linux: allow %d anywhere in the tun.dev template

overlay/tun_linux_test.go

@@ -3,7 +3,12 @@
 
 package overlay
 
-import "testing"
+import (
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
 
 var runAdvMSSTests = []struct {
 	name     string
@@ -32,3 +37,91 @@ func TestTunAdvMSS(t *testing.T) {
 		})
 	}
 }
+
+func nameSet(names ...string) map[string]struct{} {
+	used := make(map[string]struct{}, len(names))
+	for _, n := range names {
+		used[n] = struct{}{}
+	}
+	return used
+}
+
+func TestValidateTunName(t *testing.T) {
+	// A device name must be shorter than IFNAMSIZ (i.e. IFNAMSIZ-1 chars max).
+	maxLenName := strings.Repeat("a", unix.IFNAMSIZ-1)
+
+	tests := []struct {
+		name    string
+		tmpl    string
+		wantErr bool
+	}{
+		{"short literal name is fine", "nebula1", false},
+		{"literal name at the max length is fine", maxLenName, false},
+		{"literal name at IFNAMSIZ is rejected", strings.Repeat("a", unix.IFNAMSIZ), true},
+		{"trailing template is fine", "nebula%d", false},
+		{"mid-string template is fine", "neb%dprod", false},
+		{"leading template is fine", "%dnebula", false},
+		{"template at the max static length is fine", strings.Repeat("a", unix.IFNAMSIZ-2) + "%d", false},
+		{"bare %d is rejected", "%d", true},
+		{"multiple %d is rejected", "neb%d%dprod", true},
+		{"template with no room for a digit is rejected", strings.Repeat("a", unix.IFNAMSIZ-1) + "%d", true},
+		{"mid-string template with no room for a digit is rejected", "neb%d" + strings.Repeat("a", unix.IFNAMSIZ-3), true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateTunName(tt.tmpl)
+			if tt.wantErr && err == nil {
+				t.Fatalf("expected an error for %q, got none", tt.tmpl)
+			}
+			if !tt.wantErr && err != nil {
+				t.Fatalf("unexpected error for %q: %v", tt.tmpl, err)
+			}
+		})
+	}
+}
+
+func TestNextTunName(t *testing.T) {
+	// A prefix long enough that only single-digit suffixes (0-9) fit within
+	// IFNAMSIZ, so marking all ten used exercises running out of names.
+	longPrefix := strings.Repeat("a", unix.IFNAMSIZ-2)
+	longUsed := make([]string, 0, 10)
+	for i := 0; i < 10; i++ {
+		longUsed = append(longUsed, longPrefix+string(rune('0'+i)))
+	}
+
+	tests := []struct {
+		name    string
+		tmpl    string
+		used    map[string]struct{}
+		want    string
+		wantErr bool
+	}{
+		{"nothing used picks zero", "nebula%d", nil, "nebula0", false},
+		{"skips taken names", "nebula%d", nameSet("nebula0", "nebula1"), "nebula2", false},
+		{"picks the lowest free index", "nebula%d", nameSet("nebula0", "nebula2"), "nebula1", false},
+		{"ignores unrelated names", "nebula%d", nameSet("eth0", "tun5"), "nebula0", false},
+		{"mid-string placeholder picks zero", "neb%dprod", nil, "neb0prod", false},
+		{"mid-string placeholder skips taken", "neb%dprod", nameSet("neb0prod", "neb1prod"), "neb2prod", false},
+		{"leading placeholder picks zero", "%dnebula", nameSet("tun0"), "0nebula", false},
+		{"runs out of names within IFNAMSIZ", longPrefix + "%d", nameSet(longUsed...), "", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := nextTunName(tt.tmpl, tt.used)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected an error, got name %q", got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}