Which objects are finalized in Go by default and what are some of the pitfalls of it?

The function runtime.SetFinalizer(x, f interface{}) sets the finalizer associated with x to f.

What kind of objects are finalized by default?

What are some of the unintended pitfalls caused by having those objects finalized by default?

Solution

The following objects are finalized by default:

os.File: The file is automatically closed when the object is garbage collected.
os.Process: Finalization will release any resources associated with the process. On Unix, this is a no-operation. On Windows, it closes the handle associated with the process.
On Windows, it appears that package net can automatically close a network connection.

The Go standard library is not setting a finalizer on object kinds other than the ones mentioned above.

There seems to be only one potential issue that may cause problems in actual programs: When an os.File is finalized, it will make a call to the OS to close the file descriptor. In case the os.File has been created by calling function os.NewFile(fd int, name string) *File and the file descriptor is also used by another (different) os.File, then garbage collecting either one of the file objects will render the other file object unusable. For example:

package main

import (
    "fmt"
    "os"
    "runtime"
)

func open() {
    os.NewFile(1, "stdout")
}

func main() {
    open()

    // Force finalization of unreachable objects
    _ = make([]byte, 1e7)
    runtime.GC()

    _, err := fmt.Println("some text") // Print something via os.Stdout
    if err != nil {
        fmt.Fprintln(os.Stderr, "could not print the text")
    }
}

prints:

could not print the text