doctor/permissions: add new check to print process permissions

Since users can run tailscaled in a variety of ways (root, non-root,
non-root with process capabilities on Linux), this check will print the
current process permissions to the log to aid in debugging.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ida93a206123f98271a0c664775d0baba98b330c7

doctor/permissions/permissions.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package permissions provides a doctor.Check that prints the process
+// permissions for the running process.
+package permissions
+
+import (
+	"context"
+	"fmt"
+	"os/user"
+	"strings"
+
+	"golang.org/x/exp/constraints"
+	"tailscale.com/types/logger"
+)
+
+// Check implements the doctor.Check interface.
+type Check struct{}
+
+func (Check) Name() string {
+	return "permissions"
+}
+
+func (Check) Run(_ context.Context, logf logger.Logf) error {
+	return permissionsImpl(logf)
+}
+
+func formatUserID[T constraints.Integer](id T) string {
+	idStr := fmt.Sprint(id)
+	if uu, err := user.LookupId(idStr); err != nil {
+		return idStr + "(<unknown>)"
+	} else {
+		return fmt.Sprintf("%s(%q)", idStr, uu.Username)
+	}
+}
+
+func formatGroupID[T constraints.Integer](id T) string {
+	idStr := fmt.Sprint(id)
+	if g, err := user.LookupGroupId(idStr); err != nil {
+		return idStr + "(<unknown>)"
+	} else {
+		return fmt.Sprintf("%s(%q)", idStr, g.Name)
+	}
+}
+
+func formatGroups[T constraints.Integer](groups []T) string {
+	var buf strings.Builder
+	for i, group := range groups {
+		if i > 0 {
+			buf.WriteByte(',')
+		}
+		buf.WriteString(formatGroupID(group))
+	}
+	return buf.String()
+}