Fixed: Fully consume unknown CSI sequences containing unsupported parameter and intermediate bytes

Standard ECMA-48: Control Functions for Coded Character Sets specifies the format of CSI commands.
- https://en.wikipedia.org/wiki/ANSI_escape_code#Control_Sequence_Introducer_commands
- https://invisible-island.net/xterm/ecma-48-parameter-format.html#section5.4

Previously unsupported bytes would be echoed to the terminal.

```shell
$ printf '\x1b[=u' # PF
u
$ printf '\x1b[=5u' # PPF
5u
$ printf '\x1b[=5!u' # PPIF
5!u
$ printf '\x1b[=5!%u' # PPIIF
5!0
$ printf '\x1b[=?5!%u' # PPPIIF
?5!0
```

This fixes a problem with fish shell 4.0.0 which uses that sequence.

Closes termux#4338

Co-authored-by: @krobelus <[email protected]>
Co-authored-by: @agnostic-apollo  <[email protected]>

Author: krobelus

terminal-emulator/src/main/java/com/termux/terminal/TerminalEmulator.java

@@ -83,6 +83,10 @@ public final class TerminalEmulator {
     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;
+    /** Escape processing: ESC [ <parameter bytes> */
+    private static final int ESC_CSI_UNSUPPORTED_PARAMETER_BYTE = 22;
+    /** Escape processing: ESC [ <parameter bytes> <intermediate bytes> */
+    private static final int ESC_CSI_UNSUPPORTED_INTERMEDIATE_BYTE = 23;
 
     /** The number of parameter arguments including colon separated sub-parameters. */
     private static final int MAX_ESCAPE_PARAMETERS = 32;
@@ -658,6 +662,10 @@ public void processCodePoint(int b) {
                     case ESC_CSI:
                         doCsi(b);
                         break;
+                    case ESC_CSI_UNSUPPORTED_PARAMETER_BYTE:
+                    case ESC_CSI_UNSUPPORTED_INTERMEDIATE_BYTE:
+                        doCsiUnsupportedParameterOrIntermediateByte(b);
+                        break;
                     case ESC_CSI_EXCLAMATION:
                         if (b == 'p') { // Soft terminal reset (DECSTR, http://vt100.net/docs/vt510-rm/DECSTR).
                             reset();
@@ -1059,6 +1067,37 @@ private int nextTabStop(int numTabs) {
         return mRightMargin - 1;
     }
 
+    /**
+     * Process byte while in the {@link #ESC_CSI_UNSUPPORTED_PARAMETER_BYTE} or
+     * {@link #ESC_CSI_UNSUPPORTED_INTERMEDIATE_BYTE} escape state.
+     *
+     * Parse unsupported parameter, intermediate and final bytes but ignore them.
+     *
+     * > For Control Sequence Introducer, ... the ESC [ is followed by
+     * > - any number (including none) of "parameter bytes" in the range 0x30–0x3F (ASCII 0–9:;<=>?),
+     * > - then by any number of "intermediate bytes" in the range 0x20–0x2F (ASCII space and !"#$%&'()*+,-./),
+     * > - then finally by a single "final byte" in the range 0x40–0x7E (ASCII @A–Z[\]^_`a–z{|}~).
+     *
+     * - https://en.wikipedia.org/wiki/ANSI_escape_code#Control_Sequence_Introducer_commands
+     * - https://invisible-island.net/xterm/ecma-48-parameter-format.html#section5.4
+     */
+    private void doCsiUnsupportedParameterOrIntermediateByte(int b) {
+        if (mEscapeState == ESC_CSI_UNSUPPORTED_PARAMETER_BYTE && b >= 0x30 && b <= 0x3F) {
+            // Supported `0–9:;>?` or unsupported `<=` parameter byte after an
+            // initial unsupported parameter byte in `doCsi()`, or a sequential parameter byte.
+            continueSequence(ESC_CSI_UNSUPPORTED_PARAMETER_BYTE);
+        } else if (b >= 0x20 && b <= 0x2F) {
+            // Optional intermediate byte `!"#$%&'()*+,-./` after parameter or intermediate byte.
+            continueSequence(ESC_CSI_UNSUPPORTED_INTERMEDIATE_BYTE);
+        } else if (b >= 0x40 && b <= 0x7E) {
+            // Final byte `@A–Z[\]^_`a–z{|}~` after parameter or intermediate byte.
+            // Calling `unknownSequence()` would log an error with only a final byte, so ignore it for now.
+            finishSequence();
+        } else {
+            unknownSequence(b);
+        }
+    }
+
     /** Process byte while in the {@link #ESC_CSI_QUESTIONMARK} escape state. */
     private void doCsiQuestionMark(int b) {
         switch (b) {
@@ -1656,12 +1695,16 @@ private void doCsi(int b) {
                     }
                 mCursorCol = newCol;
                 break;
-            case '?': // Esc [ ? -- start of a private mode set
+            case '?': // Esc [ ? -- start of a private parameter byte
                 continueSequence(ESC_CSI_QUESTIONMARK);
                 break;
-            case '>': // "Esc [ >" --
+            case '>': // "Esc [ >" -- start of a private parameter byte
                 continueSequence(ESC_CSI_BIGGERTHAN);
                 break;
+            case '<': // "Esc [ <" -- start of a private parameter byte
+            case '=': // "Esc [ =" -- start of a private parameter byte
+                continueSequence(ESC_CSI_UNSUPPORTED_PARAMETER_BYTE);
+                break;
             case '`': // Horizontal position absolute (HPA - http://www.vt100.net/docs/vt510-rm/HPA).
                 setCursorColRespectingOriginMode(getArg0(1) - 1);
                 break;