Can Deleting a Map Entry Lead to Memory Leaks in Go?

Can Deleting a Map Entry Cause Memory Leaks in Go?

A common concern when manipulating data structures in Go is the potential for memory leaks. This question focuses on whether deleting an entry from a map of pointers can lead to such a leak.

Examining the Sources

While the documentation doesn't explicitly address this, inspecting the Go runtime source code reveals that when an entry is deleted from a map, both the key and value storage are cleared (zeroed). This means any pointers contained within are also cleared.

// Clear key storage.
memclr(k, uintptr(t.keysize))
// Clear value storage.
v := unsafe.Pointer(uintptr(unsafe.Pointer(b)) + dataOffset + bucketCnt*uintptr(t.keysize) + i*uintptr(t.valuesize))
memclr(v, uintptr(t.valuesize))

Proving it with an Example

To corroborate this, we can create a sample program with a map of pointers and register a finalizer to catch when the pointer becomes unreachable:

import (
    "fmt"
    "runtime"
    "sync"
    "time"
)

type point struct {
    X, Y int
}

func main() {
    // Create a map and put a pointer value in it, with a finalizer.
    m := map[int]*point{}
    p := &point{1, 2}
    runtime.SetFinalizer(p, func(p *point) {
        fmt.Printf("Finalized: %p %+v\n", p, p)
    })
    m[1] = p
    fmt.Printf("Put in map: %p %+v\n", p, p)

    // Delete the entry from the map.
    delete(m, 1)

    // Force garbage collection.
    runtime.GC()

    // Sleep to ensure the finalizer has a chance to run.
    time.Sleep(time.Second)

    // Print the map to verify the map itself wasn't garbage collected.
    fmt.Println(m)
}

Running this will produce the following output:

Put in map: 0x1040a128 &{X:1 Y:2}
Finalized: 0x1040a128 &{X:1 Y:2}
map[]

As you can see, the finalizer is called when the pointer is removed from the map and becomes unreachable, demonstrating that there is no memory leak.

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