Scenario: swapped-splices

Two folders trade places in one commit: what was vendor/a/ is now vendor/b/, and the other way around. Each takes its .splice along, so each splice keeps its upstream, and its history, under the other's old path. A push of either path sends only that splice's history: nothing from the folder that had the path before.

Monorepo: vendor/a/, a splice of $UPSTREAM, cloned at seed, with a commit not pushed yet, a's unpushed work. vendor/b/, a folder that grew in the monorepo.
Upstreams: $UPSTREAM has seed. $UPSTREAM-b is empty, created for vendor/b/.

A path's history alone can't tell the two splices apart: git log -- vendor/a lists the commits of the splice of $UPSTREAM before the swap and those of the splice of $UPSTREAM-b after it. Their ids can: each splice got its own when it was cloned or initialized, and it moves with the .splice. So the rebuild follows each splice back to the path where its id was before the swap, and continues there. See the design notes.

Output

setup.bash builds this state:

$ build_scenario scenario_swapped_splices

vendor/b/ becomes a splice of the empty upstream, and isn't pushed yet:

$ git splice init vendor/b "$UPSTREAM-b"
ok   vendor/b: initialized -- 'git splice push vendor/b' publishes it

$ git splice log
===  vendor/a (main)
< 8da07b1 a's unpushed work  (Test <test@example.com>)
===  vendor/b (upstream has no 'main' branch)
< 0b18046 splice: init vendor/b  (Test <test@example.com>)

The swap: three git mv, with a temporary name, in one commit.

$ git mv vendor/a tmp &&
>   git mv vendor/b vendor/a &&
>   git mv tmp vendor/b &&
>   git commit -q -m "swap a and b"

vendor/a/ is now the splice of $UPSTREAM-b, with its file and its upstream:

$ ls vendor/a
b.txt

$ git config --file vendor/a/.splice upstream.origin.url
$UPSTREAM-b

Seen from the path vendor/a/, the swap didn't add a .splice, it changed the one that was there. Only the id shows that it's another splice now, which came from vendor/b/. The path's own history before the swap is that of the splice of $UPSTREAM, and isn't for $UPSTREAM-b to get.

$ git diff --name-status HEAD^ HEAD -- vendor/a | tr '\t' ' '
M vendor/a/.splice
D vendor/a/a.txt
A vendor/a/b.txt
D vendor/a/file.txt

The history of the splice of $UPSTREAM-b continues from vendor/b/, where it was init. The swap changed none of its files, so it adds nothing:

$ git splice log vendor/a
===  vendor/a (upstream has no 'main' branch)
< 0b18046 splice: init vendor/b  (Test <test@example.com>)

$ git splice push vendor/a
??   vendor/a: upstream has no 'main' branch yet -- this push creates it
ok   vendor/a: pushed 0b18046 to main

$ git -C "$UPSTREAM-b" log --format=%s main
splice: init vendor/b

$ git -C "$UPSTREAM-b" ls-tree --name-only main
b.txt

a's unpushed work and a.txt didn't reach $UPSTREAM-b.

The other direction: vendor/b/ is the splice of $UPSTREAM now, and its history continues from vendor/a/, with a's unpushed work as a commit of its own:

$ git splice log vendor/b
===  vendor/b (main)
< 8da07b1 a's unpushed work  (Test <test@example.com>)

$ git splice push vendor/b
ok   vendor/b: pushed 8da07b1 to main

$ git -C "$UPSTREAM" log --format=%s main
a's unpushed work
seed

Output​

Output