Add support for OSC color sequences

Ref.
  - https://git.suckless.org/st/commit/8e310303903792c010d03c046ba75f8b18f7d3a7.html
  - https://git.suckless.org/st/commit/273db5ceaf392e68c2faf8f7dec14ea2e25e980d.html

st.c

@@ -168,6 +168,8 @@ static void readcolonargs(char **, int, int[][CAR_PER_ARG]);
 #endif // UNDERCURL_PATCH
 static void csiparse(void);
 static void csireset(void);
+static void osc4_color_response(int num);
+static void osc_color_response(int index, int num);
 static int eschandle(uchar);
 static void strdump(void);
 static void strhandle(void);
@@ -2393,6 +2395,42 @@ csireset(void)
 	memset(&csiescseq, 0, sizeof(csiescseq));
 }
 
+void
+osc4_color_response(int num)
+{
+	int n;
+	char buf[32];
+	unsigned char r, g, b;
+
+	if (xgetcolor(num, &r, &g, &b)) {
+		fprintf(stderr, "erresc: failed to fetch osc4 color %d\n", num);
+		return;
+	}
+
+	n = snprintf(buf, sizeof buf, "\033]4;%d;rgb:%02x%02x/%02x%02x/%02x%02x\007",
+			num, r, r, g, g, b, b);
+
+	ttywrite(buf, n, 1);
+}
+
+void
+osc_color_response(int index, int num)
+{
+	int n;
+	char buf[32];
+	unsigned char r, g, b;
+
+	if (xgetcolor(index, &r, &g, &b)) {
+		fprintf(stderr, "erresc: failed to fetch osc color %d\n", index);
+		return;
+	}
+
+	n = snprintf(buf, sizeof buf, "\033]%d;rgb:%02x%02x/%02x%02x/%02x%02x\007",
+			num, r, r, g, g, b, b);
+
+	ttywrite(buf, n, 1);
+}
+
 void
 strhandle(void)
 {
@@ -2443,44 +2481,45 @@ strhandle(void)
 				}
 			}
 			return;
-		case 10: /* foreground set */
+		case 10:
-			#if OSC_10_11_12_2_PATCH
 			if (narg < 2)
 				break;
 
 			p = strescseq.args[1];
-			if (xsetcolorname(defaultfg, p))
+
-				fprintf(stderr, "erresc: invalid foreground color %d\n", p);
+			if (!strcmp(p, "?"))
+				osc_color_response(defaultfg, 10);
+			else if (xsetcolorname(defaultfg, p))
+				fprintf(stderr, "erresc: invalid foreground color: %s\n", p);
 			else
 				redraw();
-			break;
 			return;
-			#endif // OSC_10_11_12_2_PATCH
+		case 11:
-		case 11: /* background set */
-			#if OSC_10_11_12_2_PATCH
 			if (narg < 2)
 				break;
 
 			p = strescseq.args[1];
-			if (xsetcolorname(defaultbg, p))
+
-				fprintf(stderr, "erresc: invalid background color %d\n", p);
+			if (!strcmp(p, "?"))
+				osc_color_response(defaultbg, 11);
+			else if (xsetcolorname(defaultbg, p))
+				fprintf(stderr, "erresc: invalid background color: %s\n", p);
 			else
 				redraw();
-			break;
 			return;
-			#endif // OSC_10_11_12_2_PATCH
+		case 12:
-		case 12: /* cursor color */
-			#if OSC_10_11_12_2_PATCH
 			if (narg < 2)
 				break;
 
 			p = strescseq.args[1];
-			if (xsetcolorname(defaultcs, p))
+
-				fprintf(stderr, "erresc: invalid cursor color %d\n", p);
+			if (!strcmp(p, "?"))
+				osc_color_response(defaultcs, 12);
+			else if (xsetcolorname(defaultcs, p))
+				fprintf(stderr, "erresc: invalid cursor color: %s\n", p);
 			else
 				redraw();
-			break;
+			return;
-			#endif // OSC_10_11_12_2_PATCH
 		case 4: /* color set */
 			if ((par == 4 && narg < 3) || narg < 2)
 				break;
@@ -2496,7 +2535,9 @@ strhandle(void)
 			else
 				j = (narg > 1) ? atoi(strescseq.args[1]) : -1;
 
-			if (xsetcolorname(j, p)) {
+			if (!strcmp(p, "?"))
+				osc4_color_response(j);
+			else if (xsetcolorname(j, p)) {
 				if (par == 104 && narg <= 1)
 					return; /* color reset without parameter */
 				fprintf(stderr, "erresc: invalid color j=%d, p=%s\n",

st.h

@@ -325,6 +325,9 @@ size_t utf8encode(Rune, char *);
 void *xmalloc(size_t);
 void *xrealloc(void *, size_t);
 char *xstrdup(const char *);
+
+int xgetcolor(int x, unsigned char *r, unsigned char *g, unsigned char *b);
+
 #if BOXDRAW_PATCH
 int isboxdraw(Rune);
 ushort boxdrawindex(const Glyph *);

x.c

@@ -934,6 +934,19 @@ xloadcols(void)
 }
 #endif // ALPHA_FOCUS_HIGHLIGHT_PATCH
 
+int
+xgetcolor(int x, unsigned char *r, unsigned char *g, unsigned char *b)
+{
+	if (!BETWEEN(x, 0, dc.collen))
+		return 1;
+
+	*r = dc.col[x].color.red >> 8;
+	*g = dc.col[x].color.green >> 8;
+	*b = dc.col[x].color.blue >> 8;
+
+	return 0;
+}
+
 int
 xsetcolorname(int x, const char *name)
 {