Stage and install the seed zone file in one exec

Quarantine and write were separate round-trips, so a failure between them
left the PVC with no zone data and the next reconcile reseeded at serial 1.
The write also truncated the destination in place, leaving a torn file that
PlanSeed refuses to touch.

internal/bind/seed.go

@@ -54,8 +54,7 @@ ns1 IN A %s
 // EnsureSeedZone makes path loadable without discarding live data: it probes
 // the zone file and journal, moves aside whatever cannot load, and writes a
 // skeleton only when there is nothing to preserve. Every caller that needs a
-// zone database file on disk goes through here: writeSeedZone is unexported so
-// the destructive write cannot be reached without a plan.
+// zone database file on disk goes through here.
 func (e *Executor) EnsureSeedZone(ctx context.Context, namespace, pod, zone, path, primaryIP string) error {
 	state, err := e.ZoneDiskState(ctx, namespace, pod, path)
 	if err != nil {
@@ -65,27 +64,43 @@ func (e *Executor) EnsureSeedZone(ctx context.Context, namespace, pod, zone, pat
 	if plan.Blocked != "" {
 		return fmt.Errorf("seed zone %s: %s", zone, plan.Blocked)
 	}
-	if err := e.Quarantine(ctx, namespace, pod, path, plan); err != nil {
-		return err
-	}
 	if !plan.WriteSeed {
-		return nil
+		return e.Quarantine(ctx, namespace, pod, path, plan)
 	}
-	return e.writeSeedZone(ctx, namespace, pod, zone, path, primaryIP, plan.Serial)
-}
-
-// writeSeedZone writes a seed zone file to path, creating parent directories.
-// It overwrites any existing file unconditionally.
-func (e *Executor) writeSeedZone(ctx context.Context, namespace, pod, zone, path, primaryIP string, serial int64) error {
-	content := renderSeedZone(zone, primaryIP, serial)
-	q := shellQuote(path)
-	cmd := []string{"sh", "-c", fmt.Sprintf("mkdir -p \"$(dirname %s)\" && cat > %s", q, q)}
+	content := renderSeedZone(zone, primaryIP, plan.Serial)
+	cmd := []string{"sh", "-c", seedScript(path, plan, len(content))}
 	if out, err := e.Exec(ctx, namespace, pod, cmd, content); err != nil {
 		return fmt.Errorf("seed zone %s: %w (out: %s)", zone, err, out)
 	}
 	return nil
 }
 
+// seedTempSuffix names the staging file, a sibling of the zone file so the
+// install is a same-filesystem rename.
+const seedTempSuffix = ".seed-tmp"
+
+// seedScript stages the skeleton, checks it arrived whole, then quarantines and
+// installs it in that order. Doing all of it in one exec keeps an interrupted
+// seed from leaving the PVC with no zone data, which the next reconcile would
+// read as a fresh install and reseed at serial 1; the rename means a torn write
+// is never visible at path.
+func seedScript(path string, plan SeedPlan, size int) string {
+	q, tmp := shellQuote(path), shellQuote(path+seedTempSuffix)
+	cmds := []string{
+		fmt.Sprintf("mkdir -p \"$(dirname %s)\"", q),
+		fmt.Sprintf("cat > %s", tmp),
+		// A stdin stream cut mid-transfer gives cat a short file and exit 0.
+		fmt.Sprintf("if [ \"$(wc -c < %s | tr -d ' \\n')\" -ne %d ]; then rm -f %s; exit 1; fi", tmp, size, tmp),
+	}
+	if plan.QuarantineZoneFile {
+		cmds = append(cmds, moveAside(path, plan.QuarantineSuffix))
+	}
+	if plan.QuarantineJournal {
+		cmds = append(cmds, moveAside(JournalPath(path), plan.QuarantineSuffix))
+	}
+	return strings.Join(append(cmds, fmt.Sprintf("mv -- %s %s", tmp, q)), "\n")
+}
+
 // AddCatalogMember registers a member zone in a catalog zone by adding the
 // catalog PTR record, so secondaries auto-provision it.
 func (e *Executor) AddCatalogMember(ctx context.Context, namespace, pod, catalogZone, memberZone string, creds TSIGCreds) error {