ipn/conffile: add ErrNoConfig for absent config sources

Load wraps read-phase failures with it so callers can distinguish a
missing config from an invalid one.

Updates #1866

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

ipn/conffile/conffile_test.go

@@ -0,0 +1,60 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package conffile
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestLoadAbsentIsErrNoConfig verifies that an absent config source (here a
+// missing file, the same read-phase seam the "vm:user-data" 404 goes through)
+// is reported as ErrNoConfig, so callers using tailscaled's "optional:" prefix
+// can boot unconfigured instead of failing.
+func TestLoadAbsentIsErrNoConfig(t *testing.T) {
+	_, err := Load(filepath.Join(t.TempDir(), "does-not-exist.json"))
+	if !errors.Is(err, ErrNoConfig) {
+		t.Fatalf("Load(missing) error = %v; want it to wrap ErrNoConfig", err)
+	}
+}
+
+// TestLoadInvalidIsNotErrNoConfig verifies that a config that is present but
+// invalid is NOT ErrNoConfig, so it stays fatal even in optional mode.
+func TestLoadInvalidIsNotErrNoConfig(t *testing.T) {
+	for name, data := range map[string]string{
+		"malformed":       `{"Version": "alpha0"`, // truncated JSON
+		"unknown-version": `{"Version": "bogus9"}`,
+		"unknown-field":   `{"Version": "alpha0", "Bogus": true}`,
+	} {
+		t.Run(name, func(t *testing.T) {
+			p := filepath.Join(t.TempDir(), "bad.json")
+			if err := os.WriteFile(p, []byte(data), 0600); err != nil {
+				t.Fatal(err)
+			}
+			_, err := Load(p)
+			if err == nil {
+				t.Fatalf("Load(%q) succeeded; want an error", data)
+			}
+			if errors.Is(err, ErrNoConfig) {
+				t.Fatalf("Load(%q) error = %v; want a parse error, not ErrNoConfig", data, err)
+			}
+		})
+	}
+}
+
+func TestLoadValid(t *testing.T) {
+	p := filepath.Join(t.TempDir(), "ok.json")
+	if err := os.WriteFile(p, []byte(`{"Version": "alpha0"}`), 0600); err != nil {
+		t.Fatal(err)
+	}
+	c, err := Load(p)
+	if err != nil {
+		t.Fatalf("Load(valid) error = %v; want nil", err)
+	}
+	if c.Version != "alpha0" {
+		t.Fatalf("Version = %q; want alpha0", c.Version)
+	}
+}