create DB struct

This is step one in detaching the Database layer from Headscale (h). The
ultimate goal is to have all function that does database operations in
its own package, and keep the business logic and writing separate.

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

hscontrol/util/string.go

@@ -0,0 +1,85 @@
+package util
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"strings"
+
+	"tailscale.com/tailcfg"
+)
+
+// GenerateRandomBytes returns securely generated random bytes.
+// It will return an error if the system's secure random
+// number generator fails to function correctly, in which
+// case the caller should not continue.
+func GenerateRandomBytes(n int) ([]byte, error) {
+	bytes := make([]byte, n)
+
+	// Note that err == nil only if we read len(b) bytes.
+	if _, err := rand.Read(bytes); err != nil {
+		return nil, err
+	}
+
+	return bytes, nil
+}
+
+// GenerateRandomStringURLSafe returns a URL-safe, base64 encoded
+// securely generated random string.
+// It will return an error if the system's secure random
+// number generator fails to function correctly, in which
+// case the caller should not continue.
+func GenerateRandomStringURLSafe(n int) (string, error) {
+	b, err := GenerateRandomBytes(n)
+
+	return base64.RawURLEncoding.EncodeToString(b), err
+}
+
+// GenerateRandomStringDNSSafe returns a DNS-safe
+// securely generated random string.
+// It will return an error if the system's secure random
+// number generator fails to function correctly, in which
+// case the caller should not continue.
+func GenerateRandomStringDNSSafe(size int) (string, error) {
+	var str string
+	var err error
+	for len(str) < size {
+		str, err = GenerateRandomStringURLSafe(size)
+		if err != nil {
+			return "", err
+		}
+		str = strings.ToLower(
+			strings.ReplaceAll(strings.ReplaceAll(str, "_", ""), "-", ""),
+		)
+	}
+
+	return str[:size], nil
+}
+
+func IsStringInSlice(slice []string, str string) bool {
+	for _, s := range slice {
+		if s == str {
+			return true
+		}
+	}
+
+	return false
+}
+
+func TailNodesToString(nodes []*tailcfg.Node) string {
+	temp := make([]string, len(nodes))
+
+	for index, node := range nodes {
+		temp[index] = node.Name
+	}
+
+	return fmt.Sprintf("[ %s ](%d)", strings.Join(temp, ", "), len(temp))
+}
+
+func TailMapResponseToString(resp tailcfg.MapResponse) string {
+	return fmt.Sprintf(
+		"{ Node: %s, Peers: %s }",
+		resp.Node.Name,
+		TailNodesToString(resp.Peers),
+	)
+}