x/tools/gopls: gcStart crashes while calling finishsweep_m on system stack

[adonovan]

#!stacks
"systemstack_switch" && "runtime.gcStart:+108"

Issue created by stacks.

This stack r60HUA was reported by telemetry:

• crash/crash
• runtime.systemstack_switch:+0,+0x7
• runtime.gcStart:+108,+0x41f
• runtime.forcegchelper:+15,+0x5b
• runtime.goexit:+0,+0x3

golang.org/x/tools/gopls@v0.20.0 go1.25.1 darwin/arm64 other,vscode (1)

@prattmic: not sure whether this interesting or just another symptom of memory corruption. There's not much to go on.
Dups: RePuQg

[prattmic]

Presumably there are more potentially interesting frames on the system stack, but telemetry doesn't capture them?

[adonovan]

Presumably there are more potentially interesting frames on the system stack, but telemetry doesn't capture them?

Telemetry reports only the crashing (running) goroutine. I remember that unwinding of systemstack_switch is special but I can't remember the details.

[adonovan]

This stack RePuQg was reported by telemetry:

• crash/crash
• runtime.systemstack_switch:+4,+0x7
• runtime.gcStart:+108,+0x415
• runtime.forcegchelper:+15,+0x67
• runtime.goexit:+0,+0x0

golang.org/x/tools/gopls@v0.20.0 go1.25.0 linux/amd64 vscode (1)

[adonovan]

The lack of systemstack traces is fixed by https://go.dev/cl/789880.