What does a mercurial revision with no parent mean?

I have a Mercurial repository that is in a strange state now. This is what it looks like in TortoiseHG:

I didn't think this would be possible. Revision 54 has a parent of "-1 (000000000000)" (i.e. nothing). There's clearly something I don't understand yet about Mercurial, can anyo开发者_运维问答ne let me know what this means - and what must have happened for it to get into this state. As far as I know, it's only had stuff pushed and pulled from it - and nobody has been using any wacky extensions.

Revisions 54 and 55 were just adding tags, but if I 'update -C' to revision 54 I end up with ONLY the .hgtags file.

I've made a clone from revision 53 to fix this. But I'd rather understand what happened here, so I can avoid it happening again.

When you look at the definition of a changeset, you see:

Each changeset has zero, one or two parent changesets:

• It has two parent changesets, if the commit was a merge.
• It has no parent, if the changeset is a root in the repository.
  There may be multiple roots in a repository (normally, there is only one), each representing the start of a branch.

"Updating" back to a changeset which already has a child, changing files and then committing creates a new child changeset, thus starting a new branch. Branches can be named.

So maybe this is what you did:

• updating back to 53 (which had already a child '54' of its own back then)
• changing files
• committing, thus starting a new branch from 54, with no parent
  (that would make a second commit with the same parent)

or:

• comitting 53 with a --close-branch option,
• potentially a new commit (without switching back to another branch) might begin a new one

Ry4an (an actual Mercurial specialist ;) ) chimes in and comments:

--close-branch doesn't do anything except hide a branch from a list, and it's undone next time you commit on that branch. It won't create multiple roots.

VonC is right in his diagnosis, multiple heads.
But no combination of 'update' and 'commit' will get you into that state.
To end up with multiple roots one usually has done a 'hg pull' from repo and used --force to override an "unrelated repositories" warning.

("no parent", meaning the parent ids are set to 00000, see "behind the scene":

Another way to see this is if you did hg update null after committing rev. 53. For example, consider this sequence:

hg init foo
# create some files
hg addremove
hg commit -m "Revision 0"
# edit, edit, edit
hg commit -m "Revision 1"
hg update null
hg tag -m "Create tag v1.0.0.0" "v1.0.0.0"

At this point, hg log will show revision 2's parent as -1:0000000000. Since hg update null clears out the working directory, the only file in it would be .hgtags (just like you were seeing).

Did you have other tags prior to rev. 53? If my suspicion is correct, they would not be present in your rev. 54 .hgtags.