#107 Monitor STDERR and refactor StdioClientTransport to use flows (#442)

kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/stdio/StdioClientTransportErrorHandlingTest.kt

@@ -0,0 +1,98 @@
+package io.modelcontextprotocol.kotlin.sdk.client.stdio
+
+import io.kotest.matchers.booleans.shouldBeFalse
+import io.kotest.matchers.shouldBe
+import io.modelcontextprotocol.kotlin.sdk.client.StdioClientTransport
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.test.runTest
+import kotlinx.io.Buffer
+import kotlinx.io.writeString
+import kotlin.concurrent.atomics.AtomicBoolean
+import kotlin.concurrent.atomics.ExperimentalAtomicApi
+import kotlin.test.Test
+import kotlin.time.Duration.Companion.milliseconds
+
+/**
+ * Tests for StdioClientTransport error handling: EOF, IO errors, and edge cases.
+ */
+class StdioClientTransportErrorHandlingTest {
+
+    private lateinit var transport: StdioClientTransport
+
+    @OptIn(ExperimentalAtomicApi::class)
+    @Test
+    fun `should continue on stderr EOF`() = runTest {
+        val stderrBuffer = Buffer()
+        // Empty stderr = immediate EOF
+
+        val inputBuffer = Buffer()
+        inputBuffer.writeString("""data: {"jsonrpc":"2.0","method":"ping","id":1}\n\n""")
+        val outputBuffer = Buffer()
+
+        transport = StdioClientTransport(
+            input = inputBuffer,
+            output = outputBuffer,
+            error = stderrBuffer,
+        )
+
+        val closeCalled = AtomicBoolean(false)
+        transport.onClose { closeCalled.store(true) }
+
+        transport.start()
+        delay(200.milliseconds)
+
+        // Stderr EOF should not close transport
+        closeCalled.load() shouldBe false
+
+        transport.close()
+        closeCalled.load() shouldBe true
+    }
+
+    @Test
+    fun `should call onClose exactly once on error scenarios`() = runTest {
+        val stderrBuffer = Buffer()
+        stderrBuffer.write("FATAL: critical error\n".encodeToByteArray())
+
+        val inputBuffer = Buffer()
+        val outputBuffer = Buffer()
+
+        var closeCallCount = 0
+
+        transport = StdioClientTransport(
+            input = inputBuffer,
+            output = outputBuffer,
+            error = stderrBuffer,
+            classifyStderr = { StdioClientTransport.StderrSeverity.FATAL },
+        )
+
+        transport.onClose { closeCallCount++ }
+
+        transport.start()
+        delay(100.milliseconds)
+
+        // Explicit close after error already closed it
+        transport.close()
+
+        closeCallCount shouldBe 1
+    }
+
+    @Test
+    fun `should handle empty input gracefully`() = runTest {
+        val inputBuffer = Buffer()
+        val outputBuffer = Buffer()
+
+        transport = StdioClientTransport(
+            input = inputBuffer,
+            output = outputBuffer,
+        )
+
+        var errorCalled = false
+        transport.onError { errorCalled = true }
+
+        transport.start()
+        delay(100.milliseconds)
+
+        // Empty input should close cleanly without error
+        errorCalled.shouldBeFalse()
+    }
+}