fix(go.d/isc_dhcpd): guard against out-of-range slice when parsing leases - #24122

Problem

parseDHCPdLeasesFile() in src/go/plugin/go.d/collector/isc_dhcpd/parse.go
matches lines by a loose prefix (lease, iaaddr, binding state) and then
slices a fixed number of bytes off each end to recover the value, assuming
the line is always long enough:

case !l.isAddrValid() && bytes.HasPrefix(bs, []byte("lease")):
    s := string(bs)
    if addr, err := netip.ParseAddr(s[6 : len(s)-2]); err == nil && addr.IsValid() {
        l.addr = addr
    }

dhcpd.leases is rewritten/appended by the dhcpd daemon, so the file can be
truncated mid-write at EOF (daemon killed, disk full, process crash). A
truncated line can still satisfy the prefix check while being too short for
the slice — e.g. a line cut short to lease 1 or iaaddr 2. The slice then
panics:

panic: runtime error: slice bounds out of range [6:5]

This does not crash the collector. go.d's job runtime
(src/go/plugin/framework/jobruntime/job_v1.go) already recovers panics in
both Job.collect() and Job.autoDetection(). What actually happens instead:

• During collection: the panic is recovered every cycle, logged as
  PANIC: ..., and the entire leases file yields zero metrics for that
  cycle — including any otherwise well-formed leases in the same file.
• During Check()/autodetection: the panic disables autodetection
  (disableAutoDetection()), so the job never starts at all.

So the practical impact is silent data loss on an already-running job, or a
job that silently never starts — not a crash. Still worth fixing: a
truncated/malformed leases file otherwise leaves the collector permanently
blind to all leases until the file is fixed or the agent is restarted.

Fix

Added cutAffixes(), a single bounds-checked helper shared by the three
call sites (lease address, iaaddr address, binding state), that skips the
line when it is too short to hold the expected value instead of panicking.

Testing

Added parse_test.go with malformed-line cases simulating a leases file
truncated mid-write (e.g. lease 1, iaaddr 2, a cut-off binding state
line) that reproduce the panic on the old code and pass after the fix, plus
a well-formed case. Ran the full existing isc_dhcpd package test suite
locally (temporarily lifting the darwin build exclusion to run on macOS) —
all pass with no behavior change for well-formed leases files.

Summary by cubic

Fixes the isc_dhcpd leases parser panicking when dhcpd.leases contains lines truncated mid-write (daemon killed, disk full, crash). Previously the framework-recovered panic hid the issue but made the job report zero metrics for the whole file or prevented it from starting; now malformed lines are skipped and well-formed leases still parse.

Bug Fixes

• Adds a bounds-checked cutAffixes() helper shared by the three slice call sites to skip too-short lines instead of panicking.
• Adds tests for truncated lease, iaaddr, and binding state lines plus a well-formed case.

^{Written for commit 52724c7. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of truncated DHCP lease entries to prevent parsing failures and ensure malformed lines are skipped safely.
  • Valid lease addresses and binding states continue to be parsed correctly.