Add reporting of purgeable space on MacOS

[ledeuns]

Purgeable space is space that macOS can free up when needed by removing files from your computer (you can't manually remove the files that are designated purgeable, but macOS removes them as space is required). So statfs can report low available space when in fact you can use more than that.

Tested on MacOS 14 (x86+M2) and Ubuntu 22.04
Needs to be started as root to get purgeable space info (otherwise it returns 0)

[discordianfish]

Needs to be started as root to get purgeable space info (otherwise it returns 0)

We don't support metrics that require root: https://github.com/prometheus/node_exporter/blob/master/CONTRIBUTING.md#collector-implementation-guidelines
If you find another way, that doesn't require root, I'm happy to revisit this because this does seem helpful

[ledeuns]

I remove that comment, it came from an older version of the diff 😉 Running the node_exporter with a regular user on my test machines report available space correctly.

[discordianfish]

Thanks!

[tickez]

It appears that this feature creates an issue with latest MacOS 15.3.2.
I get segmentation faults both in Intel and Apple Silicon Macs. Runs fine with 15.3
Disabling the filesystem collector resolves the crash.

SIGSEGV: segmentation violation
PC=0x19af21ff8 m=0 sigcode=2 addr=0x0
signal arrived during cgo execution

goroutine 63 gp=0x14000093c00 m=0 mp=0x100da2a80 [syscall]:
runtime.cgocall(0x1006c37c0, 0x1400017b418)
	runtime/cgocall.go:167 +0x44 fp=0x1400017b3e0 sp=0x1400017b3a0 pc=0x1001a0e34
github.com/prometheus/node_exporter/collector._Cfunc_purgeable(0x600001d54f00)
	_cgo_gotypes.go:731 +0x34 fp=0x1400017b410 sp=0x1400017b3e0 pc=0x1006708b4
github.com/prometheus/node_exporter/collector.(*filesystemCollector).GetStats(0x1400023bdc0)
	github.com/prometheus/node_exporter/collector/filesystem_macos.go:103 +0x338 fp=0x1400017b630 sp=0x1400017b410 pc=0x1006713a8
github.com/prometheus/node_exporter/collector.(*filesystemCollector).Update(0x1400023bdc0, 0x1400011f810)
	github.com/prometheus/node_exporter/collector/filesystem_common.go:186 +0x2c fp=0x1400017bd90 sp=0x1400017b630 pc=0x10066526c
github.com/prometheus/node_exporter/collector.execute({0x1006cbc7d, 0xa}, {0x100928dc0, 0x1400023bdc0}, 0x1400011f810, 0x1400011c430)
	github.com/prometheus/node_exporter/collector/collector.go:160 +0x60 fp=0x1400017bf50 sp=0x1400017bd90 pc=0x100663920
github.com/prometheus/node_exporter/collector.NodeCollector.Collect.func1({0x1006cbc7d?, 0x0?}, {0x100928dc0?, 0x1400023bdc0?})
	github.com/prometheus/node_exporter/collector/collector.go:151 +0x38 fp=0x1400017bfa0 sp=0x1400017bf50 pc=0x100663888
github.com/prometheus/node_exporter/collector.NodeCollector.Collect.gowrap1()
	github.com/prometheus/node_exporter/collector/collector.go:153 +0x3c fp=0x1400017bfd0 sp=0x1400017bfa0 pc=0x10066381c
runtime.goexit({})
	runtime/asm_arm64.s:1223 +0x4 fp=0x1400017bfd0 sp=0x1400017bfd0 pc=0x1001abf54
created by github.com/prometheus/node_exporter/collector.NodeCollector.Collect in goroutine 56
	github.com/prometheus/node_exporter/collector/collector.go:150 +0xac