Cursor.Prev() out of range issue v1.3.9

[axww]

Hi, I met an issue during dev. Here's my sample code:

package main

import (
	"fmt"
	"log"

	bolt "go.etcd.io/bbolt"
)

func main() {

	aDb, dbErr := bolt.Open("a.db", 0600, nil)
	if dbErr != nil {
		log.Fatal(dbErr)
	}
	defer aDb.Close()

	aDb.Batch(func(tx *bolt.Tx) error {
		// Create data
		b, _ := tx.CreateBucketIfNotExists([]byte("test"))
		b.Put([]byte("1"), nil)
		b.Put([]byte("2"), nil)
		b.Put([]byte("3"), nil)
		// Next test
		fmt.Println("=== Next test ===")
		cn := b.Cursor()
		cn.Seek([]byte("2")) //2
		cn.Next()            // 3
		cn.Next()            // nil (Cursor stops at 3)
		kn, _ := cn.Prev()   // 2
		fmt.Println(string(kn))
		// Prev test
		fmt.Println("=== Prev test ===")
		cp := b.Cursor()
		cp.Seek([]byte("2")) //2
		cp.Prev()            // 1
		cp.Prev()            // nil (Cursor out of range)
		kp, _ := cp.Next()   // Should be 2 but nil
		fmt.Println(string(kp))
		// End
		return nil
	})
}

The output of the program:

=== Next test ===
2
=== Prev test ===

As you can see I put 3 rows "1", "2" and "3" into the bucket.
Both of "Next test" and "Prev test", I set cursor at "2" initially.

In "Next test", I move cursor to "3", then run Next() again, it returns nil.
Then I run Prev(), the cursor targets to "2".

In "Prev test", I move cursor to "1", then run Prev() again, it returns nil.
Then I run Next(), the cursor targets to nil.

Whatever how many Next() I added in "Prev test", the cursor still targes to nil.
It seems after Cursor.Prev() out of range, it stucks there and can not be use anymore.

I don't know if this is a bug or a correct design, but it's strange.
Do you consider make these 2 situations have similiar behavior?

Thank you very much!
Best

[ahrtr]

Thanks @axww for raising this issue.

It's a minor issue. Usually when you have already reached the beginning, you should call First if you want to iterate over the elements in the reverse direction by call Next.

But I agree that users should be able to call Next directly without calling First in such scenario. Fixed it in #734.

[ahrtr]

Open to track the backport effort

[ahrtr]

Done.

[ahrtr]

The fix will be included in 1.3.10