Why is the meaning of “ours” and “theirs” reversed with git-svn

I use git-svn and I noticed that when I have to fix a merge conflict after performing a git svn rebase, the meaning of the --ours and --theirs options to e.g. git checkout is reversed. That is, if there's a conflict and I want to keep the version that came from the SVN server and throw away the changes I made locally, I have to use ours, when I would expect it to be theirs.

Why is that?

Example:

mkdir test
cd test
svnadmin create svnrepo
svn co file://$PWD/svnrepo svnwc
cd svnwc
echo foo > test.txt
svn add test.txt
svn ci -m 'svn commit 1'
cd ..
git svn clone file://$PWD/svnrepo gitwc
cd svnwc
echo bar > test.txt 
svn ci -m 'svn commit 2'
cd ..
cd gitwc
echo baz > test.txt 
git commit -a -m 'git commit 1'
git svn rebase

git checkout --ours test.txt
cat test.txt 
# shows "bar" but I expect "baz"

git checkout --theirs test.txt
cat test.txt 
# shows "baz" but I expect "bar"

Solution

That seems consistent with what a rebase does.

git svn rebase will fetch revisions from the SVN parent of the current HEAD and rebases the current (uncommitted to SVN) work against it.
git rebase does mention:
Note that a rebase merge works by replaying each commit from the working branch on top of the <upstream> branch.
Because of this, when a merge conflict happens:
- the side reported as ours is the so-far rebased series, starting with <upstream>,
- and theirs is the working branch.
  In other words, the sides are swapped.

git rebase replays each commit from the working branch on top of the <upstream> branch.

If you reconcile both definitions:

the commits coming from SVN are the ones on top of which local Git commits are replayed. They are part of the "so-far rebased series", and are referenced as "our" (in your case, the test.txt file with bar content)
the working branch (containing Git commits unknown to SVN, in your case, the test.txt file with baz content) is "their", and each of those local Git commits are being replayed.

In other words, SVN or not:

the "<upstream>" branch (on top of which anything is replayed, and which is part of the so far "rebased commits") is "ours".
what is being replayed (the working branch) is "theirs".

Good mnemonic tip by CommaToast:

whatever HEAD's pointing to is "ours"

(and the first thing a git rebase upstream does is to check out the upstream branch on top of which you want to rebase: HEAD refers to upstream -- ours now.)

This does not contradict:

this answer, where a git checkout --ours codefile.js selects a file from master during a git rebase master
this answer, where master, the upstream branch you are rebasing onto, is referenced as "ours"

The confusion is likely coming from the role of the working branch in a classic git merge.
When you are merging:

the "working branch" is the one containing what is "so far merged", and is considered as "our",
while the other commit represent what is being -- not replayed but -- merge on top of the working branch, and considered as "their".

As the git rebase man page mentions, a merge during a rebase means the side are swapped.

Another way to say the same thing is to consider that:

what we have on the checked out branch is 'ours',
what we had (and is being merged or replayed) is 'theirs'.

On a merge:

x--x--x--x--x(*) <- current branch B ('*'=HEAD)
    \
     \
      \--y--y--y <- other branch to merge

, we don't change the current branch 'B', so what we have is still what we were working on (and we merge from another branch)

x--x--x--x--x---------o(*)  MERGE, still on branch B
    \       ^        /
     \     ours     /
      \            /
       --y--y--y--/  
               ^
              their

But on a rebase, we switch side (as in git switch, mentioned in git rebase man page) because the first thing a rebase does is to check out the upstream branch! (to replay the current commits on top of it)

x--x--x--x--x(*) <- current branch B
    \
     \
      \--y--y--y <- upstream branch

A git rebase upstream will first change HEAD of B to the upstream branch HEAD (hence the switch of 'ours' and 'theirs' compared to the previous "current" working branch.)

x--x--x--x--x <- former "current" branch, new "theirs"
    \
     \
      \--y--y--y(*) <- upstream branch with B reset on it,  
                       new "ours", to replay x's on it

, and then the rebase will replay 'their' commits on the new 'our' B branch:

x--x..x..x..x <- old "theirs" commits, now "ghosts", available through reflogs
    \
     \
      \--y--y--y--x'--x'--x'(*) <-  branch B with HEAD updated ("ours")
               ^
               |
        upstream branch

The only extra step with git svn rebase is that a svn "fetch" is performed first on the Git remote branch representing SVN commits.
You have initially:

x--x--x--x--x(*) <- current branch B, "ours" for now.
    \                                   
     \
      \--y--y--y <- SVN tracking branch, "theirs for now"

, you first update the SVN tracking branch with new commits coming from SVN

x--x--x--x--x(*) <- current branch B, still "ours", not for long
    \                                   
     \
      \--y--y--y--y'--y' <- SVN tracking branch updated

, then you switch the current branch to the SVN side (which becomes "ours")

x--x--x--x--x <- for "B", now "their" during the rebase
    \                                   
     \
      \--y--y--y--y'--y'(*) <- SVN tracking branch updated, and branch B: 
                               now "ours" (this is "what we now have")

, before replaying the commits you were working on (but which are now "theirs" during that rebase)

x--x..x..x..x <- old "theirs" commits, now "ghosts", available through reflogs
    \
     \
      \--y--y--y--y'--y'--x'--x'--x'(*) <-  branch B with HEAD updated ("ours")
                      ^
                      |
        upstream SVN tracking branch