ci: mirror release artifacts to Cloudflare R2 (#1114)

Mirrors every release artifact into Cloudflare R2 alongside the existing AWS S3
upload, so both buckets carry the same objects during a parallel period. S3 is
untouched and stays authoritative for now; once R2 is confirmed complete it can
be dropped by deleting the `blobs:` block and pointing this at R2 alone.

### Why `publishers:` and not a second `blobs:` entry

goreleaser's blob pipe authenticates from the global `AWS_*` environment and has
no per-entry credential fields, so two different credential sets (AWS S3 and
Cloudflare R2) cannot coexist in `blobs:`. Custom publishers do support
per-entry `env:`, which is the supported way to isolate the two.

### Why `curl --aws-sigv4` and not `aws`/`rclone`

The CI image `docker.gitea.com/runner-images:ubuntu-latest` ships none of `aws`,
`rclone`, `s3cmd` or `mc`, but does ship curl 8.5 with `--aws-sigv4`. This keeps
the change zero-install. Credentials are fed to curl through a config file on
stdin rather than argv, so they never appear in the process list.

### Behaviour

- Object layout is identical to S3 (`gitea-runner/<version>/<artifact>`), so
  migrating consumers later means changing only the host, not the path.
- Nightly gets R2 too, matching the existing nightly-to-S3 behaviour.
- Missing R2 configuration fails the build. Because custom publishers run as the
  very last step of the publish pipeline, a preflight `--check-config` step runs
  right after checkout so the failure happens before anything is built or
  published rather than after the release already exists.
- Verified against a local MinIO instance (site region `auto`, matching R2):
  successful upload byte-compared after download, plus the missing-variable,
  wrong-credentials, missing-file and bad-argument paths.

### Required repository secrets

These must be configured before the next release run, otherwise the new
preflight step will fail the workflow:

- `R2_ENDPOINT` (full base URL, e.g. `https://<account>.r2.cloudflarestorage.com`)
- `R2_BUCKET`
- `R2_ACCESS_KEY_ID`
- `R2_SECRET_ACCESS_KEY`

### Known, deliberate wart

The publisher runs 109 times for 73 distinct object keys: goreleaser's release
pipe already registers `release.extra_files` as `UploadableFile` artifacts, and
`internal/exec` appends the publisher's own `extra_files` on top with no
de-duplication. It cannot be globbed away because `gobwas/glob` has no
substring-exclusion matcher, so `./**.sha256` cannot be narrowed to exclude
`*.xz.sha256`. It is harmless since PUT is idempotent, and the redundant
`./**.xz` glob is kept on purpose so the publisher declares its own complete
file set instead of implicitly depending on the `release:` block's globs. This
is documented in a comment above the block.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1114
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

.goreleaser.yaml

@@ -93,6 +93,37 @@ blobs:
       - glob: ./**.xz
       - glob: ./**.sha256
 
+# Mirrors the S3 `blobs:` upload above into Cloudflare R2 during the
+# parallel S3+R2 period (S3 will be removed once migration completes).
+# A second `blobs:` entry is impossible here since the blob pipe
+# authenticates from the global AWS_* env with no per-entry
+# credentials; `publishers:` supports per-entry `env:` instead, so
+# it's used to invoke scripts/upload-r2.sh once per artifact. Custom
+# publishers inherit almost nothing from the environment, hence the
+# explicit R2_* forwarding below.
+#
+# This publisher fires 109 times for 73 distinct keys because
+# goreleaser's release pipe already registers `release.extra_files`
+# as UploadableFile artifacts, and `internal/exec`'s filterArtifacts
+# appends this block's own extra_files with no de-duplication. It
+# can't be globbed away, since gobwas/glob (via goreleaser/fileglob)
+# has no substring-exclusion matcher. It's harmless: PUT is
+# idempotent, and the `./**.xz` glob below is kept deliberately so
+# this publisher declares its own complete file set rather than
+# implicitly depending on the `release:` block's globs.
+publishers:
+  - name: cloudflare-r2
+    checksum: true
+    extra_files:
+      - glob: ./**.xz
+      - glob: ./**.sha256
+    cmd: sh scripts/upload-r2.sh {{ abs .ArtifactPath }} gitea-runner/{{ .Version }}/{{ .ArtifactName }}
+    env:
+      - R2_ENDPOINT={{ index .Env "R2_ENDPOINT" }}
+      - R2_BUCKET={{ index .Env "R2_BUCKET" }}
+      - R2_ACCESS_KEY_ID={{ index .Env "R2_ACCESS_KEY_ID" }}
+      - R2_SECRET_ACCESS_KEY={{ index .Env "R2_SECRET_ACCESS_KEY" }}
+
 archives:
   - format: binary
     name_template: "{{ .Binary }}"