Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()) by azat · Pull Request #105559 · ClickHouse/ClickHouse

azat · 2026-05-21T16:12:45Z

Changelog category (leave one):

Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()). Note with enabled SSH/web terminal it may crash the server

ASan: https://pastila.nl/?000a2333/ec0eeeeba6505d52399343e5b03ff041#zHm494mXb7GNci6N9jePGw==GCM
Refs:

Version info

Merged into: 26.6.1.116
Backported to: 26.5.2.5, 26.4.4.13, 26.3.13.9

clickhouse-gh · 2026-05-21T16:13:32Z

Workflow [PR], commit [a73bb97]

Summary: ❌

job_name	test_name	status	info
Stateless tests (arm_asan_ubsan, targeted)		FAIL
	02156_async_insert_query_log	FAIL	cidb
Unit tests (asan_ubsan, function_prop_fuzzer)		FAIL
	FunctionsStress.stress	FAIL	cidb, issue
	AllTests	FAIL	cidb
Finish Workflow		FAIL
	python3 ./ci/jobs/scripts/workflow_hooks/check_report_messages.py	FAIL

AI Review

Summary

This PR fixes high-fd terminal handling by switching select-based readiness checks to poll in both replxx::Terminal::wait_for_input (via contrib/replxx update) and LineReader::hasInputData, and adds a focused integration regression test for interactive SSH PTY behavior under large fd pressure. The previously raised issues (POLLNVAL handling and false-positive test assertion) are fixed in the current code, and I did not find any remaining correctness, safety, or evidence gaps.

Final Verdict

Status: ✅ Approve

…(replace with poll()) ASan: https://pastila.nl/?000a2333/ec0eeeeba6505d52399343e5b03ff041#zHm494mXb7GNci6N9jePGw==GCM Refs: ClickHouse/replxx#39

…() limitations

…ith_high_fds

When _hold_server_fds returns, the kernel has completed all 1500 TCP handshakes, but Poco's TCPServer has only one accept thread that does accept()+dispatch() per iteration. The remaining connections sit in the kernel accept queue with no userspace fd allocated, so an immediate /proc/<pid>/fd count sees only those accepted so far (~700 in the failing run). Verified with pdb: pausing after the failure and re-checking the count shows it grew to the expected ~1500. Poll the fd count until it crosses the threshold instead of reading it once, so the test does not race Poco's accept loop.

clickhouse-gh · 2026-05-24T06:57:29Z

LLVM Coverage Report

Metric	Baseline	Current	Δ
Lines	84.10%	84.20%	+0.10%
Functions	91.40%	91.40%	+0.00%
Branches	76.50%	76.50%	+0.00%

Changed lines: 100.00% (2/2) · Uncovered code

Full report · Diff report

azat · 2026-05-24T10:25:28Z

Since this has been already deployed to a few instances, I will merge it (CI failures are not related)

Cherry pick #105559 to 26.5: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

…ZE limitation of select() (replace with poll())

Backport #105559 to 26.5: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

Cherry pick #105559 to 25.8: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

…ZE limitation of select() (replace with poll())

Cherry pick #105559 to 26.3: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

…ZE limitation of select() (replace with poll())

Cherry pick #105559 to 26.4: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

…ZE limitation of select() (replace with poll())

Backport #105559 to 26.3: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

Backport #105559 to 26.4: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

…g FD_SETSIZE limitation of select() (replace with poll())

…5559 Backport ClickHouse#105559 to 26.4: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

clickhouse-gh Bot added pr-bugfix Pull request with bugfix, not backported by default submodule changed At least one submodule changed in this PR. labels May 21, 2026

Fix crash in replxx due to hitting FD_SETSIZE limitation of select() …

98ce7de

…(replace with poll()) ASan: https://pastila.nl/?000a2333/ec0eeeeba6505d52399343e5b03ff041#zHm494mXb7GNci6N9jePGw==GCM Refs: ClickHouse/replxx#39

azat force-pushed the fix-replxx-crash branch from 47bf353 to 98ce7de Compare May 21, 2026 16:14

clickhouse-gh Bot reviewed May 21, 2026

View reviewed changes

Comment thread contrib/replxx Outdated

alexey-milovidov added the pr-must-backport Pull request should be backported intentionally. Use this label with great care! label May 21, 2026

Replace one more place with select() to poll()

5d08b88

azat mentioned this pull request May 21, 2026

RFC: Replace replxx with rustyline #105590

Draft

tests: add test for SSH handler with fds>FD_SETSIZE to trigger select…

3951f98

…() limitations

clickhouse-gh Bot reviewed May 22, 2026

View reviewed changes

Comment thread tests/integration/test_ssh/test.py Outdated

azat added 5 commits May 22, 2026 12:51

Bump replxx to handle POLLNVAL

09d0ca4

tests: raise RLIMIT_NOFILE for integration tests

f8de9a6

tests: fix checking that query finished in test_ssh_interactive_pty_w…

74662c6

…ith_high_fds

tests: better test_ssh_interactive_pty_with_high_fds

69782ab

azat requested a review from alexey-milovidov May 23, 2026 11:13

Merge remote-tracking branch 'origin/master' into fix-replxx-crash

a73bb97

azat enabled auto-merge May 24, 2026 10:24

azat added this pull request to the merge queue May 24, 2026

Merged via the queue into ClickHouse:master with commit 02e4401 May 24, 2026
162 of 166 checks passed

azat deleted the fix-replxx-crash branch May 24, 2026 10:39

robot-ch-test-poll3 added the pr-must-backport-synced The `*-must-backport` labels are synced into the cloud Sync PR label May 24, 2026

robot-ch-test-poll4 added a commit that referenced this pull request May 24, 2026

Merge pull request #105723 from ClickHouse/cherrypick/26.5/105559

3150986

Cherry pick #105559 to 26.5: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

robot-clickhouse added a commit that referenced this pull request May 24, 2026

Backport #105559 to 26.5: Fix crash in replxx due to hitting FD_SETSI…

122c643

…ZE limitation of select() (replace with poll())

robot-ch-test-poll4 mentioned this pull request May 24, 2026

Backport #105559 to 26.5: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()) #105724

Merged

robot-clickhouse-ci-2 added the pr-synced-to-cloud The PR is synced to the cloud repo label May 24, 2026

azat added a commit that referenced this pull request May 24, 2026

Merge pull request #105724 from ClickHouse/backport/26.5/105559

1a355bc

Backport #105559 to 26.5: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

robot-clickhouse-ci-1 added a commit that referenced this pull request May 24, 2026

Merge pull request #105720 from ClickHouse/cherrypick/25.8/105559

e1b2e41

Cherry pick #105559 to 25.8: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

robot-clickhouse added a commit that referenced this pull request May 24, 2026

Backport #105559 to 25.8: Fix crash in replxx due to hitting FD_SETSI…

0fa9b45

…ZE limitation of select() (replace with poll())

robot-clickhouse-ci-1 mentioned this pull request May 24, 2026

Backport #105559 to 25.8: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()) #105744

Closed

robot-clickhouse-ci-1 added a commit that referenced this pull request May 24, 2026

Merge pull request #105721 from ClickHouse/cherrypick/26.3/105559

e02e9e9

Cherry pick #105559 to 26.3: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

robot-clickhouse added a commit that referenced this pull request May 24, 2026

Backport #105559 to 26.3: Fix crash in replxx due to hitting FD_SETSI…

303a54e

…ZE limitation of select() (replace with poll())

robot-clickhouse-ci-1 mentioned this pull request May 24, 2026

Backport #105559 to 26.3: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()) #105745

Merged

robot-clickhouse-ci-1 added a commit that referenced this pull request May 24, 2026

Merge pull request #105722 from ClickHouse/cherrypick/26.4/105559

b2903bf

Cherry pick #105559 to 26.4: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

robot-clickhouse added a commit that referenced this pull request May 24, 2026

Backport #105559 to 26.4: Fix crash in replxx due to hitting FD_SETSI…

8436fe7

…ZE limitation of select() (replace with poll())

robot-clickhouse-ci-1 mentioned this pull request May 24, 2026

Backport #105559 to 26.4: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()) #105746

Merged

robot-clickhouse-ci-2 added the pr-backports-created Backport PRs are successfully created, it won't be processed by CI script anymore label May 24, 2026

fm4v added a commit that referenced this pull request May 25, 2026

Merge pull request #105745 from ClickHouse/backport/26.3/105559

0fc43bb

Backport #105559 to 26.3: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

fm4v added a commit that referenced this pull request May 25, 2026

Merge pull request #105746 from ClickHouse/backport/26.4/105559

9aef6d7

Backport #105559 to 26.4: Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

DavidHe-2008 pushed a commit to DavidHe-2008/ClickHouse that referenced this pull request Jun 1, 2026

Merge pull request ClickHouse#105559 from azat/fix-replxx-crash

48be425

Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())

azat mentioned this pull request Jun 19, 2026

Mark select() as harmful (it is unsafe and suboptimal due to FD_SETSIZE) #107999

Merged

zeekay pushed a commit to hanzoai/datastore that referenced this pull request Sep 11, 2026

Backport ClickHouse#105559 to 26.4: Fix crash in replxx due to hittin…

6c8beee

…g FD_SETSIZE limitation of select() (replace with poll())

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Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll()) - #105559

Fix crash in replxx due to hitting FD_SETSIZE limitation of select() (replace with poll())#105559
azat merged 9 commits into
ClickHouse:masterfrom
azat:fix-replxx-crash

azat commented May 21, 2026 •

edited by robot-clickhouse

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azat commented May 21, 2026 •

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