[WIP] factor out trace functionality into separate package and add Go docs

daemon/logging/trace/trace_genID.go

@@ -0,0 +1,47 @@
+package trace
+
+import (
+	"encoding/base64"
+	"math/rand"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/zrepl/zrepl/util/envconst"
+)
+
+var genIdPRNG = rand.New(rand.NewSource(1))
+
+func init() {
+	genIdPRNG.Seed(time.Now().UnixNano())
+	genIdPRNG.Seed(int64(os.Getpid()))
+}
+
+var genIdNumBytes = envconst.Int("ZREPL_TRACE_ID_NUM_BYTES", 3)
+
+func init() {
+	if genIdNumBytes < 1 {
+		panic("trace node id byte length must be at least 1")
+	}
+}
+
+func genID() string {
+	var out strings.Builder
+	enc := base64.NewEncoder(base64.RawStdEncoding, &out)
+	buf := make([]byte, genIdNumBytes)
+	for i := 0; i < len(buf); {
+		n, err := genIdPRNG.Read(buf[i:])
+		if err != nil {
+			panic(err)
+		}
+		i += n
+	}
+	n, err := enc.Write(buf[:])
+	if err != nil || n != len(buf) {
+		panic(err)
+	}
+	if err := enc.Close(); err != nil {
+		panic(err)
+	}
+	return out.String()
+}