code-golfing: cleanup osc color related code

* adds missing function prototype
* move xgetcolor() prototype to win.h (that's where all the other x.c
  func prototype seems to be declared at)
* check for snprintf error/truncation
* reduces code duplication for osc 10/11/12
* unify osc_color_response() and osc4_color_response() into a single function

the latter two was suggested by Quentin Rameau in his patch review on
the hackers list.

(cherry picked from commit 8629d9a1da72cc18568a8f146307b0e939b77ebf)

st.c

@@ -168,6 +168,7 @@ static void csidump(void);
 static void csihandle(void);
 static void csiparse(void);
 static void csireset(void);
+static void osc_color_response(int, int, int);
 static int eschandle(uchar);
 static void strdump(void);
 static void strhandle(void);
@@ -1897,39 +1898,28 @@ csireset(void)
 }
 
 void
-osc4_color_response(int num)
+osc_color_response(int num, int index, int is_osc4)
 {
 	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);
+	if (xgetcolor(is_osc4 ? num : index, &r, &g, &b)) {
+		fprintf(stderr, "erresc: failed to fetch %s color %d\n",
+		        is_osc4 ? "osc4" : "osc",
+		        is_osc4 ? num : index);
 		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]%s%d;rgb:%02x%02x/%02x%02x/%02x%02x\007",
+	             is_osc4 ? "4;" : "", num, r, r, g, g, b, b);
+	if (n < 0 || n >= sizeof(buf)) {
+		fprintf(stderr, "error: %s while printing %s response\n",
+		        n < 0 ? "snprintf failed" : "truncation occurred",
+		        is_osc4 ? "osc4" : "osc");
+	} else {
+		ttywrite(buf, n, 1);
 	}
-
-	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
@@ -1937,6 +1927,11 @@ strhandle(void)
 {
 	char *p = NULL, *dec;
 	int j, narg, par;
+	const struct { int idx; char *str; } osc_table[] = {
+		{ defaultfg, "foreground" },
+		{ defaultbg, "background" },
+		{ defaultcs, "cursor" }
+	};
 
 	term.esc &= ~(ESC_STR_END|ESC_STR);
 	strparse();
@@ -1971,43 +1966,22 @@ strhandle(void)
 			}
 			return;
 		case 10:
-			if (narg < 2)
-				break;
-
-			p = strescseq.args[1];
-
-			if (!strcmp(p, "?"))
-				osc_color_response(defaultfg, 10);
-			else if (xsetcolorname(defaultfg, p))
-				fprintf(stderr, "erresc: invalid foreground color: %s\n", p);
-			else
-				tfulldirt();
-			return;
 		case 11:
-			if (narg < 2)
-				break;
-
-			p = strescseq.args[1];
-
-			if (!strcmp(p, "?"))
-				osc_color_response(defaultbg, 11);
-			else if (xsetcolorname(defaultbg, p))
-				fprintf(stderr, "erresc: invalid background color: %s\n", p);
-			else
-				tfulldirt();
-			return;
 		case 12:
 			if (narg < 2)
 				break;
-
 			p = strescseq.args[1];
-
-			if (!strcmp(p, "?"))
-				osc_color_response(defaultcs, 12);
-			else if (xsetcolorname(defaultcs, p))
-				fprintf(stderr, "erresc: invalid cursor color: %s\n", p);
-			else
+			if ((j = par - 10) < 0 || j >= LEN(osc_table))
+				break; /* shouldn't be possible */
+
+			if (!strcmp(p, "?")) {
+				osc_color_response(par, osc_table[j].idx, 0);
+			} else if (xsetcolorname(osc_table[j].idx, p)) {
+				fprintf(stderr, "erresc: invalid %s color: %s\n",
+				        osc_table[j].str, p);
+			} else {
 				tfulldirt();
+			}
 			return;
 		case 4: /* color set */
 			if (narg < 3)
@@ -2017,9 +1991,9 @@ strhandle(void)
 		case 104: /* color reset */
 			j = (narg > 1) ? atoi(strescseq.args[1]) : -1;
 
-			if (p && !strcmp(p, "?"))
-				osc4_color_response(j);
-			else if (xsetcolorname(j, p)) {
+			if (p && !strcmp(p, "?")) {
+				osc_color_response(j, 0, 1);
+			} 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

@@ -113,8 +113,6 @@ 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);
-
 /* config.h globals */
 extern char *utmp;
 extern char *scroll;

win.h

@@ -30,6 +30,7 @@ void xdrawline(Line, int, int, int);
 void xfinishdraw(void);
 void xloadcols(void);
 int xsetcolorname(int, const char *);
+int xgetcolor(int, unsigned char *, unsigned char *, unsigned char *);
 void xseticontitle(char *);
 void xsettitle(char *);
 int xsetcursor(int);