Fix incorrect json decoding iterator's .hasNext() behavior on array-wrapped inputs: (#2268)

hasNext() shouldn't throw exception if it was called more than once
after the stream has ended.

Fixes #2267

Author: sandwwraith

formats/json-tests/jvmTest/src/kotlinx/serialization/features/JsonLazySequenceTest.kt

@@ -83,6 +83,7 @@ class JsonLazySequenceTest {
         iter.assertNext(StringData("b"))
         iter.assertNext(StringData("c"))
         assertFalse(iter.hasNext())
+        assertFalse(iter.hasNext()) // Subsequent calls to .hasNext() should not throw EOF or anything
         assertFailsWithMessage<SerializationException>("EOF") {
             iter.next()
         }
@@ -186,4 +187,11 @@ class JsonLazySequenceTest {
         assertEquals(inputList, json.decodeToSequence(paddedWs.asInputStream(), StringData.serializer(), DecodeSequenceMode.ARRAY_WRAPPED).toList())
     }
 
+    @Test
+    fun testToIteratorAndBack() = withInputs { ins ->
+        val iterator = Json.decodeToSequence(ins, StringData.serializer()).iterator()
+        val values = iterator.asSequence().take(3).toList()
+        assertEquals(inputList, values)
+        assertFalse(iterator.hasNext())
+    }
 }

formats/json/commonMain/src/kotlinx/serialization/json/Json.kt

@@ -169,7 +169,7 @@ public enum class DecodeSequenceMode {
 
     /**
      * Declares that parser itself should select between [WHITESPACE_SEPARATED] and [ARRAY_WRAPPED] modes.
-     * The selection is performed by looking on the first meaningful character of the stream.
+     * The selection is performed by looking at the first meaningful character of the stream.
      *
      * In most cases, auto-detection is sufficient to correctly parse an input.
      * If the input is _whitespace-separated stream of the arrays_, parser could select an incorrect mode,

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

@@ -68,6 +68,7 @@ private class JsonIteratorArrayWrapped<T>(
     private val deserializer: DeserializationStrategy<T>
 ) : Iterator<T> {
     private var first = true
+    private var finished = false
 
     override fun next(): T {
         if (first) {
@@ -83,7 +84,9 @@ private class JsonIteratorArrayWrapped<T>(
      * Note: if array separator (comma) is missing, hasNext() returns true, but next() throws an exception.
      */
     override fun hasNext(): Boolean {
+        if (finished) return false
         if (lexer.peekNextToken() == TC_END_LIST) {
+            finished = true
             lexer.consumeNextToken(TC_END_LIST)
             if (lexer.isNotEof()) {
                 if (lexer.peekNextToken() == TC_BEGIN_LIST) lexer.fail("There is a start of the new array after the one parsed to sequence. " +
@@ -93,7 +96,7 @@ private class JsonIteratorArrayWrapped<T>(
             }
             return false
         }
-        if (!lexer.isNotEof()) lexer.fail(TC_END_LIST)
+        if (!lexer.isNotEof() && !finished) lexer.fail(TC_END_LIST)
         return true
     }
 }

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

@@ -30,6 +30,9 @@ internal class ArrayAsSequence(private val buffer: CharArray) : CharSequence {
     fun trim(newSize: Int) {
         length = minOf(buffer.size, newSize)
     }
+
+    // source.toString() is used in JsonDecodingException
+    override fun toString(): String = substring(0, length)
 }
 
 internal class ReaderJsonLexer(