goipgrep: refactor into module; pure-Go ping/resolve; cache+CI; drop binary

projects/go-tools/go/goipgrep/internal/resolve/neigh_linux.go

@@ -0,0 +1,75 @@
+//go:build linux
+
+package resolve
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"strings"
+
+	"git.ktf.ninja/tabledevil/gists/projects/go-tools/go/goipgrep/internal/normalize"
+)
+
+type NeighborTable struct {
+	ByMAC map[string][]string
+}
+
+func LoadNeighborTable() (*NeighborTable, error) {
+	f, err := os.Open("/proc/net/arp")
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return parseProcNetARP(f), nil
+}
+
+func ResolveMACs(macs []string) ([]MACResolution, error) {
+	tab, err := LoadNeighborTable()
+	if err != nil {
+		return nil, err
+	}
+	var res []MACResolution
+	for _, mac := range macs {
+		res = append(res, MACResolution{MAC: mac, IPs: append([]string(nil), tab.ByMAC[mac]...)})
+	}
+	return res, nil
+}
+
+type MACResolution struct {
+	MAC string   `json:"mac"`
+	IPs []string `json:"ips,omitempty"`
+}
+
+func parseProcNetARP(r io.Reader) *NeighborTable {
+	nt := &NeighborTable{ByMAC: make(map[string][]string)}
+	sc := bufio.NewScanner(r)
+	first := true
+	for sc.Scan() {
+		line := strings.TrimSpace(sc.Text())
+		if line == "" {
+			continue
+		}
+		if first {
+			// header
+			first = false
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) < 4 {
+			continue
+		}
+		ip := fields[0]
+		macRaw := fields[3]
+		mac, ok := normalize.NormalizeMAC(macRaw)
+		if !ok {
+			continue
+		}
+		// Skip incomplete/placeholder entries.
+		if mac == "00:00:00:00:00:00" {
+			continue
+		}
+		nt.ByMAC[mac] = append(nt.ByMAC[mac], ip)
+	}
+	return nt
+}