Drill the network judge in a throwaway machine (hq ADR 0241)

The judge's tests run against files and fakes; the drill runs it against a
real resolver file and real resolvers, rewritten as the VPN client does,
and records the statements the controller's replay raises and clears from.

internal/network/drill_test.go

@@ -0,0 +1,101 @@
+package network
+
+import (
+	"encoding/json"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-host/internal/link"
+)
+
+// TestDrillAResolverFileRewrittenInAThrowawayMachine is the drill of novox/hq ADR 0241: on a machine of
+// its own — a throwaway container, never a machine of the mesh — the resolver file is what was declared,
+// then rewritten the way a VPN client rewrites it, then written back; the judge must say it healthy,
+// unhealthy on the second look naming the writer and the names it costs, and healthy again on the first
+// look after. Each statement is written, as the engine says it, to MESH_NETWORK_DRILL_OUT, for the
+// controller's replay of the same drill (mesh-controller testdata/network-drill.json).
+//
+// Run only where MESH_NETWORK_DRILL names the mesh name to ask, because it rewrites /etc/resolv.conf:
+//
+//	CGO_ENABLED=0 go test -c -o drill ./internal/network
+//	docker run --rm -v $PWD/drill:/drill:ro -v $PWD/out:/out -e MESH_NETWORK_DRILL=<mesh name> \
+//	    -e MESH_NETWORK_DRILL_OUT=/out/network-drill.json alpine /drill -test.run Drill -test.v
+//
+// The clock moves a look on per look; the file, the resolvers and the answers are the machine's own.
+func TestDrillAResolverFileRewrittenInAThrowawayMachine(t *testing.T) {
+	meshName := os.Getenv("MESH_NETWORK_DRILL")
+	if meshName == "" {
+		t.Skip("a drill rewrites /etc/resolv.conf: run it in a throwaway container with MESH_NETWORK_DRILL=<mesh name>")
+	}
+	declared, err := os.ReadFile(ResolvConf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	now := time.Now()
+	j := New(Machine{Now: func() time.Time { return now }, Linked: func() bool { return true }}, meshName)
+	j.Declare(string(declared), "networkmanager", true)
+
+	var said []link.Health
+	look := func(want string) Statement {
+		t.Helper()
+		now = now.Add(LookEvery)
+		st, _ := j.Look(t.Context())
+		h := link.Health{Contract: link.ReadinessContract, At: st.At.UTC(), Resources: []link.ResourceHealth{},
+			Network: &link.NetworkHealth{State: st.State, Since: st.Since.UTC(), Parts: []link.NetworkPart{}}}
+		for _, p := range st.Parts {
+			h.Network.Parts = append(h.Network.Parts, link.NetworkPart{Part: p.Part, State: p.State, Reason: p.Reason,
+				Said: p.Said, Writer: p.Writer, Owner: p.Owner, Toward: p.Toward, Since: p.Since.UTC(), Streak: p.Streak})
+		}
+		said = append(said, h)
+		raw, _ := json.Marshal(st)
+		t.Logf("%s", raw)
+		if st.State != want {
+			t.Fatalf("want %s, the judge says %s", want, st.State)
+		}
+		return st
+	}
+	write := func(content string) {
+		t.Helper()
+		// In place, as the VPN client's rename leaves a file of the same name: the judge reads by path.
+		if err := os.WriteFile(ResolvConf, []byte(content), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	defer write(string(declared))
+
+	look(Healthy)
+	look(Healthy)
+	// What the VPN client writes on connect: its header, its own resolvers — addresses that answer
+	// nothing here — and its search domains.
+	write("# Dynamic resolv.conf(5) file for glibc resolver(3) generated by forticlient\n" +
+		"# The original file is backed up and will be restored after the VPN disconnects.\n" +
+		"nameserver 192.0.2.53\nsearch corp.example\noptions timeout:1\n")
+	look(Healthy) // one look is not a finding
+	st := look(Unhealthy)
+	for _, p := range st.Parts {
+		switch p.Part {
+		case PartResolvConf:
+			if p.State != Unhealthy || p.Writer != "FortiClient" || p.Owner != "networkmanager" {
+				t.Fatalf("the file is said %+v", p)
+			}
+		case PartNames:
+			if p.State != Unhealthy || !strings.Contains(p.Said, "192.0.2.53") {
+				t.Fatalf("the names through it are said %+v", p)
+			}
+		}
+	}
+	write(string(declared))
+	look(Healthy)
+
+	if out := os.Getenv("MESH_NETWORK_DRILL_OUT"); out != "" {
+		raw, err := json.MarshalIndent(said, "", "  ")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(out, raw, 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+}