Decode: fix reuse of slice for array tables (#934)

When decoding into a non-empty slice, it needs to be emptied so that only the
tables contained in the document are present in the resulting value.

Arrays are not impacted because their unmarshal offset is tracked separately.

Fixes #931

internal/tracker/seen.go

@@ -149,8 +149,9 @@ func (s *SeenTracker) setExplicitFlag(parentIdx int) {
 
 // CheckExpression takes a top-level node and checks that it does not contain
 // keys that have been seen in previous calls, and validates that types are
-// consistent.
-func (s *SeenTracker) CheckExpression(node *unstable.Node) error {
+// consistent. It returns true if it is the first time this node's key is seen.
+// Useful to clear array tables on first use.
+func (s *SeenTracker) CheckExpression(node *unstable.Node) (bool, error) {
 	if s.entries == nil {
 		s.reset()
 	}
@@ -166,7 +167,7 @@ func (s *SeenTracker) CheckExpression(node *unstable.Node) error {
 	}
 }
 
-func (s *SeenTracker) checkTable(node *unstable.Node) error {
+func (s *SeenTracker) checkTable(node *unstable.Node) (bool, error) {
 	if s.currentIdx >= 0 {
 		s.setExplicitFlag(s.currentIdx)
 	}
@@ -192,7 +193,7 @@ func (s *SeenTracker) checkTable(node *unstable.Node) error {
 		} else {
 			entry := s.entries[idx]
 			if entry.kind == valueKind {
-				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+				return false, fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
 			}
 		}
 		parentIdx = idx
@@ -201,25 +202,27 @@ func (s *SeenTracker) checkTable(node *unstable.Node) error {
 	k := it.Node().Data
 	idx := s.find(parentIdx, k)
 
+	first := false
 	if idx >= 0 {
 		kind := s.entries[idx].kind
 		if kind != tableKind {
-			return fmt.Errorf("toml: key %s should be a table, not a %s", string(k), kind)
+			return false, fmt.Errorf("toml: key %s should be a table, not a %s", string(k), kind)
 		}
 		if s.entries[idx].explicit {
-			return fmt.Errorf("toml: table %s already exists", string(k))
+			return false, fmt.Errorf("toml: table %s already exists", string(k))
 		}
 		s.entries[idx].explicit = true
 	} else {
 		idx = s.create(parentIdx, k, tableKind, true, false)
+		first = true
 	}
 
 	s.currentIdx = idx
 
-	return nil
+	return first, nil
 }
 
-func (s *SeenTracker) checkArrayTable(node *unstable.Node) error {
+func (s *SeenTracker) checkArrayTable(node *unstable.Node) (bool, error) {
 	if s.currentIdx >= 0 {
 		s.setExplicitFlag(s.currentIdx)
 	}
@@ -242,7 +245,7 @@ func (s *SeenTracker) checkArrayTable(node *unstable.Node) error {
 		} else {
 			entry := s.entries[idx]
 			if entry.kind == valueKind {
-				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+				return false, fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
 			}
 		}
 
@@ -252,22 +255,23 @@ func (s *SeenTracker) checkArrayTable(node *unstable.Node) error {
 	k := it.Node().Data
 	idx := s.find(parentIdx, k)
 
-	if idx >= 0 {
+	firstTime := idx < 0
+	if firstTime {
+		idx = s.create(parentIdx, k, arrayTableKind, true, false)
+	} else {
 		kind := s.entries[idx].kind
 		if kind != arrayTableKind {
-			return fmt.Errorf("toml: key %s already exists as a %s,  but should be an array table", kind, string(k))
+			return false, fmt.Errorf("toml: key %s already exists as a %s,  but should be an array table", kind, string(k))
 		}
 		s.clear(idx)
-	} else {
-		idx = s.create(parentIdx, k, arrayTableKind, true, false)
 	}
 
 	s.currentIdx = idx
 
-	return nil
+	return firstTime, nil
 }
 
-func (s *SeenTracker) checkKeyValue(node *unstable.Node) error {
+func (s *SeenTracker) checkKeyValue(node *unstable.Node) (bool, error) {
 	parentIdx := s.currentIdx
 	it := node.Key()
 
@@ -281,11 +285,11 @@ func (s *SeenTracker) checkKeyValue(node *unstable.Node) error {
 		} else {
 			entry := s.entries[idx]
 			if it.IsLast() {
-				return fmt.Errorf("toml: key %s is already defined", string(k))
+				return false, fmt.Errorf("toml: key %s is already defined", string(k))
 			} else if entry.kind != tableKind {
-				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+				return false, fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
 			} else if entry.explicit {
-				return fmt.Errorf("toml: cannot redefine table %s that has already been explicitly defined", string(k))
+				return false, fmt.Errorf("toml: cannot redefine table %s that has already been explicitly defined", string(k))
 			}
 		}
 
@@ -303,30 +307,30 @@ func (s *SeenTracker) checkKeyValue(node *unstable.Node) error {
 		return s.checkArray(value)
 	}
 
-	return nil
+	return false, nil
 }
 
-func (s *SeenTracker) checkArray(node *unstable.Node) error {
+func (s *SeenTracker) checkArray(node *unstable.Node) (first bool, err error) {
 	it := node.Children()
 	for it.Next() {
 		n := it.Node()
 		switch n.Kind {
 		case unstable.InlineTable:
-			err := s.checkInlineTable(n)
+			first, err = s.checkInlineTable(n)
 			if err != nil {
-				return err
+				return false, err
 			}
 		case unstable.Array:
-			err := s.checkArray(n)
+			first, err = s.checkArray(n)
 			if err != nil {
-				return err
+				return false, err
 			}
 		}
 	}
-	return nil
+	return first, nil
 }
 
-func (s *SeenTracker) checkInlineTable(node *unstable.Node) error {
+func (s *SeenTracker) checkInlineTable(node *unstable.Node) (first bool, err error) {
 	if pool.New == nil {
 		pool.New = func() interface{} {
 			return &SeenTracker{}
@@ -339,9 +343,9 @@ func (s *SeenTracker) checkInlineTable(node *unstable.Node) error {
 	it := node.Children()
 	for it.Next() {
 		n := it.Node()
-		err := s.checkKeyValue(n)
+		first, err = s.checkKeyValue(n)
 		if err != nil {
-			return err
+			return false, err
 		}
 	}
 
@@ -352,5 +356,5 @@ func (s *SeenTracker) checkInlineTable(node *unstable.Node) error {
 	// redefinition of its keys: check* functions cannot walk into
 	// a value.
 	pool.Put(s)
-	return nil
+	return first, nil
 }