symbol interning and lookup not threadsafe

[Andy Wingo]

Symbols get interned through scm_i_str2symbol, which looks like this:

    /* Intern SYMBOL, an uninterned symbol.  */
    static void
    intern_symbol (SCM symbol)
    {
      SCM slot, cell;
      unsigned long hash;

      hash = scm_i_symbol_hash (symbol) % SCM_HASHTABLE_N_BUCKETS (symbols);
      slot = SCM_HASHTABLE_BUCKET (symbols, hash);
      cell = scm_cons (symbol, SCM_UNDEFINED);

      SCM_SET_HASHTABLE_BUCKET (symbols, hash, scm_cons (cell, slot));
      SCM_HASHTABLE_INCREMENT (symbols);

      if (SCM_HASHTABLE_N_ITEMS (symbols) > SCM_HASHTABLE_UPPER (symbols))
        scm_i_rehash (symbols, scm_i_hash_symbol, 0, "intern_symbol");
    }

    static SCM
    scm_i_str2symbol (SCM str)
    {
      SCM symbol;
      size_t raw_hash = scm_i_string_hash (str);

      symbol = lookup_interned_symbol (str, raw_hash);
      if (scm_is_false (symbol))
        {
          /* The symbol was not found, create it.  */
          symbol = scm_i_make_symbol (str, 0, raw_hash,
                                  scm_cons (SCM_BOOL_F, SCM_EOL));
          intern_symbol (symbol);
        }

      return symbol;
    }

This is not threadsafe.

Andy
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