native: the seed goes down as the word the firmware reads

internal/firmware/native/native.go

@@ -108,6 +108,31 @@ func (n *Native) PID() int {
 	return n.cmd.Process.Pid
 }
 
+// args is the child's command line.
+//
+// The seed goes down as its low 32 bits. That is all the firmware ever read:
+// the bridge parses --seed with strtoul into a uint32_t, and where a long is
+// 64 bits the cast keeps the low word. Where a long is 32 bits - every Windows
+// target - strtoul saturates at ULONG_MAX instead, so every node whose seed
+// was above 2^32 was seeded 0xFFFFFFFF, and 57 of 58 nodes booted with one
+// identity, private key included. Node 0's seed is the run seed, which fits,
+// and was the one node apart. Masking here gives every platform the low
+// word Linux and macOS always had, so no identity changes where the mesh
+// already worked, and the published binaries need no rebuild to be right.
+func (n *Native) args(bridgeAddr string) []string {
+	args := []string{"--bridge", bridgeAddr, "--seed", fmt.Sprint(uint32(n.Seed))}
+	if n.SF != 0 {
+		args = append(args, "--sf", fmt.Sprint(n.SF))
+	}
+	if n.BandwidthKHz != 0 {
+		args = append(args, "--bw-khz", strconv.FormatFloat(n.BandwidthKHz, 'f', -1, 64))
+	}
+	if n.CodingRate != 0 {
+		args = append(args, "--cr", fmt.Sprint(n.CodingRate))
+	}
+	return args
+}
+
 func (n *Native) Start(ctx context.Context, bridgeAddr string) (err error) {
 	path, err := firmware.FindNative(n.Path, n.Role)
 	if err != nil {
@@ -118,17 +143,7 @@ func (n *Native) Start(ctx context.Context, bridgeAddr string) (err error) {
 	if n.cmd != nil {
 		return errors.New("firmware: native node already started")
 	}
-	args := []string{"--bridge", bridgeAddr, "--seed", fmt.Sprint(n.Seed)}
-	if n.SF != 0 {
-		args = append(args, "--sf", fmt.Sprint(n.SF))
-	}
-	if n.BandwidthKHz != 0 {
-		args = append(args, "--bw-khz", strconv.FormatFloat(n.BandwidthKHz, 'f', -1, 64))
-	}
-	if n.CodingRate != 0 {
-		args = append(args, "--cr", fmt.Sprint(n.CodingRate))
-	}
-	cmd := exec.CommandContext(ctx, path, args...)
+	cmd := exec.CommandContext(ctx, path, n.args(bridgeAddr)...)
 	// The same attributes the emulated backend gives its children: a
 	// parent-death signal where the platform has one, and no console window
 	// on Windows, where every node is a console program and a national

internal/firmware/native/seed_internal_test.go

@@ -0,0 +1,46 @@
+package native
+
+import (
+	"math"
+	"strconv"
+	"testing"
+)
+
+// The engine strides node seeds by a 64-bit golden-ratio constant, so every
+// node past the first carries a seed above 2^32. The bridge parses --seed with
+// strtoul into a uint32_t, and on Windows, where a long is 32 bits, strtoul
+// saturates: 57 of 58 nodes were seeded 0xFFFFFFFF and booted with one
+// identity, private key included. What goes down the command line has to be
+// a number that parser cannot saturate on, and it has to be the low word
+// Linux and macOS were already keeping, so nothing changes where it worked.
+func TestTheSeedOnTheCommandLineFitsTheFirmwaresParser(t *testing.T) {
+	// The engine's stride, as a variable so the arithmetic wraps the way the
+	// engine's does rather than failing as a constant expression.
+	var stride uint64 = 0x9E3779B97F4A7C15
+	seeds := []uint64{4417, 9001 + 1*stride, 9001 + 57*stride, math.MaxUint64}
+	for _, seed := range seeds {
+		want := seed & math.MaxUint32
+		n := &Native{Seed: seed}
+		args := n.args("127.0.0.1:1")
+		var got string
+		for i, a := range args {
+			if a == "--seed" && i+1 < len(args) {
+				got = args[i+1]
+			}
+		}
+		v, err := strconv.ParseUint(got, 10, 32)
+		if err != nil {
+			t.Fatalf("seed %d went down as %q, which a 32-bit parser refuses: %v", seed, got, err)
+		}
+		if v != want {
+			t.Errorf("seed %d went down as %d, want its low word %d", seed, v, want)
+		}
+	}
+	// And two nodes of one run still get two seeds: the stride's low word is
+	// as distinct as its high one.
+	a := (&Native{Seed: 9001 + 1*stride}).args("x")[3]
+	b := (&Native{Seed: 9001 + 2*stride}).args("x")[3]
+	if a == b {
+		t.Fatalf("two nodes went down with one seed %s", a)
+	}
+}