Draft: Forkless Snapshot (Threadsave)

[nitaicaro]

Draft: Thread-based RDB snapshot (Threadsave)

Motivation

Fork-based BGSAVE relies on fork(), which on large datasets causes significant memory overhead from copy-on-write page duplication. On systems where memory is constrained, fork can fail entirely. Threadsave provides an alternative that avoids fork by performing the RDB snapshot on a background thread while the main thread continues serving clients.

How it works

Threadsave builds on the bgIteration framework (introduced in #3553). bgIteration walks the keyspace on the main thread, delivering keys to a FIFO queue. A background thread consumes entries from the queue and serializes them to an RDB file using the standard rdbSaveKeyValuePair encoding. The result is a normal RDB file - loadable by any Valkey instance.

Usage

A thread-based snapshot can be triggered in three ways:

• BGSAVE THREAD - explicit command (requires forkless-options-supported yes)
• config set threadsave-enabled-for-backup yes - makes plain BGSAVE use threadsave
• Automatic periodic saves will use threadsave when threadsave-enabled-for-backup is enabled

BGSAVE FORK explicitly forces a fork-based snapshot regardless of configuration. BGSAVE SCHEDULE THREAD / BGSAVE SCHEDULE FORK schedule a snapshot with explicit type.

Draft status

This PR is marked as draft because it includes three cherry-picked commits (the pending_command lifecycle fix, the findDbForEntry NULL check, and the hashtable bucket-chain iteration fix) that belong in the underlying bgIteration and blocked-inuse branches. Once those patches are merged into their respective base branches, this PR will be rebased and those commits will disappear from the diff. The threadsave-specific changes should mostly remain the same.

[codecov]

Codecov Report

❌ Patch coverage is 83.68421% with 62 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.98%. Comparing base (281d627) to head (fa3301a).

Files with missing lines	Patch %	Lines
src/threadsave.c	82.81%	33 Missing ⚠️
src/rdb.c	88.59%	13 Missing ⚠️
src/server.c	78.18%	12 Missing ⚠️
src/module.c	0.00%	4 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           forkless-pre-threadsave    #3648      +/-   ##
===========================================================
+ Coverage                    76.76%   76.98%   +0.22%     
===========================================================
  Files                          161      162       +1     
  Lines                        81423    81712     +289     
===========================================================
+ Hits                         62502    62906     +404     
+ Misses                       18921    18806     -115     

Files with missing lines	Coverage Δ
src/bgiteration.c	82.24% <100.00%> (+12.77%)	⬆️
src/blocked.c	92.27% <ø> (+0.21%)	⬆️
src/commands.def	100.00% <ø> (ø)
src/config.c	78.33% <ø> (ø)
src/db.c	94.61% <100.00%> (+0.29%)	⬆️
src/hashtable.c	97.86% <100.00%> (ø)
src/networking.c	92.23% <100.00%> (+0.09%)	⬆️
src/rdb.h	100.00% <ø> (ø)
src/replication.c	86.17% <100.00%> (-0.03%)	⬇️
src/server.h	100.00% <ø> (ø)
... and 4 more

... and 18 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[coderabbitai]

Important

Review skipped

Ignore keyword(s) in the title.

⛔ Ignored keywords (1)

• draft

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 8831f7e4-6ed2-4266-a282-ff8a22daf00f

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}