encoding/toml: fix decoding subtable duplicate keys

Previously, subtables with duplicate keys fail to be decoded
even though they represent valid TOML. Instead we should track
seenTableKeys by the array index to ensure duplication is local
to the given object

Fixes #3609

Signed-off-by: Lucas Charles <[email protected]>

Author: theoretick

encoding/toml/decode.go

@@ -202,11 +202,28 @@ func (d *Decoder) nextRootNode(tnode *toml.Node) error {
 	case toml.Table:
 		// Tables always begin a new line.
 		key, keyElems := d.decodeKey("", tnode.Key())
+
+		// Check if this table is a subtable of an existing array element
+		array := d.findArrayPrefix(key)
+		var actualKey string
+		if array != nil { // [last_array.new_table]
+			if array.rkey == key {
+				return d.nodeErrf(tnode.Child(), "cannot redeclare table array %q as a table", key)
+			}
+			// For subtables within array elements, we need to use the current array element's key
+			// to avoid false duplicate key errors between different array elements
+			arrayElementKey := array.rkey + "." + strconv.Itoa(len(array.list.Elts)-1)
+			subKey := key[len(array.rkey)+1:] // Remove the array prefix and dot
+			actualKey = arrayElementKey + "." + subKey
+		} else {
+			actualKey = key
+		}
+
 		// All table keys must be unique, including for the top-level table.
-		if d.seenTableKeys[key] {
+		if d.seenTableKeys[actualKey] {
 			return d.nodeErrf(tnode.Child(), "duplicate key: %s", key)
 		}
-		d.seenTableKeys[key] = true
+		d.seenTableKeys[actualKey] = true
 
 		// We want a multi-line struct with curly braces,
 		// just like TOML's tables are on multiple lines.
@@ -215,11 +232,7 @@ func (d *Decoder) nextRootNode(tnode *toml.Node) error {
 			Lbrace: token.NoPos.WithRel(token.Blank),
 			Rbrace: token.NoPos.WithRel(token.Newline),
 		}
-		array := d.findArrayPrefix(key)
 		if array != nil { // [last_array.new_table]
-			if array.rkey == key {
-				return d.nodeErrf(tnode.Child(), "cannot redeclare table array %q as a table", key)
-			}
 			subKeyElems := keyElems[array.level:]
 			topField, leafField := d.inlineFields(subKeyElems, token.Newline)
 			array.lastTable.Elts = append(array.lastTable.Elts, topField)
@@ -229,7 +242,7 @@ func (d *Decoder) nextRootNode(tnode *toml.Node) error {
 			d.topFile.Elts = append(d.topFile.Elts, topField)
 			leafField.Value = d.currentTable
 		}
-		d.currentTableKey = key
+		d.currentTableKey = actualKey
 
 	case toml.ArrayTable:
 		// Table array elements always begin a new line.

encoding/toml/decode_test.go

@@ -643,6 +643,42 @@ line two.\
 				},
 			]
 			`,
+	}, {
+		name: "ArrayTablesSubtableDuplicateKey",
+		input: `
+			[[foo]]
+			[foo.subtable]
+			name = "bar"
+			[[foo]]
+			[foo.subtable]
+			name = "bar"
+			`,
+		wantCUE: `
+			foo: [
+				{
+					subtable: {
+						name: "bar"
+					}
+				},
+				{
+					subtable: {
+						name: "bar"
+					}
+				}
+			]
+			`,
+	}, {
+		name: "ArrayTablesSubtableActualDuplicate",
+		input: `
+			[[foo]]
+			[foo.subtable]
+			name = "bar"
+			[foo.subtable]
+			`,
+		wantErr: `
+				duplicate key: foo.subtable:
+				    test.toml:4:2
+				`,
 	}, {
 		name: "ArrayTablesNested",
 		input: `