Fix MissingFieldException duplication (#2213)

Fixes #2212

Author: qwwdfsad

formats/json-tests/jvmTest/src/kotlinx/serialization/json/MissingFieldExceptionWithPathTest.kt

@@ -0,0 +1,40 @@
+package kotlinx.serialization.json
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.Json.Default.decodeFromString
+import org.junit.*
+import org.junit.Test
+import java.io.ByteArrayInputStream
+import java.io.ByteArrayOutputStream
+import java.io.PrintWriter
+import java.io.StringWriter
+import kotlin.test.*
+
+class MissingFieldExceptionWithPathTest {
+
+    @Test // Repro for #2212
+    fun testMfeIsNotReappliedMultipleTimes() {
+        val inputMalformed = """{"title": "...","cast": [{}]"""
+        try {
+            Json.decodeFromString<Movie>(inputMalformed)
+            fail("Unreacheable state")
+        } catch (e: MissingFieldException) {
+            val fullStackTrace = e.stackTraceToString()
+            val i1 = fullStackTrace.toString().indexOf("at path")
+            val i2 = fullStackTrace.toString().lastIndexOf("at path")
+            assertEquals(i1, i2)
+            assertTrue(i1 != -1)
+        }
+    }
+
+    @Serializable
+    data class Movie(
+        val title: String,
+        val cast: List<Cast>,
+    )
+
+    @Serializable
+    data class Cast(
+        val name: String
+    )
+}

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

@@ -87,6 +87,9 @@ internal open class StreamingJsonDecoder(
             return result
 
         } catch (e: MissingFieldException) {
+            // Add "at path" if and only if we've just caught an exception and it hasn't been augmented yet
+            if (e.message!!.contains("at path")) throw e
+            // NB: we could've use some additional flag marker or augment the stacktrace, but it seemed to be as too much of a burden
             throw MissingFieldException(e.missingFields, e.message + " at path: " + lexer.path.getPath(), e)
         }
     }