What memory leak patterns in Go are not goroutine leaks and how do you detect them?

Go has several memory leak patterns beyond leaked goroutines: long-lived caches without eviction, global maps that grow without bounds, slice backing arrays held by small sub-slices, and finalizers that delay GC.

// LEAK 1: growing global map without eviction
var requestMetrics = map[string]int64{} // grows forever with new paths
// Fix: use a bounded structure (LRU cache, TTL cache)
var metricsCache = cache.NewLRU[string, int64](1000) // bounded

// LEAK 2: slice sub-slice retaining large backing array
func getHeader(data []byte) []byte {
    return data[:8]  // BAD: keeps ALL of 'data' alive
}
func getHeaderSafe(data []byte) []byte {
    header := make([]byte, 8)
    copy(header, data[:8]) // GOOD: independent allocation
    return header
}

// LEAK 3: time.Ticker without Stop()
// Already discussed but worth repeating — channel and goroutine alive forever

// LEAK 4: cgo-allocated memory (outside GC's purview)
// Must manually call C.free() for C.CString(), C.malloc(), etc.

// DETECTION:
// A. runtime.ReadMemStats — watch HeapAlloc over time
func monitorHeap(ctx context.Context) {
    ticker := time.NewTicker(30 * time.Second)
    defer ticker.Stop()
    for {
        select {
        case <-ctx.Done(): return
        case <-ticker.C:
            var stats runtime.MemStats
            runtime.ReadMemStats(&stats)
            slog.Info("heap",
                "alloc_mb",    stats.HeapAlloc / (1 << 20),
                "sys_mb",      stats.HeapSys / (1 << 20),
                "num_gc",      stats.NumGC,
                "goroutines",  runtime.NumGoroutine(),
            )
        }
    }
}

// B. pprof heap profile comparison
// curl http://localhost:6060/debug/pprof/heap > heap1.out
// # wait a while
// curl http://localhost:6060/debug/pprof/heap > heap2.out
// go tool pprof -base heap1.out heap2.out
// > top10  # shows objects that grew between snapshots

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