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
+			subKey := key[len(array.rkey)+1:] // Remove the array prefix and dot
+			actualKey = fmt.Sprintf("%s.%d.%s", array.rkey, len(array.list.Elts)-1, 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: `