internal/stack: helper package for querying stack frames

Intended for #5. Also replaces a helper function in the `printer`
package.

internal/stack/stack.go

@@ -0,0 +1,72 @@
+package stack
+
+import (
+	"path"
+	"runtime"
+	"strings"
+)
+
+// Frames returns at most max callstack Frames, starting with its caller and
+// skipping skip Frames.
+func Frames(skip, max int) []runtime.Frame {
+	pc := make([]uintptr, max)
+	n := runtime.Callers(skip+2, pc)
+	if n == 0 {
+		return nil
+	}
+	pc = pc[:n]
+	frames := runtime.CallersFrames(pc)
+	var fs []runtime.Frame
+	for {
+		f, more := frames.Next()
+		fs = append(fs, f)
+		if !more {
+			break
+		}
+	}
+	return fs
+}
+
+// Match returns the first stack frame for which the predicate function returns
+// true. Returns nil if no match is found. Starts matching after skip frames,
+// starting with its caller.
+func Match(skip int, predicate func(runtime.Frame) bool) *runtime.Frame {
+	i, n := skip+1, 16
+	for {
+		fs := Frames(i, n)
+		for _, f := range fs {
+			if predicate(f) {
+				return &f
+			}
+		}
+		if len(fs) < n {
+			break
+		}
+		i += n
+	}
+	return nil
+}
+
+// Main returns the main() function Frame.
+func Main() *runtime.Frame {
+	return Match(1, func(f runtime.Frame) bool {
+		return f.Function == "main.main"
+	})
+}
+
+// ExternalCaller returns the first frame outside the callers package.
+func ExternalCaller() *runtime.Frame {
+	var first *runtime.Frame
+	return Match(1, func(f runtime.Frame) bool {
+		if first == nil {
+			first = &f
+		}
+		return pkg(first.Function) != pkg(f.Function)
+	})
+}
+
+func pkg(ident string) string {
+	dir, name := path.Split(ident)
+	parts := strings.Split(name, ".")
+	return dir + parts[0]
+}

internal/stack/stack_test.go

@@ -0,0 +1,34 @@
+package stack_test
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/mmcloughlin/avo/internal/stack"
+)
+
+const pkg = "github.com/mmcloughlin/avo/internal/stack_test"
+
+func TestFramesFirst(t *testing.T) {
+	fs := stack.Frames(0, 1)
+	if len(fs) == 0 {
+		t.Fatalf("empty slice")
+	}
+	got := fs[0].Function
+	expect := pkg + ".TestFramesFirst"
+	if got != expect {
+		t.Fatalf("bad function name %s; expect %s", got, expect)
+	}
+}
+
+func TestMatchFirst(t *testing.T) {
+	first := stack.Match(0, func(_ runtime.Frame) bool { return true })
+	if first == nil {
+		t.Fatalf("nil match")
+	}
+	got := first.Function
+	expect := pkg + ".TestMatchFirst"
+	if got != expect {
+		t.Fatalf("bad function name %s; expect %s", got, expect)
+	}
+}