fibre_channel: fix crash when attempting to dereference invalid counters

[eaibmz]

The statistics counters of a disabled FC host cannot be read from sysfs and any read attempt fails with errno ENOENT. Therefore, all statistics counters returned by procfs are nil in such a case. This results in a crash on s390x Linux when a zFCP host is disabled with chzdev -d <host id>.

Crash stacktrace in a GDB

$ gdb /root/node_exporter/node_exporter
...
Thread 4 "node_exporter" received signal SIGSEGV, Segmentation fault. [Switching to LWP 35579]
0x00000000007be86c in github.com/prometheus/node_exporter/collector.(*fibrechannelCollector).Update (c=0xc00012fef0, ch=0xc0002fc8c0, ~r0=...)
    at /root/node_exporter/collector/fibrechannel_linux.go:133
133                     c.pushCounter(ch, "dumped_frames_total", *host.Counters.DumpedFrames, *host.Name)
(gdb) bt
    at /root/node_exporter/collector/fibrechannel_linux.go:133
(gdb) p host
$1 = {Name = 0xc00038faf0, Speed = 0xc00038fb00, PortState = 0xc00038fb10, PortType = 0xc00038fb30, SymbolicName = 0xc00038fba0, NodeName = 0xc00038fb40, PortID = 0xc00038fb50,
  PortName = 0xc00038fb60, FabricName = 0xc00038fb80, DevLossTMO = 0xc00038fb90, SupportedClasses = 0xc00038fbb0, SupportedSpeeds = 0xc00038fbd0, Counters = 0xc0001cb2d0}
(gdb) p host.Name
$2 = (string *) 0xc00038faf0
(gdb) p *host.Name
$3 = "host0"
(gdb) p *host.Counters.DumpedFrames
❌️ Cannot access memory at address 0x0
(gdb) p *host.Counters
$4 = {DumpedFrames = 0x0, ErrorFrames = 0x0, InvalidCRCCount = 0x0, RXFrames = 0x0, RXWords = 0x0, TXFrames = 0x0, TXWords = 0x0, SecondsSinceLastReset = 0x0, InvalidTXWordCount = 0x0,
  LinkFailureCount = 0x0, LossOfSyncCount = 0x0, LossOfSignalCount = 0x0, NosCount = 0x0, FCPPacketAborts = 0x0}
(gdb) bt
    at /root/node_exporter/collector/fibrechannel_linux.go:133
    name="fibrechannel", c=..., ch=0xc0002fc8c0, logger=0xc0002104a0)
    at /root/node_exporter/collector/collector.go:160
(gdb) p *host.Counters
$5 = {DumpedFrames = 0x0, ErrorFrames = 0x0, InvalidCRCCount = 0x0, RXFrames = 0x0,
  RXWords = 0x0, TXFrames = 0x0, TXWords = 0x0, SecondsSinceLastReset = 0x0,
  InvalidTXWordCount = 0x0, LinkFailureCount = 0x0, LossOfSyncCount = 0x0,
  LossOfSignalCount = 0x0, NosCount = 0x0, FCPPacketAborts = 0x0}
(gdb)

[discordianfish]

I think this should be fixed in procfs to return empty (non-nil) Counters

[eaibmz]

I think this should be fixed in procfs to return empty (non-nil) Counters

The PR prometheus/procfs#623 in particular introduced references to
to distinguish between non-existing values and 0 values.
I don't think we want it undone ?