Fixes #816.

Ensure thread safety by ensuring that tag_defs[] is thread local.
This has some overhead but is negligible on modern systems.

include/tidyplatform.h

@@ -509,10 +509,10 @@ extern "C" {
 
 #  if defined(_MSC_VER)
 #    define fileno _fileno
-#if !defined(NDEBUG) && !defined(ENABLE_DEBUG_LOG) && !defined(DISABLE_DEBUG_LOG)
-#define ENABLE_DEBUG_LOG
-#endif
-#endif
+#    if !defined(NDEBUG) && !defined(ENABLE_DEBUG_LOG) && !defined(DISABLE_DEBUG_LOG)
+#      define ENABLE_DEBUG_LOG
+#    endif
+#  endif
 
 #  define access _access
 #  define strcasecmp _stricmp
@@ -553,6 +553,13 @@ extern "C" {
 #    endif
 #  endif
 
+#  ifndef TIDY_THREAD_LOCAL
+#    ifdef _MSC_VER
+#      define TIDY_THREAD_LOCAL __declspec( thread )
+#    endif
+#  endif
+
+
 #endif /* _WIN32 */
 
 
@@ -600,6 +607,10 @@ extern "C" {
 #  define TIDY_STRUCT
 #endif
 
+#ifndef TIDY_THREAD_LOCAL
+#  define TIDY_THREAD_LOCAL __thread
+#endif
+
 typedef unsigned char byte;
 
 typedef uint tchar;         /* single, full character */

src/tags.c

@@ -166,7 +166,7 @@ static CheckAttribs CheckHTML;
  * but allow this table to be ADJUSTED if NOT HTML5
  * was static const Dict tag_defs[] = 
 \*/
-static Dict tag_defs[] =
+static TIDY_THREAD_LOCAL Dict tag_defs[] =
 {
   { TidyTag_UNKNOWN,    "unknown!",   VERS_UNKNOWN,         NULL,                            (0),                                           NULL,               NULL           },