fix(cursor): Last method needs skip empty pages

[dchaofei]

In the same transaction, Detete may cause the last page to be empty. In this case, calling the Last method will return nill instead of the value of the previous page, which is incorrect.

1. suppose there is a bucket of 1000 elements
2. the structure diagram at this time

                          +---------------------+                                                                  
                          |     root bucket     |                                                                  
                +---------+---------------------+---------+                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
         +--------------------+                 +---------------------------+                                      
         |                    |                 |                           |                                      
 leaf1   |1，2，3，4.......300 |                 |301,302,303.......1000     |  leaf2                               
         |                    |                 |                           |                                      
         +--------------------+                 +---------------------------+           

3. now calling the Last method should return 1000
4. ok, now we call the Delete method to delete the last 800 elements

                          +---------------------+                                                                  
                          |     root bucket     |                                                                  
                +---------+---------------------+---------+                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
                |                                         |                                                        
         +----------------+                             +--+                                                       
         |                |                             |  |                                                       
 leaf1   |1，2，3......200 |                             |  |  leaf2 is empty                                       
         |                |                             |  |                                                       
         +----------------+                             +--+                                                       
                                                                                                                     
                                                                                                                   
   

5. bad thing happened, calling the Last method again returns nil, but I think it should return 200

[ahrtr]

Before I have a deep dive into this PR, please answer question below:

1. Do you see any real issue in production or test environment? Or can you provide a detailed step to reproduce this issue?

[dchaofei]

Before I have a deep dive into this PR, please answer question below:

1. Do you see any real issue in production or test environment? Or can you provide a detailed step to reproduce this issue?

I found this problem while reading bbolt source code.

There is a test case TestCursor_Last_EmptyPages in pr to reproduce this problem

bbolt/cursor_test.go

Line 511 in 3d8280f

func TestCursor_Last_EmptyPages(t *testing.T) {

[ahrtr]

Thanks for the feedback. Will take a look later.

Recently we added some workflow, so please rebase this PR.

[dchaofei]

@ahrtr hi, I need your approval to execute the test workflow

[ahrtr]

Please always rebase PR instead of merging master.

Please also squash the commits.

[dchaofei]

ok, it has been changed to rebase

[ahrtr]

Confirmed that it's an real issue, and overall looks good to me.

[ahrtr]

Please also add an item something like below into CHANGE,

Fix the "Last" method might return no data due to not skipping the empty pages.

[ahrtr]

LGTM

Thank you @dchaofei

cc @ptabor PTAL