[9front] libjson can print invalid json

["Dave MacFarlane" <driusan@driusan.net>]

print("%J", json); can print invalid json if a JSON string in the object has a quotation mark in it.

I just threw together this patch to try and escape the strings from printjson() if anyone's interested
in it.

diff 30c5296f32b87d83529d772732726891e1261c9c uncommitted
--- a/sys/src/libjson/printjson.c
+++ b/sys/src/libjson/printjson.c
@@ -52,6 +52,38 @@
 }
 
 static int
+printstring(Fmt *f, char *s)
+{
+	int slen = strlen(s);
+	int i, nq=0;
+	int r;
+	char *dup, lastq;
+	for(i = 0; i < slen; i++)
+		if (s[i] == '"') nq++;
+
+	if (nq == 0)
+		return fmtprint(f, "\"%s\"", s);
+
+	r = fmtprint(f, "\"");
+
+	dup = strdup(s);
+	lastq = -1;
+	for(i = 0; i < slen; i++){
+		if(dup[i] == '"') {
+			dup[i] = 0;
+			r += fmtprint(f, "%s\\\"", &dup[lastq+1]);
+			lastq = i;
+		}
+	}
+
+	if (lastq != slen-1)
+		r += fmtprint(f, "%s", &dup[lastq+1]);
+	r += fmtprint(f, "\"");
+	free(dup);
+	return r;
+}
+
+static int
 printjson(Fmt *f, JSON *j, int indent)
 {
 	switch(j->t){
@@ -65,7 +97,7 @@
 		return fmtprint(f, "%f", j->n);
 		break;
 	case JSONString:
-		return fmtprint(f, "\"%s\"", j->s);
+		return printstring(f, j->s);
 		break;
 	case JSONArray:
 		return printarray(f, j, indent+1);