, simply run hg commit as usual. evolve does not change the behaviour of commit at all.

However, it’s important to understand that new changesets are in the draft phase by default: they are mutable. This means that they can be modified by Mercurial’s existing history-editing commands (rebase, histedit, etc.), and also by the evolve extension. Specifically, evolve adds a number of commands that can be used to modify history: amend, uncommit, prune, fold, and evolve. Generally speaking, changesets remain in draft phase until they are pushed to another repository, at which point they enter public phase.

$ hg commit -m 'implement feature X'
$ hg phase -r .
1: draft

(Strictly speaking, changesets only become public when they are pushed to a publishing repository. But all repositories are publishing by default; you have to explicitly configure repositories to be non-publishing. Non-publishing repositories are an advanced topic which we’ll see when we get to sharing mutable history.)

Example 2: Amend a changeset (traditional)

Imagine you’ve just committed a new changeset, and then you discover a mistake. Maybe you forgot to run the tests and a failure slipped in. You want to modify history so that you push one perfect changeset, rather than one flawed changeset followed by an “oops” commit. (Or perhaps you made a typo in the commit message—this is really feature Y, not feature X. You can’t fix that with a followup commit.)

This is actually trivial with plain vanilla Mercurial since 2.2: fix your mistake and run

$ hg commit --amend -m 'implement feature Y'

to create a new, amended changeset. The drawback of doing this with vanilla Mercurial is that your original, flawed, changeset is removed from the repository. This is unsafe history editing. It’s probably not too serious if all you did was fix a syntax error, but still.

Figure 1: unsafe history modification with core Mercurial (not using evolve): the original revision 1 is destroyed.

(Incidentally, Mercurial’s traditional history modification mechanism isn’t really unsafe: any changeset(s) removed from the repository are kept in a backup directory, so you can manually restore them later if you change your mind. But it’s awkward and inconvenient compared to the features provided by evolve and changeset obsolescence.)

Life with evolve (basic usage)

Once you enable the evolve extension, a number of features are available to you. First, we’re going to explore several examples of painless, trouble-free history modification.

Example 3: Amend a changeset (with evolve)

Outwardly, amending a changeset with evolve can look exactly the same as it does with core Mercurial (example 2):

$ hg commit --amend -m 'implement feature Y'

Alternately, you can use the new amend command added by evolve:

$ hg amend -m 'implement feature Y'

(hg amend is nearly synonymous with hg commit --amend. The difference is that hg amend reuses the existing commit message by default, whereas hg commit --amend runs your editor if you don’t pass -m or -l.)

Under the hood, though, things are quite different. Mercurial has simply marked the old changeset obsolete, replacing it with a new one. We’ll explore what this means in detail later, after working through a few more examples.

Example 4: Prune an unwanted changeset

Sometimes you make a change, and then decide it was such a bad idea that you don’t want anyone to know about it. Or maybe it was a debugging hack that you needed to keep around for a while, but do not intend to ever push publicly.

$ echo 'debug hack' >> file1.c
$ hg commit -m 'debug hack'

In either case, hg prune is the answer. prune simply marks changesets obsolete without creating any new changesets to replace them:

$ hg prune .
1 changesets pruned
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
working directory now at 934359450037

Outwardly, it appears that your “debug hack” commit never happened; we’re right back where we started:

$ hg parents --template '{rev}:{node|short}  {desc|firstline}\n'
3:934359450037  implement feature Y

In reality, though, the “debug hack” is still there, obsolete and hidden.

Example 5: Uncommit changes to certain files

Occasionally you commit more than you intended: perhaps you made unrelated changes to different files, and thus intend to commit different files separately.

$ echo 'relevant' >> file1.c
$ echo 'irrelevant' >> file2.c

If you forget to specify filenames on the commit command line, Mercurial commits all those changes together:

$ hg commit -m 'fix bug 234'          # oops: too many files

Luckily, this mistake is easy to fix with uncommit:

$ hg uncommit file2.c
$ hg status
M file2.c

Let’s verify that the replacement changeset looks right (i.e., modifies only file1.c):

$ hg parents --template '{rev}:{node|short}  {desc|firstline}\n{files}\n'
6:c8defeecf7a4  fix bug 234

As before, the original flawed changeset is still there, but obsolete and hidden. It won’t be exchanged with other repositories by push, pull, or clone.

Example 6: Fold multiple changesets together into one

If you’re making extensive changes to fragile source code, you might commit more frequently than normal so that you can fallback on a known good state if one step goes badly.

$ echo step1 >> file1.c
$ hg commit -m 'step 1'               # revision 7
$ echo step2 >> file1.c
$ hg commit -m 'step 2'               # revision 8
$ echo step3 >> file2.c
$ hg commit -m 'step 3'               # revision 9

At the end of such a sequence, you often end up with a series of small changesets that are tedious to review individually. It might make more sense to combine them into a single changeset using the fold command.

To make sure we pass the right revisions to fold, let’s review the changesets we just created, from revision 7:

$ hg log --template '{rev}:{node|short}  {desc|firstline}\n' -r 7::
7:05e61aab8294  step 1
8:be6d5bc8e4cc  step 2
9:35f432d9f7c1  step 3

and fold them:

$ hg fold -m 'fix bug 64' -r 7::
3 changesets folded
1 files updated, 0 files merged, 0 files removed, 0 files unresolved

This time, Mercurial marks three changesets obsolete, replacing them all with a single successor.

(You might be familiar with this operation under other names, like squash or collapse.)

Changeset obsolescence under the hood

So far, everything has gone just fine: we haven’t run into merge conflicts or other trouble. Before we start exploring advanced usage that can run into trouble, let’s step back and see what happens when Mercurial marks changesets obsolete. That will make it much easier to understand the more advanced use cases we’ll see later.

When you have the evolve extension enabled, all history modification uses the same underlying mechanism: the original changesets are marked obsolete and replaced by zero or more successors. The obsolete changesets are the precursors of their successors. This applies equally to built-in commands (commit --amend), commands added by evolve (amend, prune, uncommit, fold), and commands provided by other extensions (rebase, histedit).

Another way of looking at it is that obsolescence is second-order version control, i.e. the history of your history. We’ll cover this in more detail (and mathematical precision) in the concepts guide.

Under the hood: Amend a changeset

Consider Example 2, amending a changeset with evolve. We saw above that you can do this using the exact same command-line syntax as core Mercurial, namely hg commit --amend. But the implementation is quite different, and Figure 2 shows how.

Figure 2: safe history modification using evolve: the original revision 1 is preserved as an obsolete changeset. (The “temporary amend commit”, marked with T, is an implementation detail stemming from limitations in Mercurial’s current merge machinery. Future versions of Mercurial will not create them.)

In this case, the obsolete changesets are also hidden. That is the usual end state for obsolete changesets. But many scenarios result in obsolete changesets that are still visible, which indicates your history modification work is not yet done. We’ll see examples of that later, when we cover advanced usage.

Understanding revision numbers and hidden changesets

As the name implies, hidden changesets are normally not visible. If you run hg log on the repository from Figure 2, Mercurial will show revisions 0 and 3, but not 1 and 2. That’s something you don’t see with plain vanilla Mercurial—normally, revision N is always followed by revision N + 1.

This is just the visible manifestation of hidden changesets. If revision 0 is followed by revision 3, that means there are two hidden changesets, 1 and 2, in between.

To see those hidden changesets, use the --hidden option:

$ hg --hidden log --graph --template '{rev}:{node|short}  {desc|firstline}\n'
@  3:934359450037  implement feature Y
| x  2:6c5f78d5d467  temporary amend commit for fe0ecd3bd2a4
| |
| x  1:fe0ecd3bd2a4  implement feature Y
o  0:08c4b6f4efc8  init

Note that changeset IDs are still the permanent, immutable identifier for changesets. Revision numbers are, as ever, a handy shorthand that work in your local repository, but cannot be used across repositories. They also have the useful property of showing when there are hidden changesets lurking under the covers, which is why this document uses revision numbers.

Under the hood: Prune an unwanted changeset

prune (example 4 above) is the simplest history modification command provided by evolve. All it does is mark the specified changeset(s) obsolete, with no successor/precursor relationships involved. (If the working directory parent was one of the obsolete changesets, prune updates back to a suitable ancestor.)

Figure 3: pruning a changeset marks it obsolete with no successors.

Under the hood: Uncommit changes to certain files

In one sense, uncommit is a simplified version of amend. Like amend, it obsoletes one changeset and leaves it with a single successor. Unlike amend, there is no ugly “temporary amend commit” cluttering up the repository.

In another sense, uncommit is the inverse of amend: amend takes any uncommitted changes in the working dir and “adds” them to the working directory’s parent changeset. (In reality, of course, it creates a successor changeset, marking the original obsolete.) In contrast, uncommit takes some changes in the working directory’s parent and moves them to the working dir, creating a new successor changeset in the process. Figure 4 illustrates.

Figure 4: uncommit moves some of the changes from the working directory parent into the working dir, preserving the remaining changes as a new successor changeset. (N.B. revision 4 is not shown here because it was marked obsolete in the previous example.)

Under the hood: Fold multiple changesets together into one

The last basic example is folding multiple changesets into one, which marks multiple changesets obsolete, replacing them all with a single successor.

Figure 5: fold combines multiple changesets into a single successor, marking the original (folded) changesets obsolete.

Obsolete is not hidden

So far, every obsolete changeset we have seen is also hidden. However, these are not the same thing—that’s why they have different names. It’s entirely possible to have obsolete changesets that are not hidden. We’ll see examples of that soon, when we create unstable changesets.

Note that all hidden changesets are obsolete: hidden is a subset of obsolete.

Life with evolve (advanced usage)

Now that you’ve got a solid understanding of how evolve works in concert with changeset obsolescence, let’s explore some more advanced scenarios. All of these scenarios will involve unstable changesets, which is an unavoidable consequence of obsolescence. What really sets evolve apart from other history modification mechanisms is the fact that it recognizes troubles like unstable changesets and provides a consistent way for you to get out of trouble.

(Incidentally, there are two other types of trouble that changesets can get into with evolve: they may be divergent or bumped. Both of those states are more likely to occur when sharing mutable history, so we won’t see them in this user guide.)

Example 7: Amend an older changeset

Sometimes you don’t notice your mistakes until after you have committed new changesets on top of them.

$ hg commit -m 'fix bug 17'         # rev 11 (mistake here)
$ hg commit -m 'cleanup'            # rev 12
$ hg commit -m 'feature 23'         # rev 13

Traditionally, your only option is to commit an “oops” changeset that fixes your mistake. That works, of course, but it makes you look bad: you made a mistake, and the record of that mistake is recorded in history for all eternity. (If the mistake was in the commit message, too bad: you cannot fix it.)

More subtly, there now exist changesets that are worse than what came before—the code no longer builds, the tests don’t pass, or similar. Anyone reviewing these patches will waste time on the error in the earlier patch, and then the correction later on.

You can avoid all this by amending the bad changeset and evolving subsequent history. Here’s how it works, assuming you have just committed revision 13 and noticed the mistake in revision 11:

$ hg update 11
[...fix mistake...]
$ hg amend

At this point, revision 11 is obsolete and revisions 12 and 13—the descendants of 11—are in a funny state: they are unstable.

Figure 6: amending a changeset with descendants means the amended changeset is obsolete but remains visible; its non-obsolete descendants are unstable. The temporary amend commit, revision 14, is hidden because it has no non-obsolete descendants.

All non-obsolete descendants of an obsolete changeset are unstable. An interesting consequence of this is that revision 11 is still visible, even though it is obsolete. Obsolete changesets with non-obsolete descendants are not hidden.

The fix is to evolve history:

$ hg evolve --all

This is a separate step, not automatically part of hg amend, because there might be conflicts. If your amended changeset modifies a file that one of its descendants also modified, Mercurial has to fire up your merge tool to resolve the conflict. More importantly, you have to switch contexts from “writing code” to “resolving conflicts”. That can be an expensive context switch, so Mercurial lets you decide when to do it.

The end state, after evolve finishes, is that the original revisions (11-13) are obsolete and hidden. Their successor revisions (15-17) replace them.

Figure 7: evolve your repository (hg evolve --all) to take care of instability. Unstable changesets become obsolete, and are replaced by successors just like the amended changeset was.

Example 8: Prune an older changeset

Let’s say you’ve just committed the following changesets:

$ hg commit -m 'useful work'       # rev 18
$ hg commit -m 'debug hack'        # rev 19
$ hg commit -m 'more work'         # rev 20

You want to drop revision 19, but keep 18 and 20. No problem:

$ hg prune 19
1 changesets pruned
1 new unstable changesets

As above, this leaves your repository in a funny intermediate state: revision 20 is the non-obsolete descendant of obsolete revision 19. That is, revision 20 is unstable.

Figure 8: hg prune marks a changeset obsolete without creating a successor. Just like with hg amend, non-obsolete descendants of the pruned changeset are now unstable.

As before, the solution to unstable changesets is to evolve your repository:

$ hg evolve --all

This rebases revision 20 on top of 18 as the new revision 21, leaving 19 and 20 obsolete and hidden:

Figure 9: once again, hg evolve --all takes care of instability.

Example 9: Uncommit files from an older changeset (discard changes)

As in example 5, let’s say you accidentally commit some unrelated changes together. Unlike example 5, you don’t notice your mistake immediately, but commit a new changeset on top of the bad one.

$ echo 'this fixes bug 53' >> file1.c
$ echo 'debug hack' >> file2.c
$ hg commit -m 'fix bug 53'                     # rev 22 (oops)
$ echo 'and this handles bug 67' >> file1.c
$ hg commit -m 'fix bug 67'                     # rev 23 (fine)

As with amend, you need to travel back in time and repair revision 22, leaving your changes to file2.c back in the working directory:

$ hg update 22
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg uncommit file2.c
1 new unstable changesets
$ hg status
M file2.c

Now your repository has unstable changesets, so you need to evolve it. But hg evolve requires a clean working directory to resolve merge conflicts, so you need to decide what to do with file2.c.

In this case, the change to file2.c was a temporary debugging hack, so we can discard it and immediately evolve the instability away:

$ hg revert file2.c
$ hg evolve --all
move:[23] fix bug 67
atop:[24] fix bug 53

Figure 10 illustrates the whole process.

Figure 10: hg uncommit of a changeset with descendants results in instability and a dirty working directory, both of which must be dealt with.

Example 10: Uncommit files to an older changeset (keep changes)

This is very similar to example 9. The difference that this time, our change to file2.c is valuable enough to commit, making things a bit more complicated. The setup is nearly identical:

$ echo 'fix a bug' >> file1.c
$ echo 'useful but unrelated' >> file2.c
$ hg commit -u dan -d '11 0' -m 'fix a bug'     # rev 26 (oops)
$ echo 'new feature' >> file1.c
$ hg commit -u dan -d '12 0' -m 'new feature'   # rev 27 (fine)

As before, we update back to the flawed changeset (this time, revision 26) and uncommit, leaving uncommitted changes to file2.c in the working dir:

$ hg update -q 26
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg uncommit -q file2.c                        # obsoletes rev 26, creates rev 28
1 new unstable changesets
$ hg status
M file2.c

This time, let’s save that useful change before evolving:

$ hg commit -m 'useful tweak'                   # rev 29

Figure 11 shows the story so far: uncommit obsoleted revision 26 and created revision 28, the successor of 26. Then we committed revision 29, a child of 28. We still have to deal with the unstable revision 27.

Figure 11: Uncommitting a file and then committing that change separately will soon result in a two-headed repository.

This is where things get tricky. As usual when a repository has unstable changesets, we want to evolve it:

$ hg evolve --all

The problem is that hg evolve rebases revision 27 onto revision 28, creating 30 (the successor of 27). This is entirely logical: 27 was the child of 26, and 26’s successor is 28. So of course 27’s successor (30) should be the child of 26’s successor (28). Unfortunately, that leaves us with a two-headed repository:

Figure 12: evolve takes care of unstable changesets; it does not solve all the world’s problems.

As usual when faced with a two-headed repository, you can either merge or rebase. It’s up to you.

Example 11: Recover an obsolete changeset

Sometimes you might obsolete a changeset, and then change your mind. You’ll probably start looking for an “unobsolete” command to restore a changeset to normal state. For complicated implementation reasons, that command doesn’t exist. (If you have already pushed an obsolescence marker to another repo, then Mercurial would need a way to revoke that remote obsolesence marker. That’s a hard problem.)

Instead, evolve provides a touch command to resurrect an obsolete changeset. An unexpected quirk: you almost certainly need to use --hidden, since obsolete changesets tend to be hidden, and you can’t reference a hidden changeset otherwise. Typical usage thus looks like

$ hg --hidden touch REV

This creates a new, normal changeset which is the same as REV—except with a different changeset ID. The new changeset will have the same parent as REV, and will be a successor of REV.

The current implementation of hg touch is not ideal, and is likely to change in the future. Consider the history in Figure 12, where revision 27 is obsolete and the child of 26, also obsolete. If we hg touch 27, that creates a new revision which is a non-obsolete child of 26—i.e., it is unstable. It’s also divergent, another type of trouble that we’ll learn about in the next section.