Friday, September 07, 2012

A Go Gotcha that Got Me

Here's a little Go program that has a surprising output:
package main

import (
 "fmt"
 "net"
 "crypto/tls"
)
 
func main() {
 var c net.Conn

 c, err := tls.Dial("tcp", "www.jgc.org:80", nil)

 fmt.Printf("%v %v\n", c, err)

 if c == nil {
  fmt.Printf("Nil\n")
 } else {
  fmt.Printf("Not nil\n")
 }
}
This program tries to connect to my web site using TLS on the non-TLS port 80. That's done to force there to be a TLS error. The output is a little surprising:
  <nil> local error: record overflow
   Not nil
The Printf gives the value of c as but when the test c == nil is performed it's non-nil. 

So, what's going on? 

The answer is in the Go FAQ: Why is my nil error value not equal to nil?

In short, c is an interface (a net.Conn). The implementation of an interface is a type and a value. The type gives the actual type that implements that interface (in the case above it's a *tls.Conn) and the value is a pointer to the concrete example of that type. 

When the error occurs in the code above tls.Dial returns nil, err. The nil is a nil pointer to a tls.Conn. When the assignment to c happens c becomes non-nil (its type is now *tls.Conn) but its value is nil (since tls.Dial returned a nil pointer). Thus the nil test fails. 

The bottom line is that my code above is the wrong thing to do. Don't do nil tests on interface variables.

PS A lot of people have been asking me why I wasn't checking the value of err. In the real code I was, but a defer statement was acting on c and in it I had the code

if c != nil {
  c.Close()
}

In figuring out why that failed sometimes I discovered the gotcha.

If you enjoyed this blog post, you might enjoy my travel book for people interested in science and technology: The Geek Atlas. Signed copies of The Geek Atlas are available.

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