ci: detect and prevent empty case statements in Go code (#11593)

One of the security patches released 2026-03-09 [fixed a vulnerability](https://codeberg.org/forgejo/forgejo/pulls/11513/commits/d1c7b04d09f6a13896eaa1322ac690b2021539da) caused by a misapplication of Go `case` statements, where the implementation would have been correct if Go `case` statements automatically fall through to the next case block, but they do not.  This PR adds a semgrep rule which detects any empty `case` statement and raises an error, in order to prevent this coding mistake in the future.

For example, code like this will now trigger a build error:
```go
	switch setting.Protocol {
	case setting.HTTPUnix:
	case setting.FCGI:
	case setting.FCGIUnix:
	default:
		defaultLocalURL := string(setting.Protocol) + "://"
	}
```

Example error:
```
    cmd/web.go
   ❯❯❱ semgrep.config.forgejo-switch-empty-case
          switch has a case block with no content. This is treated as "break" by Go, but developers may
          confuse it for "fallthrough".  To fix this error, disambiguate by using "break" or
          "fallthrough".

          279┆ switch setting.Protocol {
          280┆ case setting.HTTPUnix:
          281┆ case setting.FCGI:
          282┆ case setting.FCGIUnix:
          283┆ default:
          284┆   defaultLocalURL := string(setting.Protocol) + "://"
          285┆   if setting.HTTPAddr == "0.0.0.0" {
          286┆           defaultLocalURL += "localhost"
          287┆   } else {
          288┆           defaultLocalURL += setting.HTTPAddr
```

As described in the error output, this error can be fixed by explicitly listing `break` (the real Go behaviour, to do nothing in the block), or by listing `fallthrough` (if the intent was to fall through).

All existing code triggering this detection has been changed to `break` (or, rarely, irrelevant cases have been removed), which should maintain the same code functionality.  While performing this fixup, a light analysis was performed on each case and they *appeared* correct, but with ~65 cases I haven't gone into extreme depth.

Tests are present for the semgrep rule in `.semgrep/tests/go.go`.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11593
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

.semgrep/tests/go.go

@@ -0,0 +1,44 @@
+// Copyright 2014 The Gogs Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"context"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+
+	_ "net/http/pprof" // Used for debugging if enabled and a web server is running
+
+	"forgejo.org/modules/setting"
+)
+
+func setPortEmptyCaseBad(port string) error {
+	setting.AppURL = strings.Replace(setting.AppURL, setting.HTTPPort, port, 1)
+	setting.HTTPPort = port
+
+	// ruleid:forgejo-switch-empty-case
+	switch setting.Protocol {
+	case setting.HTTPUnix:
+	case setting.FCGI:
+	case setting.FCGIUnix:
+	default:
+		defaultLocalURL := string(setting.Protocol) + "://"
+	}
+
+	// ok:forgejo-switch-empty-case
+	switch setting.Protocol {
+	case setting.HTTPUnix:
+		break
+	case setting.FCGI:
+		break
+	case setting.FCGIUnix:
+		break
+	default:
+		defaultLocalURL := string(setting.Protocol) + "://"
+	}
+
+	return nil
+}