Go runtime error when an unused func is commented out

Here is the code:

package main
import (
"database/sql"
"errors"
"fmt"
_ "github.com/lib/pq"
)
type Post struct {
Id       int
Content  string
Author   string
Comments []Comment
}
type Comment struct {
Id      int
Content string
Author  string
Post    *Post
}
var Db *sql.DB
// connect to the Db
func init() {
var err error
//Db, err = sql.Open("postgres", "user=gwp dbname=gwp password=gwp sslmode=disable")
Db, err = sql.Open("postgres", "user=gwp dbname=gwp sslmode=disable")
if err != nil {
panic(err)
}
}
func (comment *Comment) Create() (err error) {
if comment.Post == nil {
err = errors.New("You need to specify the post that the comment belongs to.")
return
}
err = Db.QueryRow("insert into comments (content, author, post_id) values ($1, $2, $3) returning id", comment.Content, comment.Author, comment.Post.Id).Scan(&comment.Id)
return
}
// Get a single post
func GetPost(id int) (post Post, err error) {
post = Post{}
post.Comments = []Comment{}
err = Db.QueryRow("select id, content, author from posts where id = $1", id).Scan(&post.Id, &post.Content, &post.Author)
rows, err := Db.Query("select id, content, author from comments where post_id = $1", id)
if err != nil {
return
}
for rows.Next() {
comment := Comment{Post: &post}
err = rows.Scan(&comment.Id, &comment.Content, &comment.Author)
if err != nil {
return
}
post.Comments = append(post.Comments, comment)
}
rows.Close()
return
}
// Create a new post
func (post *Post) Create() (err error) {
err = Db.QueryRow("insert into posts (content, author) values ($1, $2) returning id", post.Content, post.Author).Scan(&post.Id)
return
}
func main() {
post := Post{Content: "Hello World!", Author: "Sau Sheong"}
post.Create()
// Add a comment
comment := Comment{Content: "Good post!", Author: "Joe", Post: &post}
comment.Create()
readPost, _ := GetPost(post.Id)
fmt.Println(readPost)                  // {1 Hello World! Sau Sheong [{1 Good post! Joe 0xc20802a1c0}]}
fmt.Println(readPost.Comments)         // [{1 Good post! Joe 0xc20802a1c0}]
fmt.Println(readPost.Comments[0].Post) // &{1 Hello World! Sau Sheong [{1 Good post! Joe 0xc20802a1c0}]}
}

The init function is not used anywhere, but if I comment it out, the program crashes with the following error:

/usr/local/Cellar/go/1.7.4_2/libexec/bin/go run /Users/kaiyin/golang/src/github.com/sausheong/gwp/Chapter_6_Storing_Data/sql_store2/store.go
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x5fa9a]
goroutine 1 [running]:
panic(0x1d62c0, 0xc42000c0e0)
/usr/local/Cellar/go/1.7.4_2/libexec/src/runtime/panic.go:500 +0x1a1
database/sql.(*DB).conn(0x0, 0x1, 0x22, 0x1c, 0x0)
/usr/local/Cellar/go/1.7.4_2/libexec/src/database/sql/sql.go:781 +0x3a
database/sql.(*DB).query(0x0, 0x216def, 0x40, 0xc42004fd80, 0x2, 0x2, 0x1, 0x10, 0xc42000cba0, 0x0)
/usr/local/Cellar/go/1.7.4_2/libexec/src/database/sql/sql.go:1074 +0x3b
database/sql.(*DB).Query(0x0, 0x216def, 0x40, 0xc42004fd80, 0x2, 0x2, 0x1, 0xc42000cba0, 0xc42004fd28)
/usr/local/Cellar/go/1.7.4_2/libexec/src/database/sql/sql.go:1062 +0x90
database/sql.(*DB).QueryRow(0x0, 0x216def, 0x40, 0xc42004fd80, 0x2, 0x2, 0x11200b7b18)
/usr/local/Cellar/go/1.7.4_2/libexec/src/database/sql/sql.go:1143 +0x70
main.(*Post).Create(0xc420018380, 0xc420018380, 0xc42004fe20)
/Users/kaiyin/golang/src/github.com/sausheong/gwp/Chapter_6_Storing_Data/sql_store2/store.go:69 +0xfe
main.main()
/Users/kaiyin/golang/src/github.com/sausheong/gwp/Chapter_6_Storing_Data/sql_store2/store.go:75 +0xd7
exit status 2
Process finished with exit code 1

Not very helpful for debugging.

Any idea what's going on here? Thanks!

All init functions are called automatically when the program starts. It must be doing some initialization that other code depends on. See https://golang.org/doc/effective_go.html#init.