Bug #20908: Crash if select @@""

Zero-length variables caused failures when using the length to look
up the name in a hash.  Instead, signal that no zero-length name can
ever be found and that to encounter one is a syntax error.


mysql-test/r/variables.result:
  Results for test.
mysql-test/t/variables.test:
  Insert tests to prove that zero-length variable names do not cause
  faults.
sql/gen_lex_hash.cc:
  If the length is zero, then there is nothing to look-up in the 
  hash.
sql/sql_lex.cc:
  Names of variables must not be empty.  Signal an error of that 
  happens.

1 files changed, 9 insertions, 2 deletions

diff --git a/sql/gen_lex_hash.cc b/sql/gen_lex_hash.cc
index 7e0b178f7af..89af0d30076 100644
--- a/sql/gen_lex_hash.cc
+++ b/sql/gen_lex_hash.cc
@@ -447,8 +447,9 @@ int main(int argc,char **argv)
    and you are welcome to modify and redistribute it under the GPL license\n\
    \n*/\n\n");
 
-  printf("/* This code is generated by gen_lex_hash.cc that seeks for\
- a perfect\nhash function */\n\n");
+  /* Broken up to indicate that it's not advice to you, gentle reader. */
+  printf("/* Do " "not " "edit " "this " "file!  This is generated by "
+         "gen_lex_hash.cc\nthat seeks for a perfect hash function */\n\n");
   printf("#include \"lex.h\"\n\n");
 
   calc_length();
@@ -468,6 +469,12 @@ static inline SYMBOL *get_hash_symbol(const char *s,\n\
 {\n\
   register uchar *hash_map;\n\
   register const char *cur_str= s;\n\
+\n\
+  if (len == 0) {\n\
+    DBUG_PRINT(\"warning\", (\"get_hash_symbol() received a request for a zero-length symbol, which is probably a mistake.\"));\
+    return(NULL);\n\
+  }\
+\n\
   if (function){\n\
     if (len>sql_functions_max_len) return 0;\n\
     hash_map= sql_functions_map;\n\