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martinvonz / jj

A Git-compatible VCS that is both simple and powerful
https://martinvonz.github.io/jj/
Apache License 2.0
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Conflicts at the end of files that don't end in a newline are not materialized correctly #3968

Open ilyagr opened 6 days ago

ilyagr commented 6 days ago 
jj new
echo -n "aa" > qq
jj new
echo -n "bb" > qq
jj new @-
echo -n "cc" > qq
jj new all:@-+
cat qq

Actual Result

<<<<<<< Conflict 1 of 1
%%%%%%% Changes from base to side #1
-aa+cc+++++++ Contents of side #2
bb>>>>>>> Conflict 1 of 1 ends

If the file is subsequently edited, jj cannot parse the conflict correctly.

Expected Behavior

I'd materialize the conflict as though the files ended in newlines, if we decide this is acceptable. We could perhaps notify the user about this in the labels:

<<<<<<< Conflict 1 of 1 (extra newlines inserted)
%%%%%%% Changes from base to side #1
-aa
+cc
+++++++ Contents of side #2
bb
>>>>>>> Conflict 1 of 1 ends

If this is not acceptable, I'm not sure what is best to do.

ilyagr commented 6 days ago

I think pretending that the file had newlines is my preference. This will only matter if the user edits the conflicts manually, in which case the file was probably meant to end in a newline anyway.

There is one alternative on my mind, but I don't like it:

The fish shell prints ⏎\n when a command's output does not end in a newline:

$ echo -n aa
aa⏎
$ # Next command here

We could in principle use a trick like that (either with ⏎\n or with \nJJ: NO NEWLINE\n), but I'm not sure it's worth it. Also, we then lose the ability to represent a file that ends with ⏎\n. It's also a curious question what to do if a conflict not at the end of file includes ⏎\n.

martinvonz commented 6 days ago

Mercurial adds "\ No newline at end of file" in diffs. We could copy that. I'm fine with either that or adding just a newline as you suggested.

emilazy commented 6 days ago

I like the idea of using the existing label syntax; that avoids introducing any more ambiguity with change contents that isn’t already present. The two sides could disagree on the presence of the newline, right? If putting it at the end of the label line attached to the relevant text is too subtle, maybe another label line could be added?

arxanas commented 6 days ago

IIUC the main idea in the thread so far is to only render the lack of a newline, but not necessarily be able to operate on it in a principled fashion?

Shouldn't the lack of a newline be encoded in the conflict marker syntax itself? It seems syntactically important. Wouldn't you want to preserve the invariant that you can reconstruct any of the file sides directly from the conflicted contents, rather than having to consult an out-of-band store?

ilyagr commented 6 days ago

UPDATE: I decided to promote this to a separate issue, https://github.com/martinvonz/jj/issues/3975.

Wouldn't you want to preserve the invariant that you can reconstruct any of the file sides directly from the conflicted contents, rather than having to consult an out-of-band store?

This issue has been nerd-sniping me for a while; in addition to trailing newlines, it's also a matter of encoding conflict markers inside files

TODO: find link. I remember discussing it with Martin recently in some PR, haven't found it yet. TLDR: jj currently can't handle conflicts in out own tutorial.md because of

https://github.com/martinvonz/jj/blob/638649b435795bbb991d840e9ad1750648d9b74a/docs/tutorial.md?plain=1#L284-L291

In the shortest term, I'll probably ignore both issues and just add newlines to the end of files.

However, I think I finally figured out a syntax that will work, will not require a complicated parser, and even be somewhat readable. Before computing conflicts (and only if a file is conflicted), we'd encode file contents as follows:

  1. A mildly pathological file
JJ Verbatim Line:<<<<<<<<<<<
blahblah
JJ EOF: No newline at the end of file
  1. A moderately pathological file
JJ Verbatim Line:<<<<<<<<<<<
JJ Verbatim Line:JJ Verbatim Line:<<<<<<<<<<<
blahblah
JJ Verbatim Line:JJ EOF: No newline at the end of file
  1. Another fun file
blah
JJ Verbatim Line:JJ EOF: No newline at the end of file
JJ EOF: No newline at the end of file

Fun, huh?

This relies on conflict markers having to start at the beginning of the line. If we ever have a notion of conflicts that happen in the middle of lines, we'd need a different syntax, of course. I think that would be nice, but also would probably need a whole different UI to be usable; I'm not sure it can be very usable in the terminal or as a text file.