decode: moves PosLoopDetector into its own package

format/apple/bookmark/apple_bookmark.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/wader/fq/format"
+	"github.com/wader/fq/format/apple"
 	"github.com/wader/fq/pkg/decode"
 	"github.com/wader/fq/pkg/interp"
 	"github.com/wader/fq/pkg/scalar"
@@ -299,32 +300,12 @@ const (
 	dictEntrySize  = 4
 )
 
-type posLoopDetector []int64
-
-func (pld *posLoopDetector) push(i int64, detect func()) {
-	for _, o := range *pld {
-		if i == o {
-			detect()
-		}
-	}
-	*pld = append(*pld, i)
-}
-
-func (pld *posLoopDetector) pop() {
-	*pld = (*pld)[:len(*pld)-1]
-}
-
-func (pld *posLoopDetector) pushAndPop(i int64, detect func()) func() {
-	pld.push(i, detect)
-	return pld.pop
-}
-
 func makeDecodeRecord() func(d *decode.D) {
-	var pld posLoopDetector
+	var pld apple.PosLoopDetector
 
 	var decodeRecord func(d *decode.D)
 	decodeRecord = func(d *decode.D) {
-		defer pld.pushAndPop(
+		defer pld.PushAndPop(
 			d.Pos(),
 			func() { d.Fatalf("infinite recursion detected in record decode function") },
 		)()

format/apple/bplist/bplist.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/wader/fq/format"
+	"github.com/wader/fq/format/apple"
 	"github.com/wader/fq/pkg/decode"
 	"github.com/wader/fq/pkg/interp"
 	"github.com/wader/fq/pkg/scalar"
@@ -185,26 +186,6 @@ func decodeItem(d *decode.D, p *plist) bool {
 	return false
 }
 
-type indexStack []uint64
-
-func (i *indexStack) pop() {
-	*i = (*i)[:len(*i)-1]
-}
-
-func (i *indexStack) push(idx uint64, handler func()) {
-	for _, v := range *i {
-		if v == idx {
-			handler()
-		}
-	}
-	*i = append(*i, idx)
-}
-
-func (i *indexStack) pushAndPop(idx uint64, handler func()) func() {
-	i.push(idx, handler)
-	return i.pop
-}
-
 // decodeReference looks up and decodes an object based on its index in the
 // offset table. Returns a bool indicating whether or not the decoded item is
 // a string (necessary for checking dictionary key validity).
@@ -216,7 +197,7 @@ func (pl *plist) decodeReference(d *decode.D, idx uint64) bool {
 	}
 	pl.consumed[idx] = true
 
-	defer pl.idxStack.pushAndPop(idx, func() { d.Fatalf("infinite recursion detected") })()
+	defer pl.pld.PushAndPop(int64(idx), func() { d.Fatalf("infinite recursion detected") })()
 
 	itemOffset := pl.o[idx]
 	if itemOffset >= pl.t.offsetTableStart {
@@ -243,7 +224,7 @@ type plist struct {
 	t        trailer
 	o        []uint64
 	consumed map[uint64]bool
-	idxStack indexStack
+	pld      apple.PosLoopDetector
 }
 
 func bplistDecode(d *decode.D, _ any) any {

format/apple/loop_detector.go

@@ -0,0 +1,29 @@
+package apple
+
+// PosLoopDetector is used for detecting loops when writing decoders, and can
+// short-circuit infinite recursion that can cause stack overflows.
+type PosLoopDetector []int64
+
+// Push adds the current offset to the stack and executes the supplied
+// detection function
+func (pld *PosLoopDetector) Push(offset int64, detect func()) {
+	for _, o := range *pld {
+		if offset == o {
+			detect()
+		}
+	}
+	*pld = append(*pld, offset)
+}
+
+// Pop removes the most recently added offset from the stack.
+func (pld *PosLoopDetector) Pop() {
+	*pld = (*pld)[:len(*pld)-1]
+}
+
+// PushAndPop adds the current offset to the stack, executes the supplied
+// detection function, and returns the Pop method. A good usage of this is to
+// pair this method call with a defer statement.
+func (pld *PosLoopDetector) PushAndPop(i int64, detect func()) func() {
+	pld.Push(i, detect)
+	return pld.Pop
+}