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No need to handle freelist as a specical case when freeing a page #788

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merged 1 commit into from
Jul 22, 2024

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ahrtr
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@ahrtr ahrtr commented Jul 8, 2024

Read #67 (comment)

When the page ID to be freed isn't included in f.allocs, then it means it must be allocated before opening the db this time, and the write transaction is the very first write TXN after opening the db. So it (the page ID to be freed) must be visible to all readonly transactions. We just need to use the default value 0 for such case, in the same way as we did for other page types.

Will rebase this PR once #775 gets merged.

The existing test cases already cover the sanity test. We will add more dedicated test cases for freelist management later.

cc @tjungblu @fuweid @ivanvc

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LGTM. However, I'd suggest having @tjungblu review this PR, too :), as I'm not an expert on freelist.

@@ -67,9 +67,6 @@ func (t *shared) Free(txid common.Txid, p *common.Page) {
allocTxid, ok := t.allocs[p.Id()]
if ok {
delete(t.allocs, p.Id())
} else if p.IsFreelistPage() {
// Freelist is always allocated by prior tx.
allocTxid = txid - 1
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@fuweid fuweid Jul 18, 2024

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The Line 79 txp.alloctx = append(txp.alloctx, allocTxid) could append zero into alloctx.

There are two cases we should consider:

Rollback

The zero value will be covered by the rollback. So it looks fine.

tx := txp.alloctx[i]
if tx == 0 {
continue
}

Release by opening new writable TX

This change looks safe to me because the page will be free after all the read-only TX are closed.
I think the original Freelist is always allocated by prior tx. might free the page which can be read by opening read-only TX.

for i := 0; i < len(txp.ids); i++ {
if atx := txp.alloctx[i]; atx < begin || atx > end {
continue
}
m = append(m, txp.ids[i])

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LGTM

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, ivanvc

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@tjungblu
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/lgtm

Thanks @ahrtr - I'll adjust the testcases accordingly.

@ahrtr ahrtr merged commit d72e6bf into etcd-io:main Jul 22, 2024
18 checks passed
@ahrtr ahrtr deleted the simply_free_20240708 branch July 22, 2024 11:56
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5 participants