Frozen: fix delegation to system printf

Previously, the output length was limited by 20 chars

PUBLISHED_FROM=f4ceb624fa1d0f46833181ac4de9a3a6c4f14741

frozen.c

@@ -514,21 +514,37 @@ int json_vprintf(struct json_out *out, const char *fmt, va_list xap) {
           len += out->printer(out, quote, 1);
         }
       } else {
+        /*
+         * we delegate printing to the system printf.
+         * The goal here is to delegate all modifiers parsing to the system
+         * printf, as you can see below we still have to parse the format
+         * types.
+         *
+         * Currently, %s with strings longer than 20 chars will require
+         * double-buffering (an auxiliary buffer will be allocated from heap).
+         * TODO(dfrank): reimplement %s and %.*s in order to avoid that.
+         */
+
         const char *end_of_format_specifier = "sdfFgGlhuI.*-0123456789";
         size_t n = strspn(fmt + 1, end_of_format_specifier);
+        char *pbuf = buf;
+        size_t need_len;
         char fmt2[20];
         va_list sub_ap;
         strncpy(fmt2, fmt, n + 1 > sizeof(fmt2) ? sizeof(fmt2) : n + 1);
         fmt2[n + 1] = '\0';
 
-        /*
-         * we delegate printing to the system printf.
-         * The goal here is to delegate all modifiers parsing to the system
-         * printf, as you can see below we still have to parse the format
-         * types.
-         */
         va_copy(sub_ap, ap);
-        vsnprintf(buf, sizeof(buf), fmt2, sub_ap);
+        need_len = vsnprintf(buf, sizeof(buf), fmt2, sub_ap) + 1 /* null-term */;
+        if (need_len > sizeof(buf)) {
+          /*
+           * resulting string doesn't fit into a stack-allocated buffer `buf`,
+           * so we need to allocate a new buffer from heap and use it
+           */
+          pbuf = (char *)malloc(need_len);
+          va_copy(sub_ap, ap);
+          vsnprintf(pbuf, need_len, fmt2, sub_ap);
+        }
 
         /*
          * however we need to parse the type ourselves in order to advance
@@ -562,8 +578,14 @@ int json_vprintf(struct json_out *out, const char *fmt, va_list xap) {
           }
         }
 
-        len += out->printer(out, buf, strlen(buf));
+        len += out->printer(out, pbuf, strlen(pbuf));
         skip = n + 1;
+
+        /* If buffer was allocated from heap, free it */
+        if (pbuf != buf) {
+          free(pbuf);
+          pbuf = NULL;
+        }
       }
       fmt += skip;
     } else if (is_alpha(*fmt)) {

unit_test.c

@@ -269,6 +269,17 @@ static const char *test_json_printf(void) {
     ASSERT(memcmp(buf, "\"foo\"", 5) == 0);
   }
 
+  {
+    /*
+     * Check long string (which forces frozen to allocate a temporary buffer
+     * from heap)
+     */
+    struct json_out out = JSON_OUT_BUF(buf, sizeof(buf));
+    const char *result = "{\"foo\": \"1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890\"}";
+    json_printf(&out, "{foo: %s}", "\"1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890\"");
+    ASSERT(strcmp(buf, result) == 0);
+  }
+
   return NULL;
 }