Fix mg_get_http_var retval

PUBLISHED_FROM=257deff5ea20302627e77a0f29cf2359276a41d4

docs/c-api/http_server.h/mg_get_http_var.md

@@ -12,5 +12,6 @@ Fetches a HTTP form variable.
 Fetches a variable `name` from a `buf` into a buffer specified by `dst`,
 `dst_len`. The destination is always zero-terminated. Returns the length of
 a fetched variable. If not found, 0 is returned. `buf` must be valid
-url-encoded buffer. If destination is too small, `-1` is returned. 
+url-encoded buffer. If destination is too small or an error occured,
+negative number is returned. 
 

mongoose.c

@@ -6492,6 +6492,13 @@ int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
   size_t name_len;
   int len;
 
+  /*
+   * According to the documentation function returns negative
+   * value in case of error. For debug purposes it returns:
+   * -1 - src is wrong (NUUL)
+   * -2 - dst is wrong (NULL)
+   * -3 - failed to decode url or dst is to small
+   */
   if (dst == NULL || dst_len == 0) {
     len = -2;
   } else if (buf->p == NULL || name == NULL || buf->len == 0) {
@@ -6500,7 +6507,7 @@ int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
   } else {
     name_len = strlen(name);
     e = buf->p + buf->len;
-    len = -1;
+    len = 0;
     dst[0] = '\0';
 
     for (p = buf->p; p + name_len < e; p++) {
@@ -6512,8 +6519,9 @@ int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
           s = e;
         }
         len = mg_url_decode(p, (size_t)(s - p), dst, dst_len, 1);
+        /* -1 means: failed to decode or dst is too small */
         if (len == -1) {
-          len = -2;
+          len = -3;
         }
         break;
       }

mongoose.h

@@ -4516,7 +4516,8 @@ size_t mg_parse_multipart(const char *buf, size_t buf_len, char *var_name,
  * Fetches a variable `name` from a `buf` into a buffer specified by `dst`,
  * `dst_len`. The destination is always zero-terminated. Returns the length of
  * a fetched variable. If not found, 0 is returned. `buf` must be valid
- * url-encoded buffer. If destination is too small, `-1` is returned.
+ * url-encoded buffer. If destination is too small or an error occured,
+ * negative number is returned.
  */
 int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
                     size_t dst_len);