reduce allocations when printing stack traces

stack.go

@@ -1,6 +1,7 @@
 package errors
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"path"
@@ -61,25 +62,27 @@ func (f Frame) name() string {
 //    %+s   function name and path of source file relative to the compile time
 //          GOPATH separated by \n\t (<funcname>\n\t<path>)
 //    %+v   equivalent to %+s:%d
-func (f Frame) Format(s fmt.State, verb rune) {
+func (f Frame) Format(s fmt.State, verb rune) { f.format(s, s, verb) }
+
+func (f Frame) format(w io.Writer, s fmt.State, verb rune) {
 	switch verb {
 	case 's':
 		switch {
 		case s.Flag('+'):
-			io.WriteString(s, f.name())
-			io.WriteString(s, "\n\t")
-			io.WriteString(s, f.file())
+			io.WriteString(w, f.name())
+			io.WriteString(w, "\n\t")
+			io.WriteString(w, f.file())
 		default:
-			io.WriteString(s, path.Base(f.file()))
+			io.WriteString(w, path.Base(f.file()))
 		}
 	case 'd':
-		io.WriteString(s, strconv.Itoa(f.line()))
+		io.WriteString(w, strconv.Itoa(f.line()))
 	case 'n':
-		io.WriteString(s, funcname(f.name()))
+		io.WriteString(w, funcname(f.name()))
 	case 'v':
-		f.Format(s, 's')
-		io.WriteString(s, ":")
-		f.Format(s, 'd')
+		f.format(w, s, 's')
+		io.WriteString(w, ":")
+		f.format(w, s, 'd')
 	}
 }
 
@@ -136,6 +139,11 @@ func (st StackTrace) formatSlice(s fmt.State, verb rune) {
 	io.WriteString(s, "]")
 }
 
+// stackMinLen is a best-guess at the minimum length of a stack trace. It
+// doesn't need to be exact, just give a good enough head start for the buffer
+// to avoid the expensive early growth.
+const stackMinLen = 96
+
 // stack represents a stack of program counters.
 type stack []uintptr
 
@@ -144,10 +152,14 @@ func (s *stack) Format(st fmt.State, verb rune) {
 	case 'v':
 		switch {
 		case st.Flag('+'):
+			var b bytes.Buffer
+			b.Grow(len(*s) * stackMinLen)
 			for _, pc := range *s {
 				f := Frame(pc)
-				fmt.Fprintf(st, "\n%+v", f)
+				b.WriteByte('\n')
+				f.format(&b, st, 'v')
 			}
+			io.Copy(st, &b)
 		}
 	}
 }