dashboard/app: add a helper to fix up build arches

[a-nogikh]

---

Before sending a pull request, please review Contribution Guidelines:
https://github.com/google/syzkaller/blob/master/docs/contributing.md

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[gemini-code-assist]

Code Review

This pull request introduces a new admin endpoint patchBuildArchs to update build architectures for a specified namespace. The review feedback highlights critical improvements for the implementation: optimizing the Datastore query by filtering on the architecture directly to prevent potential OOMs, postponing HTTP header writes until after fallible operations to ensure proper error handling, and simplifying the update logic by removing the temporary mapping.

[gemini-code-assist]

There are a few significant improvement opportunities in this helper function:

1. Efficiency: Instead of querying all builds for the namespace and filtering them in memory, you can filter by Arch = "x86_64" directly in the Datastore query. For namespaces with a large history (like upstream), loading all builds can cause Out Of Memory (OOM) errors or request timeouts.
2. Correctness: Setting headers and writing to the response writer before performing fallible operations (like Datastore queries) is an anti-pattern. If the query fails, the error handler will try to write an HTML error page, but the Content-Type header is already set to text/plain, causing the browser to render raw HTML.
3. Simplicity: The newArchs map and key stringification can be completely eliminated by re-evaluating the target architecture inside the updateBatch callback.

	var builds []*Build
	keys, err := db.NewQuery("Build").
		Filter("Namespace=", ns).
		Filter("Arch=", "x86_64").
		GetAll(ctx, &builds)
	if err != nil {
		return fmt.Errorf("failed to query builds: %w", err)
	}

	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	fmt.Fprintf(w, "Patching builds for namespace %q (dry_run=%v)\n", ns, dryRun)

	var toUpdate []*db.Key
	for i, build := range builds {
		targetArch := "amd64"
		if arm64Re.MatchString(build.Manager) {
			targetArch = "arm64"
		}

		key := keys[i]
		toUpdate = append(toUpdate, key)

		fmt.Fprintf(w, "Build %q (Manager: %q) %q -> %q\n", build.ID, build.Manager, build.Arch, targetArch)
	}

	fmt.Fprintf(w, "Found %d builds to patch.\n", len(toUpdate))

	if dryRun {
		fmt.Fprintf(w, "Dry run, skipping datastore update.\n")
		return nil
	}

	return updateBatch(ctx, toUpdate, func(key *db.Key, b *Build) {
		if arm64Re.MatchString(b.Manager) {
			b.Arch = "arm64"
		} else {
			b.Arch = "amd64"
		}
	})

[codecov]

Codecov Report

❌ Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.7%. Comparing base (9a5a7e5) to head (1da7d27).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
dashboard/app/admin.go	0.0%	44 Missing ⚠️
dashboard/app/main.go	0.0%	2 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
dashboard/app/main.go	66.9% <0.0%> (-1.6%)	⬇️
dashboard/app/admin.go	1.9% <0.0%> (-0.3%)	⬇️

... and 7 files with indirect coverage changes

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