注释掉未使用的功能时发生运行时错误

Question

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!

Answer 1

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.