Fibre Channel collector panics when sysfs statistics counters are missing

[itjustnathan]

Problem

The Fibre Channel collector can panic during a scrape when an FC host does not expose all expected statistics files under /sys/class/fc_host/<host>/statistics.

The procfs library represents unavailable counters as nil pointers, but the collector dereferences them unconditionally.

Panic

node_exporter[1070004]: panic: runtime error: invalid memory address or nil pointer dereference
node_exporter[1070004]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xa2f259]
node_exporter[1070004]: goroutine 89 [running]:
node_exporter[1070004]: github.com/prometheus/node_exporter/collector.(*fibrechannelCollector).Update(0xc00014cf90, 0xc00037fd50)
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/fibrechannel_linux.go:133 +0x479
node_exporter[1070004]: github.com/prometheus/node_exporter/collector.execute({0xc098f3, 0xc}, {0xd34e00, 0xc00014cf90}, 0xc00037fd50, 0xc00003e480)
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/collector.go:160 +0x82
node_exporter[1070004]: github.com/prometheus/node_exporter/collector.NodeCollector.Collect.func1({0xc098f3?, 0x0?}, {0xd34e00?, 0xc00014cf90?})
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/collector.go:151 +0x33
node_exporter[1070004]: created by github.com/prometheus/node_exporter/collector.NodeCollector.Collect in goroutine 60
node_exporter[1070004]:         /Volumes/Home/go/node_exporter/collector/collector.go:150 +0xce