add TraceFrom(pcs) for "on-demand" StackTrace instantiation

Allows to record just the pcs from runtime.Callers when an error
occurs and convert them to a StackTrace only if the trace is actually
needed, e.g. when printing to a log.

Improves performance about 4x.

Author: lukseven

stack.go

@@ -208,14 +208,24 @@ func (cs CallStack) Format(s fmt.State, verb rune) {
 	s.Write(closeBracketBytes)
 }
 
+// Convenience wrapper around runtime.Callers()
+func Callers(skip int) []uintptr {
+	var pcs [512]uintptr
+	n := runtime.Callers(skip+1, pcs[:])
+	return pcs[:n]
+}
+
 // Trace returns a CallStack for the current goroutine with element 0
 // identifying the calling function.
 func Trace() CallStack {
-	var pcs [512]uintptr
-	n := runtime.Callers(1, pcs[:])
+	return TraceFrom(Callers(1))
+}
 
-	frames := runtime.CallersFrames(pcs[:n])
-	cs := make(CallStack, 0, n)
+// TraceFrom creates a CallStack from the given program counters (as generated
+// by runtime.Callers)
+func TraceFrom(pcs []uintptr) CallStack {
+	frames := runtime.CallersFrames(pcs)
+	cs := make(CallStack, 0, len(pcs))
 
 	// Skip extra frame retrieved just to make sure the runtime.sigpanic
 	// special case is handled.

stack_test.go

@@ -497,6 +497,14 @@ func deepStack(depth int, b *testing.B) stack.CallStack {
 	return s
 }
 
+func deepCallers(depth int, b *testing.B) []uintptr {
+	if depth > 0 {
+		return deepCallers(depth-1, b)
+	}
+	b.StartTimer()
+	return stack.Callers(1)
+}
+
 func BenchmarkTrace10(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		b.StopTimer()
@@ -518,6 +526,33 @@ func BenchmarkTrace100(b *testing.B) {
 	}
 }
 
+func BenchmarkCallers(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		stack.Callers(1)
+	}
+}
+
+func BenchmarkCallers10(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		deepCallers(10, b)
+	}
+}
+
+func BenchmarkCallers50(b *testing.B) {
+	b.StopTimer()
+	for i := 0; i < b.N; i++ {
+		deepCallers(50, b)
+	}
+}
+
+func BenchmarkCallers100(b *testing.B) {
+	b.StopTimer()
+	for i := 0; i < b.N; i++ {
+		deepCallers(100, b)
+	}
+}
+
 ////////////////
 // Benchmark functions followed by formatting
 ////////////////