lightning: change the implementation of `ScannedPos`

[joechenrh]

What problem does this PR solve?

Issue Number: ref https://github.com/pingcap/tidb/issues/61088

Problem Summary:

What changed and how does it work?

In the encode step, we will track the amount of data read and show it on the panel. This is achieved by calling Parser.ScannedPos to get the latest read position. However, the current implement is wrong:

https://github.com/pingcap/tidb/blob/11276faa9dc05ed7047f766a4c1fb45cd2b83109/pkg/lightning/mydump/parquet_parser.go#L377-L379

Because we will open one reader for each column, Seek can't reflect the actual bytes we read. Below is a simple illustration.

File layout (Dict page and meta are ignored)
--------------------------------------------------------------------------------------------------
|   Column 0 Group 0    |    Column 1 Group 0    |   Column 0 Group 1    |   Column 1 Group 1    | 
--------------------------------------------------------------------------------------------------
                ^                       ^
                |                       |
            Reader 0                 Reader 1

Since it's hard to retrieve total scanned bytes for the parser, we used the size of read rows instead which is better than using Seek.

Additionally, we use ScannedPos to update metrics for all types of data.

Check List

Tests

• [ ] Unit test
• [ ] Integration test
• [ ] Manual test (add detailed scripts or steps below)
• [X] No need to test

  • [ ] I checked and no code files have been changed.

As mentioned before, this interface is used to update the metrics, so changes in this PR won't affect the execution of import jobs.

Side effects

• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Breaking backward compatibility

Documentation

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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Codecov Report

:x: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 73.4832%. Comparing base (4419a28) to head (e951863). :warning: Report is 305 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61670        +/-   ##
================================================
+ Coverage   73.0715%   73.4832%   +0.4116%     
================================================
  Files          1729       1760        +31     
  Lines        481120     492737     +11617     
================================================
+ Hits         351562     362079     +10517     
- Misses       108028     108373       +345     
- Partials      21530      22285       +755     

Flag	Coverage Δ
integration	44.9431% <36.0000%> (?)
unit	72.2741% <72.2222%> (-0.0552%)	:arrow_down:

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Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	45.9046% <ø> (-1.0389%)	:arrow_down:

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[joechenrh]

/retest

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[joechenrh]

To some extent, the value get from ScannedPos won't affect the progress we calculate too much.

Because for each parser, the ScannedPos points to the start position of meta part and never changed after opening (because we will open a new reader for eacn column). As the meta is small, the delta we get is almost the same as the file size. 😂

So just keep the current logic and fix it after switching to new parse library.