ci: improve go caches (#38730)

Cache saves have been rejected since June because the repo sat above its
10 GB allowance, so every PR run fell back to a cache built with an
older toolchain and rebuilt the backend from scratch.

- go version in the `gobuild` and `golint` keys and `restore-keys`
- seeder triggers on `go.mod` and gained `workflow_dispatch`
- single writer for the `gomod` cache, the two were racing
- new daily `cache-prune` workflow holding the total under a limit
- `cache: false` for `setup-go` in release and cron workflows, it held
815 MB
- add `workflow_dispatch` so the cache workflows can be triggered
on-demand

Assisted-by: Claude Code:claude-opus-5

.github/workflows/cache-prune.yml

@@ -0,0 +1,38 @@
+name: cache-prune
+
+# Keeps the repository's total cache size below a fixed limit, so that GitHub never
+# rejects a save for being over the allowance.
+
+on:
+  schedule:
+    - cron: "37 2 * * *" # every day at 02:37 UTC
+  workflow_dispatch:
+  workflow_call:
+
+permissions:
+  actions: write # to delete caches
+
+concurrency:
+  group: cache-prune
+
+jobs:
+  prune:
+    runs-on: ubuntu-latest
+    if: github.repository == 'go-gitea/gitea'
+    env:
+      GH_TOKEN: ${{ github.token }}
+      GH_REPO: ${{ github.repository }}
+    steps:
+      # Deletes least recently used first, the order GitHub itself evicts in, which takes
+      # superseded generations first as those stop being restored once a newer one exists.
+      - name: delete caches over the size limit
+        run: |
+          caches=$(gh cache list --limit 1000 --sort last_accessed_at --order asc --json id,key,sizeInBytes)
+          size=$(jq '[.[].sizeInBytes] | add // 0' <<< "$caches")
+          echo "cache usage: $((size / 1000000)) MB"
+          jq -r '.[] | "\(.id) \(.sizeInBytes) \(.key)"' <<< "$caches" |
+            while [ "$size" -gt 6500000000 ] && read -r id bytes key; do
+              echo "deleting $key"
+              gh cache delete "$id"
+              size=$((size - bytes))
+            done

.github/workflows/cache-seeder.yml

@@ -8,10 +8,12 @@
 name: cache-seeder
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main
     paths:
+      - "go.mod" # a toolchain bump invalidates the build caches
       - "go.sum"
       - ".golangci.yml"
       - ".github/actions/go-cache/action.yml"
@@ -70,3 +72,10 @@ jobs:
       - run: make ${{ matrix.target }}
         env:
           TAGS: ${{ matrix.tags }}
+
+  # reclaims the caches this run superseded, so the next save still fits in the allowance
+  prune:
+    needs: [gobuild, lint]
+    permissions:
+      actions: write
+    uses: ./.github/workflows/cache-prune.yml

.github/workflows/cron-licenses.yml

@@ -17,6 +17,7 @@ jobs:
         with:
           go-version-file: go.mod
           check-latest: true
+          cache: false
       - run: make generate-gitignore
         timeout-minutes: 40
       - name: push translations to repo

.github/workflows/release-nightly.yml

@@ -23,6 +23,7 @@ jobs:
         with:
           go-version-file: go.mod
           check-latest: true
+          cache: false
       - uses: ./.github/actions/node-setup
       - run: make deps-frontend deps-backend
       # xgo build

.github/workflows/release-tag-rc.yml

@@ -24,6 +24,7 @@ jobs:
         with:
           go-version-file: go.mod
           check-latest: true
+          cache: false
       - uses: ./.github/actions/node-setup
       - run: make deps-frontend deps-backend
       # xgo build

.github/workflows/release-tag-version.yml

@@ -27,6 +27,7 @@ jobs:
         with:
           go-version-file: go.mod
           check-latest: true
+          cache: false
       - uses: ./.github/actions/node-setup
       - run: make deps-frontend deps-backend
       # xgo build