stack: compress shared stacks for clearer output

Combines the shared parts of stacktraces so they
take up less space. For example if error is wrapped
from similar codepath to the main error, the main error
gets the stacktrace and wrapping gets only to stacktrace
up to the point where same frame exists in main error as well.

This also means we need to be less careful about WithStack as
if error already has a longer stack from current position, it
would be ignored.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

util/stack/compress.go

@@ -0,0 +1,67 @@
+package stack
+
+import (
+	"slices"
+)
+
+func compressStacks(st []*Stack) []*Stack {
+	if len(st) == 0 {
+		return nil
+	}
+
+	slices.SortFunc(st, func(a, b *Stack) int {
+		return len(b.Frames) - len(a.Frames)
+	})
+
+	out := []*Stack{st[0]}
+
+loop0:
+	for _, st := range st[1:] {
+		maxIdx := -1
+		for _, prev := range out {
+			idx := subFrames(st.Frames, prev.Frames)
+			if idx == -1 {
+				continue
+			}
+			// full match, potentially skip all
+			if idx == len(st.Frames)-1 {
+				if st.Pid == prev.Pid && st.Version == prev.Version && slices.Compare(st.Cmdline, st.Cmdline) == 0 {
+					continue loop0
+				}
+			}
+			if idx > maxIdx {
+				maxIdx = idx
+			}
+		}
+
+		if maxIdx > 0 {
+			st.Frames = st.Frames[:len(st.Frames)-maxIdx]
+		}
+		out = append(out, st)
+	}
+
+	return out
+}
+
+func subFrames(a, b []*Frame) int {
+	idx := -1
+	i := len(a) - 1
+	j := len(b) - 1
+	for i >= 0 {
+		if j < 0 {
+			break
+		}
+		if a[i].Equal(b[j]) {
+			idx++
+			i--
+			j--
+		} else {
+			break
+		}
+	}
+	return idx
+}
+
+func (a *Frame) Equal(b *Frame) bool {
+	return a.File == b.File && a.Line == b.Line && a.Name == b.Name
+}

util/stack/compress_test.go

@@ -0,0 +1,54 @@
+package stack
+
+import (
+	"testing"
+
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/require"
+)
+
+func testcall1() error {
+	return errors.Errorf("error1")
+}
+
+func testcall2() error {
+	return errors.WithStack(testcall1())
+}
+
+func testcall3() error {
+	err := testcall2()
+	// this is from a different line
+	return errors.WithStack(err)
+}
+
+func TestCompressStacks(t *testing.T) {
+	err := testcall2()
+	st := Traces(err)
+
+	// full trace match, shorter is removed
+	require.Len(t, st, 1)
+	require.GreaterOrEqual(t, len(st[0].Frames), 2)
+
+	f := st[0].Frames
+	require.Contains(t, f[0].Name, "testcall1")
+	require.Contains(t, f[1].Name, "testcall2")
+}
+
+func TestCompressMultiStacks(t *testing.T) {
+	err := testcall3()
+	st := Traces(err)
+
+	require.Len(t, st, 2)
+	require.GreaterOrEqual(t, len(st[0].Frames), 4)
+
+	f1 := st[0].Frames
+	require.Contains(t, f1[0].Name, "testcall1")
+	require.Contains(t, f1[1].Name, "testcall2")
+	require.Contains(t, f1[2].Name, "testcall3")
+
+	f2 := st[1].Frames
+	require.Contains(t, f2[0].Name, "testcall3")
+	// next line is shared and everything after is removed
+	require.Len(t, f2, 2)
+	require.Equal(t, f1[3], f2[1])
+}

util/stack/stack.go

@@ -44,6 +44,10 @@ func Helper() {
 }
 
 func Traces(err error) []*Stack {
+	return compressStacks(traces(err))
+}
+
+func traces(err error) []*Stack {
 	var st []*Stack
 
 	switch e := err.(type) {