merr/mctx: move stack code into merr

merr/merr.go

@@ -47,7 +47,10 @@ func wrap(e error, cp bool) *err {
 
 	er, ok := e.(*err)
 	if !ok {
-		return &err{err: e}
+		return &err{
+			err:  e,
+			attr: make(map[interface{}]interface{}, 1),
+		}
 	} else if !cp {
 		return er
 	}
@@ -100,19 +103,21 @@ func Value(e error, k interface{}) interface{} {
 // WrapSkip is like Wrap but also allows for skipping extra stack frames when
 // embedding the stack into the error.
 func WrapSkip(ctx context.Context, e error, skip int, kvs ...interface{}) error {
-	prevCtx, _ := Value(e, attrKeyCtx).(context.Context)
-	if prevCtx != nil {
+	er := wrap(e, true)
+	if _, ok := getStack(er); !ok {
+		setStack(er, skip+1)
+	}
+
+	if prevCtx, _ := er.attr[attrKeyCtx].(context.Context); prevCtx != nil {
 		ctx = mctx.MergeAnnotations(prevCtx, ctx)
 	}
 
-	if _, ok := mctx.Stack(ctx); !ok {
-		ctx = mctx.WithStack(ctx, skip+1)
-	}
 	if len(kvs) > 0 {
 		ctx = mctx.Annotate(ctx, kvs...)
 	}
 
-	return WithValue(e, attrKeyCtx, ctx)
+	er.attr[attrKeyCtx] = ctx
+	return er
 }
 
 // Wrap takes in an error and returns one which wraps it in merr's inner type,
@@ -122,9 +127,9 @@ func WrapSkip(ctx context.Context, e error, skip int, kvs ...interface{}) error
 // For convenience, extra annotation information can be passed in here as well
 // via the kvs argument. See mctx.Annotate for more information.
 //
-// This function automatically embeds stack information into the Context as it's
-// being stored, using mctx.WithStack, unless the error already has stack
-// information in it.
+// This function automatically embeds stack information into the error as it's
+// being stored, using WithStack, unless the error already has stack information
+// in it.
 func Wrap(ctx context.Context, e error, kvs ...interface{}) error {
 	return WrapSkip(ctx, e, 1, kvs...)
 }
@@ -146,7 +151,7 @@ func ctx(e error) context.Context {
 		ctx = context.Background()
 	}
 
-	if stack, ok := mctx.Stack(ctx); ok {
+	if stack, ok := Stack(e); ok {
 		ctx = mctx.Annotate(ctx, annotateKey("errLoc"), stack.String())
 	}
 	return ctx
@@ -156,9 +161,9 @@ func ctx(e error) context.Context {
 // or New. If none is embedded this uses context.Background().
 //
 // The returned Context will have annotated on it (see mctx.Annotate) the
-// underlying error's string (as returned by Error()) and the stack location in
-// the Context. Stack locations are automatically added by New and Wrap via
-// mctx.WithStack.
+// underlying error's string (as returned by Error()) and the error's stack
+// location. Stack locations are automatically added by New and Wrap via
+// WithStack.
 //
 // If this error is nil this returns context.Background().
 func Context(e error) context.Context {

merr/stack.go

@@ -1,7 +1,6 @@
-package mctx
+package merr
 
 import (
-	"context"
 	"fmt"
 	"path/filepath"
 	"runtime"
@@ -13,7 +12,7 @@ import (
 // stored when embedding stack traces in errors.
 var MaxStackSize = 50
 
-type ctxStackKey int
+type stackKey int
 
 // Stacktrace represents a stack trace at a particular point in execution.
 type Stacktrace struct {
@@ -73,22 +72,34 @@ func (s Stacktrace) FullString() string {
 	return sb.String()
 }
 
-// WithStack returns a Context with the current stacktrace embedded in it (as a
+func setStack(er *err, skip int) {
+	stackSlice := make([]uintptr, MaxStackSize+skip)
+	// incr skip once for WithStack, and once for runtime.Callers
+	l := runtime.Callers(skip+2, stackSlice)
+	er.attr[stackKey(0)] = Stacktrace{frames: stackSlice[:l]}
+}
+
+// WithStack returns an error with the current stacktrace embedded in it (as a
 // Stacktrace type). If skip is non-zero it will skip that many frames from the
 // top of the stack. The frame containing the WithStack call itself is always
 // excluded.
-func WithStack(ctx context.Context, skip int) context.Context {
-	stackSlice := make([]uintptr, MaxStackSize)
-	// incr skip once for WithStack, and once for runtime.Callers
-	l := runtime.Callers(skip+2, stackSlice)
-	stack := Stacktrace{frames: stackSlice[:l]}
-
-	return context.WithValue(ctx, ctxStackKey(0), stack)
+//
+// This call always overwrites any previously existing stack information on the
+// error, as opposed to Wrap which only does so if the error didn't already have
+// any.
+func WithStack(e error, skip int) error {
+	er := wrap(e, true)
+	setStack(er, skip+1)
+	return er
 }
 
-// Stack returns the Stacktrace instance which was embedded by WithStack, or false if
-// none ever was.
-func Stack(ctx context.Context) (Stacktrace, bool) {
-	stack, ok := ctx.Value(ctxStackKey(0)).(Stacktrace)
+func getStack(er *err) (Stacktrace, bool) {
+	stack, ok := er.attr[stackKey(0)].(Stacktrace)
 	return stack, ok
 }
+
+// Stack returns the Stacktrace instance which was embedded by Wrap/WrapSkip, or
+// false if none ever was.
+func Stack(e error) (Stacktrace, bool) {
+	return getStack(wrap(e, false))
+}

merr/stack_test.go

@@ -1,4 +1,4 @@
-package mctx
+package merr
 
 import (
 	"context"
@@ -9,7 +9,7 @@ import (
 )
 
 func TestStack(t *T) {
-	foo := WithStack(context.Background(), 0)
+	foo := New(context.Background(), "test")
 	fooStack, ok := Stack(foo)
 	massert.Fatal(t, massert.Equal(true, ok))