Fail Faster
For a long time I have heard complaints that Xdebug, with its debugger enabled, slows down applications, even if there is no IDE listening.
I have never been able to reproduce this, as on Linux (and macOS), TCP connections to a localhost ports fail immediately if that port is not open.
Xdebug waits for up to 200ms when making a debug connection. This is configurable with the xdebug.connect_timeout_ms setting.
But if there is nothing listening, there should not be any delay, as it ought to be an immediate error.
I wrote a comment in an issue from 2024:
The error message
ERR: Time-out connecting to debugging client, waited: 200 ms.indicates that your network stack doesn't immediately reject the connection if it can't be made. That's usually due to a (misconfigured) firewall.
It turns out, that it is not a firewall, but that Windows itself behaves bad here. Instead of failing immediately, it does retry sending TCP SYN packets to try to establish a connection. This is pointless on the local loopback interface, as network packets cannot get lost.
Through a post by Daniel Stenberg on Mastodon, I found out the cause, and a solution.
He explains how it affects curl's Happy Eyeballs algorithm in a blog post from 2024. Although Xdebug does not use Happy Eyeballs, the underlying problem is the same.
Last week, Marcel Jamin wrote to the curl mailing list, with a solution. This solution disables the retry mechanism to establish a connection. And in turn that means, that there is no longer a delay on Windows either.
I have today merged a pull request for Xdebug which implements this solution. The patch is part of the upcoming Xdebug 3.6 release.
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@blog I think it probably is a firewall issue really, unless Windows is ignoring the RST packets that tell it nothing is listening.
That's what happens on linux - the SYN is sent and gets an RST and that is reported as a connection failure with EPORTUNREACH but if you firewall loopback so it doesn't get the RST then you get the Windows behaviour of it retrying SYN packets.
There's nothing special about the loopback handling in linux.
Life Line
Great Egret Has Caught a Fish
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#photography #Birds #BirdPhotography #BirdsOfFediverse #NaturePhotography #wildlife #wwt #Arundel
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#photography #Birds #BirdPhotography #BirdsOfFediverse #NaturePhotography #wildlife #Arundel #WWT
I hiked 6.6km in 5h47m41s
A Juvenile Great Crested Grebe Having Lunch
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#photography #Birds #BirdPhotography #BirdsOfFediverse #NaturePhotography #wildlife #london
I walked 6.8km in 1h4m56s
Stonechat on a Branch
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#photography #Birds #BirdPhotography #BirdsOfFediverse #NaturePhotography #wildlife
I walked 1.4km in 34m31s
Merge branch 'v2022'
Update data to 2026f
Updated a building
Specify cycleways width
Survey if vacant places are still vacant
Specify whether places take fees to visit
Specify maximum allowed weights
Specify whether pedestrian crossings have islands
Specify whether kerbs have tactile paving
Determine the heights of kerbs
Specify street/place names to addresses
Survey housenumbers
Specify whether there are crossings at intersections of paths and roads
Specify whether traffic signals have sound signals
Specify whether traffic signals have tactile indications that it's safe to cross
Specify whether public transport stops have tactile paving
Specify whether public transport stops have bins
Specify path surfaces
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