util/syspolicy: implement a syspolicy store that reads settings from environment variables

In this PR, we implement (but do not use yet, pending tailscale#13727 review) a syspolicy/source.Store
that reads policy settings from environment variables. It converts a CamelCase setting.Key,
such as AuthKey or ExitNodeID, to a SCREAMING_SNAKE_CASE, TS_-prefixed environment
variable name, such as TS_AUTH_KEY and TS_EXIT_NODE_ID. It then looks up the variable
and attempts to parse it according to the expected value type. If the environment variable
is not set, the policy setting is considered not configured in this store (the syspolicy package
will still read it from other sources). Similarly, if the environment variable has an invalid value
for the setting type, it won't be used (though the reported/logged error will differ).

Updates tailscale#13193
Updates tailscale#12687

Signed-off-by: Nick Khyl <[email protected]>

Author: nickkhyl

util/syspolicy/internal/metrics/metrics.go

@@ -284,7 +284,7 @@ func SetHooksForTest(tb internal.TB, addMetric, setMetric metricFn) {
 }
 
 func newSettingMetric(key setting.Key, scope setting.Scope, suffix string, typ clientmetric.Type) metric {
-	name := strings.ReplaceAll(string(key), setting.KeyPathSeparator, "_")
+	name := strings.ReplaceAll(string(key), string(setting.KeyPathSeparator), "_")
 	return newMetric([]string{name, metricScopeName(scope), suffix}, typ)
 }
 

util/syspolicy/setting/key.go

@@ -10,4 +10,4 @@ package setting
 type Key string
 
 // KeyPathSeparator allows logical grouping of policy settings into categories.
-const KeyPathSeparator = "/"
+const KeyPathSeparator = '/'

util/syspolicy/source/env_policy_store.go

@@ -0,0 +1,159 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package source
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"unicode/utf8"
+
+	"github.com/pkg/errors"
+	"tailscale.com/util/syspolicy/setting"
+)
+
+var lookupEnv = os.LookupEnv // test hook
+
+var _ Store = (*EnvPolicyStore)(nil)
+
+// EnvPolicyStore is a [Store] that reads policy settings from environment variables.
+type EnvPolicyStore struct{}
+
+// ReadString implements [Store].
+func (s *EnvPolicyStore) ReadString(key setting.Key) (string, error) {
+	_, str, err := s.lookupSettingVariable(key)
+	if err != nil {
+		return "", err
+	}
+	return str, nil
+}
+
+// ReadUInt64 implements [Store].
+func (s *EnvPolicyStore) ReadUInt64(key setting.Key) (uint64, error) {
+	name, str, err := s.lookupSettingVariable(key)
+	if err != nil {
+		return 0, err
+	}
+	if str == "" {
+		return 0, setting.ErrNotConfigured
+	}
+	value, err := strconv.ParseUint(str, 0, 64)
+	if err != nil {
+		return 0, fmt.Errorf("%s: %w: %q is not a valid uint64", name, setting.ErrTypeMismatch, str)
+	}
+	return value, nil
+}
+
+// ReadBoolean implements [Store].
+func (s *EnvPolicyStore) ReadBoolean(key setting.Key) (bool, error) {
+	name, str, err := s.lookupSettingVariable(key)
+	if err != nil {
+		return false, err
+	}
+	if str == "" {
+		return false, setting.ErrNotConfigured
+	}
+	value, err := strconv.ParseBool(str)
+	if err != nil {
+		return false, fmt.Errorf("%s: %w: %q is not a valid bool", name, setting.ErrTypeMismatch, str)
+	}
+	return value, nil
+}
+
+// ReadStringArray implements [Store].
+func (s *EnvPolicyStore) ReadStringArray(key setting.Key) ([]string, error) {
+	_, str, err := s.lookupSettingVariable(key)
+	if err != nil || str == "" {
+		return nil, err
+	}
+	var dst int
+	res := strings.Split(str, ",")
+	for src := range res {
+		res[dst] = strings.TrimSpace(res[src])
+		if res[dst] != "" {
+			dst++
+		}
+	}
+	return res[0:dst], nil
+}
+
+func (s *EnvPolicyStore) lookupSettingVariable(key setting.Key) (name, value string, err error) {
+	name, err = keyToEnvVarName(key)
+	if err != nil {
+		return "", "", err
+	}
+	value, ok := lookupEnv(name)
+	if !ok {
+		return name, "", setting.ErrNotConfigured
+	}
+	return name, value, nil
+}
+
+var (
+	errEmptyKey   = errors.New("key must not be empty")
+	errInvalidKey = errors.New("key must consist of alphanumeric characters and slashes")
+)
+
+// keyToEnvVarName returns the environment variable name for a given policy
+// setting key, or an error if the key is invalid. It converts CamelCase keys into
+// underscore-separated words and prepends the variable name with the TS prefix.
+// For example: AuthKey => TS_AUTH_KEY, ExitNodeAllowLANAccess => TS_EXIT_NODE_ALLOW_LAN_ACCESS, etc.
+//
+// It's fine to use this in [EnvPolicyStore] without caching variable names since it's not a hot path.
+// [EnvPolicyStore] is not a [Changeable] policy store, so the conversion will only happen once.
+func keyToEnvVarName(key setting.Key) (string, error) {
+	if len(key) == 0 {
+		return "", errEmptyKey
+	}
+
+	isLower := func(c byte) bool { return 'a' <= c && c <= 'z' }
+	isUpper := func(c byte) bool { return 'A' <= c && c <= 'Z' }
+	isLetter := func(c byte) bool { return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') }
+	isDigit := func(c byte) bool { return '0' <= c && c <= '9' }
+
+	words := make([]string, 0, 8)
+	words = append(words, "TS")
+	var currentWord strings.Builder
+	for i := 0; i < len(key); i++ {
+		c := key[i]
+		if c >= utf8.RuneSelf {
+			return "", errInvalidKey
+		}
+
+		var split bool
+		switch {
+		case isLower(c):
+			c -= 'a' - 'A' // make upper
+			split = currentWord.Len() > 0 && !isLetter(key[i-1])
+		case isUpper(c):
+			if currentWord.Len() > 0 {
+				prevUpper := isUpper(key[i-1])
+				nextLower := i < len(key)-1 && isLower(key[i+1])
+				split = !prevUpper || nextLower // split on case transition
+			}
+		case isDigit(c):
+			split = currentWord.Len() > 0 && !isDigit(key[i-1])
+		case c == setting.KeyPathSeparator:
+			words = append(words, currentWord.String())
+			currentWord.Reset()
+			continue
+		default:
+			return "", errInvalidKey
+		}
+
+		if split {
+			words = append(words, currentWord.String())
+			currentWord.Reset()
+		}
+
+		currentWord.WriteByte(c)
+	}
+
+	if currentWord.Len() > 0 {
+		words = append(words, currentWord.String())
+	}
+
+	return strings.Join(words, "_"), nil
+}