轉：運行postgres查詢時出現無效的內存地址錯誤

Question

當我運行我的Go代碼來查詢我的本地postgres數據庫時，我不斷收到此錯誤。

錯誤：

panic serving [::1]:56708: runtime error: invalid memory address or nil pointer dereference 
goroutine 23 [running]: 
net/http.func·011() 
    /usr/local/go/src/pkg/net/http/server.go:1100 +0xb7 
runtime.panic(0x2ef0a0, 0x4d8ee4) 
    /usr/local/go/src/pkg/runtime/panic.c:248 +0x18d 
database/sql.(*DB).conn(0x0, 0x277a1, 0x0, 0x0) 
    /usr/local/go/src/pkg/database/sql/sql.go:625 +0x751 
database/sql.(*DB).Ping(0x0, 0x0, 0x0) 
    /usr/local/go/src/pkg/database/sql/sql.go:452 +0x39 
main.firstHandler(0x58e9a8, 0xc208052320, 0xc2080284e0) 
    /Users/Tommy/Documents/gocode/server/server.go:122 +0x35 
net/http.HandlerFunc.ServeHTTP(0x3c6be8, 0x58e9a8, 0xc208052320, 0xc2080284e0) 
    /usr/local/go/src/pkg/net/http/server.go:1235 +0x40 
github.com/gorilla/mux.(*Router).ServeHTTP(0xc2080186e0, 0x58e9a8, 0xc208052320, 0xc2080284e0) 
    /Users/Audrey/gocode/src/github.com/gorilla/mux/mux.go:98 +0x292 
net/http.(*ServeMux).ServeHTTP(0xc208022660, 0x58e9a8, 0xc208052320, 0xc2080284e0) 
    /usr/local/go/src/pkg/net/http/server.go:1511 +0x1a3 
net/http.serverHandler.ServeHTTP(0xc208004660, 0x58e9a8, 0xc208052320, 0xc2080284e0) 
    /usr/local/go/src/pkg/net/http/server.go:1673 +0x19f 
net/http.(*conn).serve(0xc208050500) 
    /usr/local/go/src/pkg/net/http/server.go:1174 +0xa7e 
created by net/http.(*Server).Serve 
    /usr/local/go/src/pkg/net/http/server.go:1721 +0x313 

轉到：

func firstHandler(w http.ResponseWriter, r *http.Request) { 
    err := db.Ping() 
    if err != nil { 
     log.Fatal(err) 
    } 
    rows, err := db.Query("SELECT id, created_at, updated_at FROM script WHERE updated_at = $1", 3) 
    if err != nil { 
     log.Fatal(err) 
    } 
    defer rows.Close() 

    var created_at, updated_at, id int 
    for rows.Next() { 
     err := rows.Scan(&id, &created_at, &updated_at) 
     if err != nil { 
      log.Fatal(err) 
     } 
     fmt.Fprintf("%s %s %s", id, created_at, updated_at) 
    } 
} 

var r = mux.NewRouter() 
var db *sql.DB 

func main() { 
    db, err := sql.Open("postgres", "user=Tommy host=localhost dbname=dbgo sslmode=verify-full") 
    if err != nil { 
     log.Fatal(err) 
    } 
    defer db.Close() 

    r.HandleFunc("/ping", firstHandler) 

    http.Handle("/", r) 

    http.ListenAndServe(":8080", nil) 
} 

幫助。我究竟做錯了什麼？我也提到這個：https://gophercasts.io/lessons/4-postgres-basics。

Answer 1

其實，聲明與連接：

var db *sql.DB 

，但你打開一個連接：

db, err := sql.Open("postgres", "user=Tommy host=localhost dbname=dbgo sslmode=verify-full") 

注：=（它結合了變量聲明與賦值）。這實際上會影響全局數據庫變量的本地。連接已打開但分配給本地變量。所以全局數據庫變量的值是零。

當firstHandler函數被調用時，它的值仍然是零，這觸發了恐慌。

替換：= a =（並在之前聲明err對象）。