Is there any way to use hg copy from a specific revision?

Question

Is there any way to use hg copy from a specific revision?

I want to copy an earlier version of a file and work on it while preserving the history of the copied file at the point it was copied.

In other words if I have some program foo.py at rev 23 but I want to fork it into bar.py from rev 6 of foo.py, is there a way to specify:

hg copy foo.py@6 bar.py

Answer 1

I can't think of a single command that would do this because you have to add a commit as a child of the older revision. But it can certainly be done in a series of commands.

Example:

hg up 6
hg copy foo.py bar.py
hg commit -m "Copied foo to bar because reasons"
hg up tip
hg merge

This will create file bar.py which would be the contents of foo.py at revision 6.

(For private/unpublished changes you could also use rebase and/or histedit but that seems out of scope for this question).

bar.py may receive merges intended for foo.py which can be undesirable, especially if a lot of changes have occurred since rev 6. You may just have to discard conflicts.

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If it is acceptable to have the copied file's lifetime start from the current revision its simpler:

Example:

hg cat --rev=6 --output bar.py foo.py
hg copy -A foo.py bar.py
hg commit -m "Create bar.py from earlier revision of foo.py"

In many cases this would be OK - for instance if there were no content changes to foo.py in all that time, but maybe only its location or filename changed.

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FYI the reason hg copy by itself can't be used for this is its basic definition:

mark files as copied for the next commit

(according to hg cp --help) so that is not what you should use; its just not designed for what you had in mind.