Query terminal for bce, rep and tsl capabilities via XTGETTCAP

This is supported by xterm, foot, ghostty, iterm2 and perhaps a few
other terminals. It serves a similar purpose to querying `terminfo(5)`
capabilities from the filesystem, but with the following advantages:

* Doesn't require a terminfo(5) entry (or database) to be installed
* Doesn't require linking to libtinfo (or reimplementing parts of it)
* Works reliably over SSH
* Works reliably (in principle, at least) when spoofing `$TERM`, as
  is done automatically in some contexts (e.g. Docker)

See also:

* https://invisible-island.net/xterm/ctlseqs/ctlseqs.html
* https://codeberg.org/dnkl/foot/src/commit/a1ac37e771edf9fe907ee09dbec0e90d447679e1/doc/foot-ctlseqs.7.scd#L756-L757
* moby/moby#42323
* https://docs.docker.com/engine/reference/run/#environment-variables

mk/build.mk

@@ -53,8 +53,8 @@ syntax_objects := $(call prefix-obj, build/syntax/, \
     color highlight merge state syntax )
 
 terminal_objects := $(call prefix-obj, build/terminal/, \
-    color cursor input ioctl key linux mode osc52 output parse paste rxvt \
-    style terminal )
+    color cursor input ioctl key linux mode osc52 output parse paste \
+    query rxvt style terminal )
 
 editor_objects := $(call prefix-obj, build/, \
     bind block block-iter bookmark buffer change cmdline commands \

src/terminal/input.c

@@ -165,12 +165,14 @@ static bool is_text(const char *str, size_t len)
 
 static const char *tflag_to_str(TermFeatureFlags flag)
 {
-    // Note: this only handles a small subset of individual flags,
-    // as returned by parse_csi_query_reply()
-    if (flag == TFLAG_KITTY_KEYBOARD) {
-        return "KITTYKBD";
-    } else if (flag == TFLAG_SYNC) {
-        return "SYNC";
+    // This only handles a subset of individual flags, as returned
+    // by parse_csi_query_reply() and parse_dcs_query_reply()
+    switch ((unsigned int)flag) {
+    case TFLAG_KITTY_KEYBOARD: return "KITTYKBD";
+    case TFLAG_SYNC: return "SYNC";
+    case TFLAG_BACK_COLOR_ERASE: return "BCE";
+    case TFLAG_ECMA48_REPEAT: return "REP";
+    case TFLAG_SET_WINDOW_TITLE: return "TITLE";
     }
     return "??";
 }
@@ -179,7 +181,7 @@ static KeyCode handle_query_reply(Terminal *term, KeyCode key)
 {
     TermFeatureFlags flag = key & ~KEYCODE_QUERY_REPLY_BIT;
     BUG_ON(!IS_POWER_OF_2(flag)); // Only 1 flag should be set
-    LOG_INFO("detected terminal feature '%s' via query", tflag_to_str(flag));
+    LOG_INFO("detected terminal feature %s via query", tflag_to_str(flag));
     term->features |= flag;
     return KEY_IGNORE;
 }

src/terminal/output.c

@@ -151,6 +151,9 @@ void term_put_queries(TermOutputBuffer *obuf)
         "\033[?u" // Kitty keyboard protocol flags
         "\033[?4m" // XTQMODKEYS 4 (xterm modifyOtherKeys mode)
         "\033[?2026$p" // DECRQM 2026 (terminal-wg synchronized updates)
+        "\033P+q626365\033\\" // XTGETTCAP "bce"
+        "\033P+q726570\033\\" // XTGETTCAP "rep"
+        "\033P+q74736C\033\\" // XTGETTCAP "tsl"
     ;
 
     LOG_INFO("querying terminal");

src/terminal/parse.c

@@ -10,6 +10,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 #include "parse.h"
+#include "query.h"
 #include "terminal.h"
 #include "util/ascii.h"
 #include "util/debug.h"
@@ -717,7 +718,7 @@ static ssize_t parse_dcs(const char *buf, size_t len, size_t i, KeyCode *k)
                 *k = KEY_IGNORE;
                 return i;
             case 0x1B: // ESC (https://vt100.net/emu/dec_ansi_parser#STESC)
-                *k = KEY_IGNORE;
+                *k = parse_dcs_query_reply(data, pos, pos >= sizeof(data));
                 return i - 1;
             }
             continue;

src/terminal/query.c

@@ -0,0 +1,66 @@
+#include "query.h"
+#include "cursor.h"
+#include "terminal.h"
+#include "util/log.h"
+#include "util/string-view.h"
+#include "util/strtonum.h"
+
+KeyCode parse_dcs_query_reply(const char *data, size_t len, bool truncated)
+{
+    const char *note = "";
+    if (unlikely(len == 0 || truncated)) {
+        note = truncated ? " (truncated)" : " (empty)";
+        goto unhandled;
+    }
+
+    StringView seq = string_view(data, len);
+    if (strview_has_prefix(&seq, "1+r")) {
+        strview_remove_prefix(&seq, 3);
+        if (strview_equal_cstring(&seq, "626365")) { // "bce"
+            return KEYCODE_QUERY_REPLY_BIT | TFLAG_BACK_COLOR_ERASE;
+        }
+        if (strview_equal_cstring_icase(&seq, "74736C=1B5D323B")) { // "tsl=\e]2;"
+            return KEYCODE_QUERY_REPLY_BIT | TFLAG_SET_WINDOW_TITLE;
+        }
+        if (
+            strview_has_prefix(&seq, "726570=") // "rep="
+            && strview_has_suffix(&seq, "62") // "b"
+            && (seq.length & 1) // Odd length (2x even hex strings + "=")
+        ) {
+            return KEYCODE_QUERY_REPLY_BIT | TFLAG_ECMA48_REPEAT;
+        }
+        LOG_DEBUG("unhandled XTGETTCAP reply: %.*s", (int)len, data);
+        return KEY_IGNORE;
+    }
+
+    if (strview_has_prefix(&seq, "1$r")) {
+        strview_remove_prefix(&seq, 3);
+        if (strview_has_suffix(&seq, " q")) {
+            size_t n = seq.length - 2;
+            unsigned int x;
+            if (n >= 1 && n == buf_parse_uint(seq.data, n, &x)) {
+                const char *str = (x <= CURSOR_STEADY_BAR) ? cursor_type_to_str(x) : "??";
+                LOG_DEBUG("DECRQSS DECSCUSR (cursor style) reply: %u (%s)", x, str);
+                return KEY_IGNORE;
+            }
+        }
+        // TODO: Detect RGB and italic support from DECRQSS "1$r0;3;38:2::60:70:80m" reply
+        // TODO: To facilitate the above, convert Terminal::color_type to TermFeatureFlags bits
+        LOG_DEBUG("unhandled DECRQSS reply: %.*s", (int)len, data);
+        return KEY_IGNORE;
+    }
+
+    if (strview_has_prefix(&seq, "0+r")) {
+        note = " (XTGETTCAP; 'invalid request')";
+        goto unhandled;
+    }
+
+    if (strview_equal_cstring(&seq, "0$r")) {
+        note = " (DECRQSS; 'invalid request')";
+        goto unhandled;
+    }
+
+unhandled:
+    LOG_DEBUG("unhandled DCS string%s: %.*s", note, (int)len, data);
+    return KEY_IGNORE;
+}

src/terminal/query.h

@@ -0,0 +1,11 @@
+#ifndef TERMINAL_QUERY_H
+#define TERMINAL_QUERY_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include "key.h"
+#include "util/macros.h"
+
+KeyCode parse_dcs_query_reply(const char *data, size_t len, bool truncated) NONNULL_ARGS WARN_UNUSED_RESULT;
+
+#endif