feat: move comma check to lexer and improve trailing comma reports

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonParserTest.kt

@@ -6,7 +6,6 @@ package kotlinx.serialization.json
 
 import kotlinx.serialization.*
 import kotlinx.serialization.builtins.*
-import kotlinx.serialization.json.internal.*
 import kotlinx.serialization.test.*
 import kotlin.test.*
 
@@ -79,11 +78,11 @@ class JsonParserTest : JsonTestBase() {
     fun testTrailingComma() {
         testTrailingComma("{\"id\":0,}")
         testTrailingComma("{\"id\":0  ,}")
-        testTrailingComma("{\"id\":0  , ,}")
+        testTrailingComma("{\"id\":0  , ,}", message = "Multiple consecutive commas are not allowed in JSON")
     }
 
-    private fun testTrailingComma(content: String) {
-        assertFailsWithSerialMessage("JsonDecodingException", "Trailing comma before the end of JSON object") {  Json.parseToJsonElement(content) }
+    private fun testTrailingComma(content: String, message: String = "Trailing comma before the end of JSON object") {
+        assertFailsWithSerialMessage("JsonDecodingException", message) { Json.parseToJsonElement(content) }
     }
 
     @Test

formats/json-tests/commonTest/src/kotlinx/serialization/json/MissingCommaTest.kt

@@ -26,7 +26,7 @@ class MissingCommaTest : JsonTestBase() {
     @Test
     fun missingCommaBetweenFieldsAfterPrimitive() {
         val message =
-            "Unexpected JSON token at offset 8: Expected comma after the key-value pair at path: \$.i"
+            "Unexpected JSON token at offset 8: Expected end of the object or comma at path: \$"
         val json = """{"i":42 "c":{"i":"string"}}"""
 
         assertFailsWithSerialMessage("JsonDecodingException", message) {
@@ -37,7 +37,7 @@ class MissingCommaTest : JsonTestBase() {
     @Test
     fun missingCommaBetweenFieldsAfterObject() {
         val message =
-            "Unexpected JSON token at offset 19: Expected comma after the key-value pair at path: \$.c"
+            "Unexpected JSON token at offset 19: Expected end of the object or comma at path: \$"
         val json = """{"c":{"i":"string"}"i":42}"""
 
         assertFailsWithSerialMessage("JsonDecodingException", message) {
@@ -48,7 +48,7 @@ class MissingCommaTest : JsonTestBase() {
     @Test
     fun allowTrailingCommaDoesNotApplyToCommaBetweenFields() {
         val message =
-            "Unexpected JSON token at offset 8: Expected comma after the key-value pair at path: \$.i"
+            "Unexpected JSON token at offset 8: Expected end of the object or comma at path: \$"
         val json = """{"i":42 "c":{"i":"string"}}"""
 
         assertFailsWithSerialMessage("JsonDecodingException", message) {
@@ -59,7 +59,7 @@ class MissingCommaTest : JsonTestBase() {
     @Test
     fun lenientSerializeDoesNotAllowToSkipCommaBetweenFields() {
         val message =
-            "Unexpected JSON token at offset 8: Expected comma after the key-value pair at path: \$.i"
+            "Unexpected JSON token at offset 8: Expected end of the object or comma at path: \$"
         val json = """{"i":42 "c":{"i":"string"}}"""
 
         assertFailsWithSerialMessage("JsonDecodingException", message) {
@@ -69,7 +69,7 @@ class MissingCommaTest : JsonTestBase() {
 
     @Test
     fun missingCommaInDynamicMap() {
-        val m = "Unexpected JSON token at offset 9: Expected end of the object or comma at path: \$"
+        val m = "Unexpected JSON token at offset 8: Expected end of the object or comma at path: \$"
         val json = """{"i":42 "c":{"i":"string"}}"""
         assertFailsWithSerialMessage("JsonDecodingException", m) {
             default.parseToJsonElement(json)
@@ -78,7 +78,7 @@ class MissingCommaTest : JsonTestBase() {
 
     @Test
     fun missingCommaInArray() {
-        val m = "Unexpected JSON token at offset 3: Expected end of the array or comma at path: \$[0]"
+        val m = "Unexpected JSON token at offset 3: Expected end of the array or comma at path: \$"
         val json = """[1 2 3 4]"""
 
         assertFailsWithSerialMessage("JsonDecodingException", m) {
@@ -88,11 +88,21 @@ class MissingCommaTest : JsonTestBase() {
 
     @Test
     fun missingCommaInStringMap() {
-        val m = "Unexpected JSON token at offset 9: Expected comma after the key-value pair at path: \$['a']"
+        val m = "Unexpected JSON token at offset 9: Expected end of the object or comma at path: \$"
         val json = """{"a":"1" "b":"2"}"""
 
         assertFailsWithSerialMessage("JsonDecodingException", m) {
             default.decodeFromString<Map<String, String>>(json)
         }
     }
+
+    @Test
+    fun missingCommaInUnknownKeys() {
+        val m = "Unexpected JSON token at offset 17: Expected end of the object or comma at path: \$"
+        val json = """{"i":"1","b":"2" "c":"2"}"""
+
+        assertFailsWithSerialMessage("JsonDecodingException", m) {
+            lenient.decodeFromString<Child>(json)
+        }
+    }
 }

formats/json-tests/commonTest/src/kotlinx/serialization/json/TrailingCommaTest.kt

@@ -125,4 +125,45 @@ class TrailingCommaTest : JsonTestBase() {
             "wl":{"l":[1, 2, 3,],},}"""
         assertEquals(Mixed(multipleFields, withMap, withList), tj.decodeFromString(input, mode))
     }
+
+    @Test
+    fun testCommaReportedPosition() = parametrizedTest { mode ->
+        val sd = """{"a":"1",    }"""
+
+        checkSerializationException({
+            default.decodeFromString<Map<String, String>>(sd, mode)
+        }, { message ->
+            assertContains(
+                message,
+                """Unexpected JSON token at offset 8: Trailing comma before the end of JSON object"""
+            )
+        })
+    }
+
+    @Test
+    fun testMultipleTrailingCommasAreNotAllowedEvenWhenTrailingIsEnabled() = parametrizedTest { mode ->
+        val sd = """{"a":"1",,}"""
+        checkSerializationException({
+            tj.decodeFromString<Map<String, String>>(sd, mode)
+        }, { message ->
+            assertContains(
+                message,
+                """Unexpected JSON token at offset 9: Multiple consecutive commas are not allowed in JSON"""
+            )
+        })
+    }
+
+    @Test
+    fun testMultipleTrailingCommasError() = parametrizedTest { mode ->
+        val sd = """{"a":"1",,,}"""
+
+        checkSerializationException({
+            default.decodeFromString<Map<String, String>>(sd, mode)
+        }, { message ->
+            assertContains(
+                message,
+                """Unexpected JSON token at offset 9: Multiple consecutive commas are not allowed in JSON"""
+            )
+        })
+    }
 }

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonExceptions.kt

@@ -46,13 +46,6 @@ internal fun AbstractJsonLexer.throwInvalidFloatingPointDecoded(result: Number):
         hint = specialFlowingValuesHint)
 }
 
-internal fun AbstractJsonLexer.invalidTrailingComma(entity: String = "object"): Nothing {
-    fail("Trailing comma before the end of JSON $entity",
-        position = currentPosition - 1,
-        hint = "Trailing commas are non-complaint JSON and not allowed by default. Use 'allowTrailingComma = true' in 'Json {}' builder to support them."
-    )
-}
-
 @OptIn(ExperimentalSerializationApi::class)
 internal fun InvalidKeyKindException(keyDescriptor: SerialDescriptor) = JsonEncodingException(
     "Value of type '${keyDescriptor.serialName}' can't be used in JSON as a key in the map. " +

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonTreeReader.kt

@@ -4,7 +4,7 @@
 
 package kotlinx.serialization.json.internal
 
-import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
 @OptIn(ExperimentalSerializationApi::class)
@@ -24,53 +24,52 @@ internal class JsonTreeReader(
         readObjectImpl { callRecursive(Unit) }
 
     private inline fun readObjectImpl(reader: () -> JsonElement): JsonObject {
-        var lastToken = lexer.consumeNextToken(TC_BEGIN_OBJ)
+        lexer.consumeNextToken(TC_BEGIN_OBJ)
+        lexer.path.pushDescriptor(JsonObjectSerializer.descriptor)
+
         if (lexer.peekNextToken() == TC_COMMA) lexer.fail("Unexpected leading comma")
         val result = linkedMapOf<String, JsonElement>()
         while (lexer.canConsumeValue()) {
+            lexer.path.resetCurrentMapKey()
             // Read key and value
             val key = if (isLenient) lexer.consumeStringLenient() else lexer.consumeString()
             lexer.consumeNextToken(TC_COLON)
+
+            lexer.path.updateCurrentMapKey(key)
             val element = reader()
             result[key] = element
             // Verify the next token
-            lastToken = lexer.consumeNextToken()
-            when (lastToken) {
-                TC_COMMA -> Unit // no-op, can continue with `canConsumeValue` that verifies the token after comma
-                TC_END_OBJ -> break // `canConsumeValue` can return incorrect result, since it checks token _after_ TC_END_OBJ
-                else -> lexer.fail("Expected end of the object or comma")
-            }
+            lexer.consumeCommaOrPeekEnd(
+                allowTrailing = trailingCommaAllowed,
+                expectedEnd = '}',
+            )
         }
-        // Check for the correct ending
-        if (lastToken == TC_BEGIN_OBJ) { // Case of empty object
-            lexer.consumeNextToken(TC_END_OBJ)
-        } else if (lastToken == TC_COMMA) { // Trailing comma
-            if (!trailingCommaAllowed) lexer.invalidTrailingComma()
-            lexer.consumeNextToken(TC_END_OBJ)
-        } // else unexpected token?
+
+        lexer.consumeNextToken(TC_END_OBJ)
+        lexer.path.popDescriptor()
+
         return JsonObject(result)
     }
 
     private fun readArray(): JsonElement {
-        var lastToken = lexer.consumeNextToken()
+        lexer.consumeNextToken(TC_BEGIN_LIST)
+        lexer.path.pushDescriptor(JsonArraySerializer.descriptor)
         // Prohibit leading comma
         if (lexer.peekNextToken() == TC_COMMA) lexer.fail("Unexpected leading comma")
         val result = arrayListOf<JsonElement>()
         while (lexer.canConsumeValue()) {
+            lexer.path.updateDescriptorIndex(result.size)
             val element = read()
             result.add(element)
-            lastToken = lexer.consumeNextToken()
-            if (lastToken != TC_COMMA) {
-                lexer.require(lastToken == TC_END_LIST) { "Expected end of the array or comma" }
-            }
-        }
-        // Check for the correct ending
-        if (lastToken == TC_BEGIN_LIST) { // Case of empty object
-            lexer.consumeNextToken(TC_END_LIST)
-        } else if (lastToken == TC_COMMA) { // Trailing comma
-            if (!trailingCommaAllowed) lexer.invalidTrailingComma("array")
-            lexer.consumeNextToken(TC_END_LIST)
+            lexer.consumeCommaOrPeekEnd(
+                allowTrailing = trailingCommaAllowed,
+                expectedEnd = ']',
+                entity = "array"
+            )
         }
+        lexer.consumeNextToken(TC_END_LIST)
+        lexer.path.popDescriptor()
+
         return JsonArray(result)
     }
 
@@ -103,6 +102,7 @@ internal class JsonTreeReader(
                 --stackDepth
                 result
             }
+
             TC_BEGIN_LIST -> readArray()
             else -> lexer.fail("Cannot read Json element because of unexpected ${tokenDescription(token)}")
         }

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt

@@ -123,7 +123,6 @@ internal open class StreamingJsonDecoder(
         if (descriptor.elementsCount == 0 && descriptor.ignoreUnknownKeys(json)) {
             skipLeftoverElements(descriptor)
         }
-        if (lexer.tryConsumeComma() && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma("")
         // First consume the object so we know it's correct
         lexer.consumeNextToken(mode.end)
         // Then cleanup the path
@@ -185,24 +184,21 @@ internal open class StreamingJsonDecoder(
     }
 
     private fun decodeMapIndex(): Int {
-        var hasComma = false
         val decodingKey = currentIndex % 2 != 0
         if (decodingKey) {
             if (currentIndex != -1) {
-                hasComma = lexer.tryConsumeComma()
+                lexer.consumeCommaOrPeekEnd(
+                    allowTrailing = json.configuration.allowTrailingComma,
+                    expectedEnd = mode.end
+                )
             }
         } else {
             lexer.consumeNextToken(COLON)
         }
 
         return if (lexer.canConsumeValue()) {
-            if (decodingKey) {
-                if (currentIndex == -1) lexer.require(!hasComma) { "Unexpected leading comma" }
-                else lexer.require(hasComma) { "Expected comma after the key-value pair" }
-            }
             ++currentIndex
         } else {
-            if (hasComma && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma()
             CompositeDecoder.DECODE_DONE
         }
     }
@@ -220,19 +216,10 @@ internal open class StreamingJsonDecoder(
     private fun decodeObjectIndex(descriptor: SerialDescriptor): Int {
         while (true) {
             if (currentIndex != -1) {
-                val next = lexer.peekNextToken()
-                if (next == TC_END_OBJ) {
-                    currentIndex = CompositeDecoder.DECODE_DONE
-                    return elementMarker?.nextUnmarkedIndex() ?: CompositeDecoder.DECODE_DONE
-                }
-
-                lexer.require(next == TC_COMMA) { "Expected comma after the key-value pair" }
-                val commaPosition = lexer.currentPosition
-                lexer.consumeNextToken()
-                lexer.require(
-                    configuration.allowTrailingComma || lexer.peekNextToken() != TC_END_OBJ,
-                    position = commaPosition
-                ) { "Trailing comma before the end of JSON object" }
+                lexer.consumeCommaOrPeekEnd(
+                    allowTrailing = json.configuration.allowTrailingComma,
+                    expectedEnd = mode.end
+                )
             }
 
             if (!lexer.canConsumeValue()) break
@@ -270,13 +257,17 @@ internal open class StreamingJsonDecoder(
     }
 
     private fun decodeListIndex(): Int {
-        // Prohibit leading comma
-        val hasComma = lexer.tryConsumeComma()
+        if (currentIndex != -1) {
+            lexer.consumeCommaOrPeekEnd(
+                allowTrailing = json.configuration.allowTrailingComma,
+                expectedEnd = mode.end,
+                entity = "array"
+            )
+        }
+
         return if (lexer.canConsumeValue()) {
-            if (currentIndex != -1 && !hasComma) lexer.fail("Expected end of the array or comma")
             ++currentIndex
         } else {
-            if (hasComma && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma("array")
             CompositeDecoder.DECODE_DONE
         }
     }

formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt

@@ -168,17 +168,6 @@ internal abstract class AbstractJsonLexer {
 
     abstract fun consumeNextToken(): Byte
 
-    fun tryConsumeComma(): Boolean {
-        val current = skipWhitespaces()
-        val source = source
-        if (current >= source.length || current == -1) return false
-        if (source[current] == ',') {
-            ++currentPosition
-            return true
-        }
-        return false
-    }
-
     protected fun isValidValueStart(c: Char): Boolean {
         return when (c) {
             '}', ']', ':', ',' -> false
@@ -759,4 +748,37 @@ internal abstract class AbstractJsonLexer {
             currentPosition = snapshot
         }
     }
+
+    fun consumeCommaOrPeekEnd(
+        allowTrailing: Boolean,
+        expectedEnd: Char,
+        entity: String = "object",
+    ) {
+        val nextToken = peekNextToken()
+        if (nextToken == charToTokenClass(expectedEnd)) return
+        if (nextToken != TC_COMMA) {
+            path.popDescriptor()
+            fail("Expected end of the $entity or comma")
+        }
+        val commaPosition = currentPosition++ // consumes peeked comma
+        val peeked = peekNextToken()
+
+        if (peeked == charToTokenClass(expectedEnd) && !allowTrailing) {
+            path.popDescriptor()
+
+            fail(
+                "Trailing comma before the end of JSON $entity",
+                position = commaPosition,
+                hint = "Trailing commas are non-complaint JSON and not allowed by default. Use 'allowTrailingComma = true' in 'Json {}' builder to support them."
+            )
+        }
+        if (peeked == TC_COMMA) {
+            path.popDescriptor()
+            fail(
+                "Multiple consecutive commas are not allowed in JSON",
+                position = currentPosition,
+                hint = "Trailing commas are non-complaint JSON and not allowed by default. Use 'allowTrailingComma = true' in 'Json {}' builder to support them."
+            )
+        }
+    }
 }