subassconvert: make subrip attribute parsing more robust

Add general code to separate the HTML-like attribute=value syntax used
in srt font tags into attribute and value parts. This simplifies some
of the parsing code, makes detection of malformed input more robust,
and allows warning about unrecognized attributes.

Author: Uoti Urpala

sub/subassconvert.c

@@ -106,25 +106,30 @@ static const struct {
 
 #define SUBRIP_MAX_STACKED_FONT_TAGS    16
 
-/* Read the attribute value starting at *s, and skip *s past the value.
- * Set out_value to the parsed value, with possible '"' stripped.
- * Return whether the attribute is well formed. */
-static bool read_value(char **s, struct bstr *out_value)
+/* Read the HTML-style attribute starting at *s, and skip *s past the value.
+ * Set attr and val to the parsed attribute name and value.
+ * Return 0 on success, or -1 if no valid attribute was found.
+ */
+static int read_attr(char **s, struct bstr *attr, struct bstr *val)
 {
-    char term = 0;
-    if (**s == '"') {
-        term = '"';
-        (*s)++;
-    }
-    out_value->start = *s;
-    out_value->len = 0;
-    unsigned char *start = *s;
-    unsigned char *end = term ? strchr(start, term) : strpbrk(start, " >");
+    char *eq = strchr(*s, '=');
+    if (!eq)
+        return -1;
+    attr->start = *s;
+    attr->len = eq - *s;
+    for (int i = 0; i < attr->len; i++)
+        if (!isalnum(attr->start[i]))
+            return -1;
+    val->start = eq + 1;
+    bool quoted = val->start[0] == '"';
+    if (quoted)
+        val->start++;
+    unsigned char *end = strpbrk(val->start, quoted ? "\"" : " >");
     if (!end)
-        return false;
-    out_value->len = end - out_value->start;
-    *s = end + (term ? 1 : 0);
-    return true;
+        return -1;
+    val->len = end - val->start;
+    *s = end + quoted;
+    return 0;
 }
 
 void subassconvert_subrip(const char *orig, char *dest, int dest_buffer_size)
@@ -195,22 +200,21 @@ void subassconvert_subrip(const char *orig, char *dest, int dest_buffer_size)
             line += 6;
 
             while (*line && *line != '>') {
-                if (strncmp(line, "size=", 5) == 0) {
-                    line += 5;
-                    struct bstr val;
-                    if (!read_value(&line, &val))
-                        break;
+                if (*line == ' ') {
+                    line++;
+                    continue;
+                }
+                struct bstr attr, val;
+                if (read_attr(&line, &attr, &val) < 0)
+                    break;
+                if (!bstrcmp0(attr, "size")) {
                     tag->size = bstrtoll(val, &val, 10);
                     if (val.len)
                         break;
                     append_text(&new_line, "{\\fs%d}", tag->size);
                     tag->has_size = true;
                     has_valid_attr = true;
-                } else if (strncmp(line, "color=", 6) == 0) {
-                    line += 6;
-                    struct bstr val;
-                    if (!read_value(&line, &val))
-                        break;
+                } else if (!bstrcmp0(attr, "color")) {
                     if (bstr_eatstart(&val, bstr("#"))) {
                         // #RRGGBB format
                         tag->color = bstrtoll(val, &val, 16) & 0x00ffffff;
@@ -240,18 +244,15 @@ void subassconvert_subrip(const char *orig, char *dest, int dest_buffer_size)
                     append_text(&new_line, "{\\c&H%06X&}", tag->color);
                     tag->has_color = true;
                     has_valid_attr = true;
-                } else if (strncmp(line, "face=", 5) == 0) {
+                } else if (!bstrcmp0(attr, "face")) {
                     /* Font face attribute */
-                    line += 5;
-                    struct bstr val;
-                    if (!read_value(&line, &val))
-                        break;
                     tag->face = val;
                     append_text(&new_line, "{\\fn%.*s}", BSTR_P(tag->face));
                     tag->has_face = true;
                     has_valid_attr = true;
                 } else
-                    line++;
+                    mp_tmsg(MSGT_SUBREADER, MSGL_WARN,"SubRip: unrecognized "
+                            "attribute \"%.*s\" in font tag\n", BSTR_P(attr));
             }
 
             if (!has_valid_attr || *line != '>') { /* Not valid font tag */