[Bug 1764311] [NEW] KIO sftp high CPU usage after being left idle (endless loop?)

[Macros (Roger Rösch)]

Public bug reported:

Dolphin was left open overnight being connected via sftp://user@host:port/folder/path.
(Two targets, one continuously reachable under host:port, one went offline during the night)
Returning to the PC I found 4 sftp.so processes consuming 50% of a CPU core each.
They continued to do so after the folders in Dolphin were closed, and even after Dolphin itself was closed.
They responded to SIGTERM.

This is strace output from 1 of the processes, the same which apport was
used on:

ppoll([{fd=4, events=POLLIN}], 1, {tv_sec=0, tv_nsec=1000000}, NULL, 8) = 0 (Timeout)
write(3, "\1\0\0\0\0\0\0\0", 8)         = 8
ppoll([{fd=4, events=POLLIN}], 1, {tv_sec=0, tv_nsec=1000000}, NULL, 8) = 0 (Timeout)
write(3, "\1\0\0\0\0\0\0\0", 8)         = 8
ppoll([{fd=4, events=POLLIN}], 1, {tv_sec=0, tv_nsec=1000000}, NULL, 8) = 0 (Timeout)
write(3, "\1\0\0\0\0\0\0\0", 8)         = 8
ppoll([{fd=4, events=POLLIN}], 1, {tv_sec=0, tv_nsec=1000000}, NULL, 8) = 0 (Timeout)
write(3, "\1\0\0\0\0\0\0\0", 8)         = 8
ppoll([{fd=4, events=POLLIN}], 1, {tv_sec=0, tv_nsec=1000000}, NULL, 8) = 0 (Timeout)
write(3, "\1\0\0\0\0\0\0\0", 8)         = 8
(continues to repeat, seems like its stuck in a loop)

Kubuntu 18.04 Beta 2, kde-runtime (4:17.08.3-0ubuntu1)

What I expected to happen:
No high CPU usage, maybe no sftp.so process at all as the conenction was idle.

What happened instead:
4 sftp.so processes using 200% CPU of my quad core, consuming energy and shortening the lifespan of my system.

Feel free to contact me with questions, I can try to reproduces the problem.
Also I will leave one of the processes suspended for 1-3 days after filing this report, so I can send a SIGCONT and give further information on it if requested.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: kde-runtime 4:17.08.3-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu4
Architecture: amd64
Date: Mon Apr 16 09:54:24 2018
ExecutablePath: /usr/bin/kdeinit5
ProcEnviron:
 
SourcePackage: kde-runtime
UpgradeStatus: Upgraded to bionic on 2018-04-10 (5 days ago)

** Affects: kde-runtime (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug bionic

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Title:
  KIO sftp high CPU usage after being left idle (endless loop?)

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