From b94d93dd820f2f47a1437180253ce0b616a911cd Mon Sep 17 00:00:00 2001
From: Kyle Brown <crashbox360@gmail.com>
Date: Wed, 26 Aug 2026 19:19:51 -0400
Subject: [PATCH] fix: DSR replies to bare CSI n (missing default status-report
 parameter)
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DSR (CSI Ps n) declared no defaultParameters, so a bare CSI n (Ps omitted
-> args[0]==0) matched neither the args[0]==5 (status) nor args[0]==6 (CPR)
branch and produced no reply. A guest sending a bare status query then
hangs waiting for \033[0n that never comes (§36 m4).

Add defaultParameters returning {5}. CSIManager substitutes a 0/omitted
slot with the handler's declared default BEFORE execute runs (it iterates
defaults.length, so the default fires even at argCount==0), so bare CSI n
now resolves to Ps=5 -> \033[0n. This matches ECMA-48 / common terminal
behavior (Ps=0 treated as Ps=5 for status). CSI 5 n is unchanged (args[0]
==5, no substitution); CSI 6 n (CPR) is unchanged; CSI ? n (DECDSR) path
is untouched.

Tests: dsrBareCsiNRepliesStatus (bare CSI n -> \033[0n) and dsrExplicit-
FiveStillRepliesStatus (explicit 5 unchanged), reading the reply via
terminal.io.getInput(). Revert-and-fail proven: removing defaultParameters
fails the bare-CSI-n test (no reply -> assertNotNull fails). 102/0.

QA: clean build (compileJava + compileTestJava ran, not cached), full
suite green; PMD main 0 (preserved zero-violation baseline), Checkstyle 0
in touched files (the 1 main warning is pre-existing in DeltaFrameCodec),
SpotBugs delta 0.

Assisted by: GLM (syn:large:text on synthetic.new) — code generation and review.
---
 .../common/vm/terminal/escapes/csi/DSR.java   |  8 ++++++
 .../vm/terminal/TerminalBufferTest.java       | 26 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/src/main/java/li/cil/oc2/common/vm/terminal/escapes/csi/DSR.java b/src/main/java/li/cil/oc2/common/vm/terminal/escapes/csi/DSR.java
index 3ef6a558..706f3458 100644
--- a/src/main/java/li/cil/oc2/common/vm/terminal/escapes/csi/DSR.java
+++ b/src/main/java/li/cil/oc2/common/vm/terminal/escapes/csi/DSR.java
@@ -7,6 +7,14 @@ public DSR(final Terminal terminal) {
         super(terminal);
     }
 
+    @Override
+    public int[] defaultParameters(CSIState state) {
+        // Ps omitted (or 0) defaults to 5 — device status report. CSIManager substitutes
+        // this before execute, so a bare CSI n replies \033[0n instead of hanging the guest
+        // waiting for a status response that never comes (§36 m4).
+        return new int[] {5};
+    }
+
     @Override
     public void execute(int[] args, int argCount, CSIState state) {
         if (args[0] == 5) {
diff --git a/src/test/java/li/cil/oc2/common/vm/terminal/TerminalBufferTest.java b/src/test/java/li/cil/oc2/common/vm/terminal/TerminalBufferTest.java
index 4bd87707..6e392814 100644
--- a/src/test/java/li/cil/oc2/common/vm/terminal/TerminalBufferTest.java
+++ b/src/test/java/li/cil/oc2/common/vm/terminal/TerminalBufferTest.java
@@ -130,6 +130,32 @@ void vpaMovesRowOnly() {
         assertEquals(6, terminal.y);
     }
 
+    @Test
+    void dsrBareCsiNRepliesStatus() {
+        // §36 m4: DSR (CSI Ps n) with no Ps defaults to Ps=5 (device status). CSIManager
+        // substitutes the handler's defaultParameters before execute, so a bare CSI n
+        // replies \033[0n instead of hanging a guest waiting for a status response.
+        write(terminal, CSI + "n");
+        final ByteBuffer reply = terminal.io.getInput();
+        assertNotNull(reply, "bare CSI n must produce a status-report reply");
+        final byte[] bytes = new byte[reply.remaining()];
+        reply.get(bytes);
+        assertEquals("\033[0n", new String(bytes, StandardCharsets.US_ASCII),
+                "bare CSI n replies operating-status (\\033[0n)");
+    }
+
+    @Test
+    void dsrExplicitFiveStillRepliesStatus() {
+        // Explicit Ps=5 is unchanged by the defaultParameters fix — still a status report.
+        write(terminal, CSI + "5n");
+        final ByteBuffer reply = terminal.io.getInput();
+        assertNotNull(reply, "CSI 5n must produce a status-report reply");
+        final byte[] bytes = new byte[reply.remaining()];
+        reply.get(bytes);
+        assertEquals("\033[0n", new String(bytes, StandardCharsets.US_ASCII),
+                "CSI 5n replies operating-status (\\033[0n)");
+    }
+
     @Test
     void cudMovesCursorDownAndClampsSaturatedCount() {
         // parseArgument saturates at Integer.MAX_VALUE; terminal.y + args[0] must not overflow to a