core/toposort: Do not calculate cycles within an SCC

Previously, the toposort algorithm would calculate all the cycles within
a multi-node SCC, and then rank the cycles in order to figure out which
cycle should be broken and entered first.

It turns out that the number of cycles in an SCC can be a factorial of
the number of nodes. Consequently, it is felt that this approach could
lead to problems.

Instead, when we reach an SCC with several nodes, we simply sort those
nodes lexically. This is easier to explain and understand, and still
allows the user to be aware of where there are cycles, and that they can
be addressed. By not calculating cycles, performance is greatly
improved.

The changes to our own tests, in terms of ordering of output, are
surprisingly small. Analysis of unity suggests there will likely be
impact on users: maybe around 10% of the lines that changed when
toposort was first introduced will change again.

Fixes #3786.

Signed-off-by: Matthew Sackman <[email protected]>
Change-Id: I28edde9b3e8fb33a9fa5e97c93f1c7db1248c55d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209497
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: cuematthew

.github/workflows/trybot.yaml

@@ -128,27 +128,27 @@ jobs:
       - name: Test with -tags=cuewasm
         run: go test -tags cuewasm ./cmd/cue/cmd ./cue/interpreter/wasm
       - id: auth
-        uses: google-github-actions/auth@v2
-        with:
-          credentials_json: ${{ secrets.E2E_GCLOUD_KEY }}
         if: |-
           github.repository == 'cue-lang/cue' && (((github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/heads/release-branch.')) && (! (contains(github.event.head_commit.message, '
           Dispatch-Trailer: {"type":"')))) || (github.ref == 'refs/heads/ci/test')) && (matrix.go-version == '1.24.x' && matrix.runner == 'ubuntu-24.04')
         name: gcloud auth for end-to-end tests
+        uses: google-github-actions/auth@v2
+        with:
+          credentials_json: ${{ secrets.E2E_GCLOUD_KEY }}
       - if: |-
           github.repository == 'cue-lang/cue' && (((github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/heads/release-branch.')) && (! (contains(github.event.head_commit.message, '
           Dispatch-Trailer: {"type":"')))) || (github.ref == 'refs/heads/ci/test')) && (matrix.go-version == '1.24.x' && matrix.runner == 'ubuntu-24.04')
         name: gcloud setup for end-to-end tests
         uses: google-github-actions/setup-gcloud@v2
-      - if: |-
+      - env:
+          CUE_TEST_TOKEN: ${{ secrets.E2E_PORCUEPINE_CUE_TOKEN }}
+        if: |-
           github.repository == 'cue-lang/cue' && (((github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/heads/release-branch.')) && (! (contains(github.event.head_commit.message, '
           Dispatch-Trailer: {"type":"')))) || (github.ref == 'refs/heads/ci/test')) && (matrix.go-version == '1.24.x' && matrix.runner == 'ubuntu-24.04')
         name: End-to-end test
         run: |-
           cd internal/_e2e
           go test -race
-        env:
-          CUE_TEST_TOKEN: ${{ secrets.E2E_PORCUEPINE_CUE_TOKEN }}
       - if: (matrix.go-version == '1.24.x' && matrix.runner == 'ubuntu-24.04')
         name: Go checks
         run: |-

cmd/cue/cmd/testdata/script/issue986.txtar

@@ -31,6 +31,6 @@ cmp stdout expect-stdout
 
 -- expect-stdout --
 a: aaa
-b: bbb
 ab: aaabbb
+b: bbb
 cd: cccddd