Fix tor_strtok_r_impl and test cases per bug #5091

==

Nick here. I tweaked this patch a little to make it apply cleanly to
master, to extract some common code into a function, and to replace
snprintf with tor_snprintf.

-- nickm

src/common/compat.c

@@ -546,25 +546,43 @@ const char TOR_TOLOWER_TABLE[256] = {
   240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,
 };
 
+/** Helper for tor_strtok_r_impl: Advances cp past all characters in
+ * <b>sep</b>, and returns its new value. */
+static char *
+strtok_helper(char *cp, const char *sep)
+{
+  if (sep[1]) {
+    while (*cp && strchr(sep, *cp))
+      ++cp;
+  } else {
+    while (*cp && *cp == *sep)
+      ++cp;
+  }
+  return cp;
+}
+
 /** Implementation of strtok_r for platforms whose coders haven't figured out
  * how to write one.  Hey guys!  You can use this code here for free! */
 char *
 tor_strtok_r_impl(char *str, const char *sep, char **lasts)
 {
   char *cp, *start;
-  if (str)
-    start = cp = *lasts = str;
-  else if (!*lasts)
-    return NULL;
-  else
-    start = cp = *lasts;
-
   tor_assert(*sep);
+  if (str) {
+    str = strtok_helper(str, sep);
+    if (!*str)
+      return NULL;
+    start = cp = *lasts = str;
+  } else if (!*lasts || !**lasts) {
+    return NULL;
+  } else {
+    start = cp = *lasts;
+  }
+
   if (sep[1]) {
     while (*cp && !strchr(sep, *cp))
       ++cp;
   } else {
-    tor_assert(strlen(sep) == 1);
     cp = strchr(cp, *sep);
   }
 
@@ -572,7 +590,7 @@ tor_strtok_r_impl(char *str, const char *sep, char **lasts)
     *lasts = NULL;
   } else {
     *cp++ = '\0';
-    *lasts = cp;
+    *lasts = strtok_helper(cp, sep);
   }
   return start;
 }