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Fix how we cancel the context in the builtin backup engine #17285

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merged 2 commits into from
Nov 27, 2024
Merged

Fix how we cancel the context in the builtin backup engine #17285

merged 2 commits into from
Nov 27, 2024

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frouioui
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@frouioui frouioui commented Nov 26, 2024 •
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Description

This PR makes sure we cancel the context only at the correct time. It was getting canceled too early which could lead to incomplete/failing backups with storage implementations that write concurrently (Ceph and S3).

This regression was introduced by: #16856, backporting this to release-21.0 as the issue was introduced there. It would lead to the following error:

commerce/0 (zone1-0000000102): time:{seconds:1732657021 nanoseconds:695573000} file:"builtinbackupengine.go" line:509 value:"resetting mysqld super_read_only to true"
commerce/0 (zone1-0000000102): time:{seconds:1732657021 nanoseconds:696651000} file:"builtinbackupengine.go" line:526 value:"restarting mysql replication"
E1126 15:37:02.757871   98619 main.go:56] rpc error: code = Unknown desc = TabletManager.Backup on zone1-0000000102: operation error S3: PutObject, https response error StatusCode: 0, RequestID: , HostID: , canceled, context canceled;operation error S3: PutObject, https response error StatusCode: 0, RequestID: , HostID: , canceled, context canceled;operation error S3: PutObject

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vitess-bot bot commented Nov 26, 2024

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@vitess-bot vitess-bot bot added NeedsBackportReason If backport labels have been applied to a PR, a justification is required NeedsDescriptionUpdate The description is not clear or comprehensive enough, and needs work NeedsIssue A linked issue is missing for this Pull Request NeedsWebsiteDocsUpdate What it says labels Nov 26, 2024
@github-actions github-actions bot added this to the v22.0.0 milestone Nov 26, 2024
@frouioui frouioui removed NeedsDescriptionUpdate The description is not clear or comprehensive enough, and needs work NeedsWebsiteDocsUpdate What it says NeedsIssue A linked issue is missing for this Pull Request NeedsBackportReason If backport labels have been applied to a PR, a justification is required labels Nov 26, 2024
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codecov bot commented Nov 26, 2024 •
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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.39%. Comparing base (8648264) to head (4cc10e5).
Report is 2 commits behind head on main.

Additional details and impacted files 
@@            Coverage Diff             @@
##             main   #17285      +/-   ##
==========================================
- Coverage   67.40%   67.39%   -0.02%     
==========================================
  Files        1574     1574              
  Lines      253205   253217      +12     
==========================================
- Hits       170676   170643      -33     
- Misses      82529    82574      +45

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shlomi-noach
shlomi-noach reviewed Nov 27, 2024
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Please see inline question - I unfortunately do not follow the logic.

go/vt/mysqlctl/builtinbackupengine.go
if finalErr != nil {
cancel()
}
}()
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@shlomi-noach shlomi-noach Nov 27, 2024 •
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I'm not sure I follow:

  • It looks as if the function can exit and still leave the context active, in which case, what is cancelling it?
  • If we still have operations in flight, why would we exit the function? Should we not wait until everything is complete?

Something feels off here, as an anti-pattern.

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The S3 and Ceph uploads may be incomplete by the time we return from this function. Writing to the buffers used by these storage implementation will be complete (which is how we're able to return from be.backupFile and be.backupFiles), but the actual reading from the buffer and uploading to the remote storage will/may not be complete. If we cancel the context too early, unfinished uploads will be canceled and marked as failed.

It is only at a later stage where we wait for all the uploads to be finished with the EndBackup method on the backup handle:

vitess/go/vt/mysqlctl/backup.go

Lines 190 to 191 in 16fa7a3

case BackupUsable:
finishErr = bh.EndBackup(ctx)

At this stage we will observe the failures created by the S3 or Ceph storage implementation and will decide to fail - even though we have already uploaded the backup (including the MANIFEST) and restarted MySQL.

There is definitely something off with this code. We must wait for the full backup (writing to the backend storage included) to complete before going forward with writing the MANIFEST and assuming the backup is useable. This is something that I am implementing along with a retry mechanism on: #17271.

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what is cancelling it?

The caller of ExecuteBackup will eventually cancel it, whether it is through a gRPC call or through vtbackup, the context always gets canceled.

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Thank you for the clarity! ❤️

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@frouioui How can it ever cancel it? ctx might be cancelled, but ctxCancel is not? That leads to a memory leak.

So even if the outer context is cancelled, we still need to cancel this inner one to avoid the memory leak.

@frouioui frouioui merged commit 0726ea6 into vitessio:main Nov 27, 2024
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@frouioui frouioui deleted the fix-context-cancel-builtin branch November 27, 2024 19:58
vitess-bot pushed a commit that referenced this pull request Nov 27, 2024
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Fix how we cancel the context in the builtin backup engine (#17285)
f31c28e 
Signed-off-by: Florent Poinsard <[email protected]>
@vitess-bot vitess-bot bot mentioned this pull request Nov 27, 2024
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deepthi pushed a commit that referenced this pull request Nov 28, 2024
@vitess-bot
[release-21.0] Fix how we cancel the context in the builtin backup en…
985a501 
…gine (#17285) (#17291)

Signed-off-by: Florent Poinsard <[email protected]>
Co-authored-by: vitess-bot[bot] <108069721+vitess-bot[bot]@users.noreply.github.com>
@frouioui frouioui mentioned this pull request Dec 2, 2024
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@dbussink dbussink dbussink left review comments

@rohit-nayak-ps rohit-nayak-ps rohit-nayak-ps approved these changes

@shlomi-noach shlomi-noach shlomi-noach approved these changes

@deepthi deepthi Awaiting requested review from deepthi deepthi is a code owner

@mattlord mattlord Awaiting requested review from mattlord mattlord is a code owner

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Backport to: release-21.0 Needs to be backport to release-21.0 Component: Backup and Restore Type: Bug
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