Fixed: Parse (but ignore for now) terminal APC sequences

fornwall · agnostic-apollo · commit f35063da7491 · 2025-03-28T10:06:48.000+05:00
diff --git a/terminal-emulator/src/main/java/com/termux/terminal/TerminalEmulator.java b/terminal-emulator/src/main/java/com/termux/terminal/TerminalEmulator.java
@@ -79,6 +79,10 @@ public final class TerminalEmulator {
     private static final int ESC_CSI_SINGLE_QUOTE = 18;
     /** Escape processing: CSI ! */
     private static final int ESC_CSI_EXCLAMATION = 19;
+    /** Escape processing: "ESC _" or Application Program Command (APC). */
+    private static final int ESC_APC = 20;
+    /** Escape processing: "ESC _" or Application Program Command (APC), followed by Escape. */
+    private static final int ESC_APC_ESCAPE = 21;
 
     /** The number of parameter arguments. This name comes from the ANSI standard for terminal escape codes. */
     private static final int MAX_ESCAPE_PARAMETERS = 16;
@@ -545,6 +549,15 @@ private void processByte(byte byteToProcess) {
     }
 
     public void processCodePoint(int b) {
+        // The Application Program-Control (APC) string might be arbitrary non-printable characters, so handle that early.
+        if (mEscapeState == ESC_APC) {
+            doApc(b);
+            return;
+        } else if (mEscapeState == ESC_APC_ESCAPE) {
+            doApcEscape(b);
+            return;
+        }
+
         switch (b) {
             case 0: // Null character (NUL, ^@). Do nothing.
                 break;
@@ -1001,6 +1014,30 @@ private void doDeviceControl(int b) {
         }
     }
 
+    /**
+     * When in {@link #ESC_APC} (APC, Application Program Command) sequence.
+     */
+    private void doApc(int b) {
+        if (b == 27) {
+            continueSequence(ESC_APC_ESCAPE);
+        }
+        // Eat APC sequences silently for now.
+    }
+
+    /**
+     * When in {@link #ESC_APC} (APC, Application Program Command) sequence.
+     */
+    private void doApcEscape(int b) {
+        if (b == '\\') {
+            // A String Terminator (ST), ending the APC escape sequence.
+            finishSequence();
+        } else {
+            // The Escape character was not the start of a String Terminator (ST),
+            // but instead just data inside of the APC escape sequence.
+            continueSequence(ESC_APC);
+        }
+    }
+
     private int nextTabStop(int numTabs) {
         for (int i = mCursorCol + 1; i < mColumns; i++)
             if (mTabStop[i] && --numTabs == 0) return Math.min(i, mRightMargin);
@@ -1396,6 +1433,9 @@ private void doEsc(int b) {
             case '>': // DECKPNM
                 setDecsetinternalBit(DECSET_BIT_APPLICATION_KEYPAD, false);
                 break;
+            case '_': // APC - Application Program Command.
+                continueSequence(ESC_APC);
+                break;
             default:
                 unknownSequence(b);
                 break;
diff --git a/terminal-emulator/src/test/java/com/termux/terminal/ApcTest.java b/terminal-emulator/src/test/java/com/termux/terminal/ApcTest.java
@@ -0,0 +1,21 @@
+package com.termux.terminal;
+
+public class ApcTest extends TerminalTestCase {
+
+    public void testApcConsumed() {
+        // At time of writing this is part of what yazi sends for probing for kitty graphics protocol support:
+        // https://github.com/sxyazi/yazi/blob/0cdaff98d0b3723caff63eebf1974e7907a43a2c/yazi-adapter/src/emulator.rs#L129
+        // This should not result in anything being written to the screen: If kitty graphics protocol support
+        // is implemented it should instead result in an error code on stdin, and if not it should be consumed
+        // silently just as xterm does. See https://sw.kovidgoyal.net/kitty/graphics-protocol/.
+        withTerminalSized(2, 2)
+            .enterString("\033_Gi=31,s=1,v=1,a=q,t=d,f=24;AAAA\033\\")
+            .assertLinesAre("  ", "  ");
+
+        // It is ok for the APC content to be non printable characters:
+        withTerminalSized(12, 2)
+            .enterString("hello \033_some\023\033_\\apc#end\033\\ world")
+            .assertLinesAre("hello  world", "            ");
+    }
+
+}
