Fix windows dataloss issue

[jarifibrahim]

Windows doesn't allow memory mapping a file to a size greater than the file's actual size. To circumvent this, we increase the file size by truncating it.

badger/y/mmap_windows.go

Lines 41 to 48 in f5b6321

	// In windows, we cannot mmap a file more than it's actual size.
	// So truncate the file to the size of the mmap.
	if fi.Size() < size {
	if err := fd.Truncate(size); err != nil {
	return nil, fmt.Errorf("truncate: %s", err)
	}
	}

When badger would re-open, we try to replay this "truncated" file. Since this truncated file consists of all zeros, the replay would return the last offset as zero and then we would truncate the original file to size zero.

The replay function would return zero as the last valid offset which was wrong. The last valid offset is start offset plus the forward movements of the file offset.
So instead of

badger/value.go

Line 433 in f5b6321

var validEndOffset uint32

var validEndOff uint32 // notice we're starting from zero, not the start point.

we should be doing

var validEndOff uint32 = offset

Fixes - #1126

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This change is 

[coveralls]

Coverage increased (+0.4%) to 77.942% when pulling d300dbd on ibrahim/windows-dataloss into f5b6321 on master.

[coveralls]

Coverage increased (+0.4%) to 77.974% when pulling d300dbd on ibrahim/windows-dataloss into f5b6321 on master.

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ashish-goswami)

[zhenruyan]

I see that this issue has been resolved, but currently I am encountering the same phenomenon on Windows 10 Golang 1.21.0. When the program interrupts abnormally and fails to close properly, the vlog size becomes 1kb and prompts "Value log truncate required to run DB. This might result in data loss" @jarifibrahim