feat(ops/builderball): introduce bouncing Nix cache proxy

Adds a Nix cache proxy which can be used to send a Nix cache lookup to the first
available cache that has the given NAR. We will use this for dynamically created
builders.

Relates to b/432.

Change-Id: If970d2393e43ba032b5b7d653f2b92f6ac0eab63
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12949
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/builderball/main.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"os"
+	"time"
+
+	"tvl.fyi/ops/builderball/config"
+	"tvl.fyi/ops/builderball/proxy"
+)
+
+func printStats() {
+	hits, misses := proxy.GetStats()
+	if hits > 0 || misses > 0 {
+		slog.Info("served cache requests", "hits", hits, "misses", misses)
+	}
+}
+
+func main() {
+	flag.Parse()
+	slog.Info("launching builderball proxy", "host", *config.Host, "port", *config.Port)
+
+	logConfig := slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}
+
+	if *config.Debug {
+		logConfig.Level = slog.LevelDebug
+	}
+
+	if *config.JSON {
+		logConfig.AddSource = true
+		slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stderr, &logConfig)))
+	} else {
+		slog.SetDefault(slog.New(slog.NewTextHandler(os.Stderr, &logConfig)))
+	}
+
+	slog.Debug("debug logging enabled") // prints only then, of course.
+
+	if len(config.Caches) > 0 {
+		slog.Info("static binary caches configured", "caches", config.Caches)
+	}
+
+	if *config.Tailscale {
+		slog.Info("tailscale discovery is enabled", "tag", *config.TSTag)
+	} else if len(config.Caches) == 0 {
+		slog.Error("no static binary caches configured, and tailscale discovery is disabled")
+		os.Exit(1)
+	}
+
+	http.HandleFunc("GET /nix-cache-info", func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintf(w, `StoreDir: /nix/store
+WantMassQuery: 1
+Priority: %d
+`, *config.Priority)
+	})
+
+	http.HandleFunc("GET /", proxy.Handler)
+
+	go func() {
+		for {
+			printStats()
+			time.Sleep(time.Duration(*config.Ticker) * time.Second)
+		}
+	}()
+
+	err := http.ListenAndServe(fmt.Sprintf("%s:%d", *config.Host, *config.Port), nil)
+	if err != nil {
+		slog.Error("HTTP server failed", "error", err.Error())
+		os.Exit(1)
+	}
+}