scanner: fix buffer stack popping in yylex_destroy

[happyCoder92]

Fixes #392.

[jannick0]

Memory leakage issue confirmed and that the proposed commit remedies it. Appears to be in the world for decades now. Currently only the top buffer is freed up, all others left alone:

flex/src/flex.skl

Lines 2899 to 2903 in 98018e3

	while(YY_CURRENT_BUFFER){
	yy_delete_buffer( YY_CURRENT_BUFFER M4_YY_CALL_LAST_ARG );
	YY_CURRENT_BUFFER_LVALUE = NULL;
	yypop_buffer_state(M4_YY_CALL_ONLY_ARG);
	}

A simple example:

/* buffer-stack-memory-leakage.l  */
%option noyywrap noinput nounput
%%
.
%%
int main ( int argc, char** argv )
{
  int i, n, rc = 0;
  char s[20];
  FILE* fp;
  if ( argc == 1 || ( n = atoi ( argv[1] ) ) <=0 ) {
    fprintf ( stderr, "First command line argument expected to be an integer > 0"
                      " (number of buffers to be created).\n" );
    return 1;
  }

  for ( i=1; i<=n; i++ ) {
    sprintf ( s, "%d.txt", i );
    fp = fopen ( s, "w" );
    if ( !fp ) {
      perror ( s );
      rc = 1;
      goto cleanup;
    }
    fclose ( fp );
    fp = fopen ( s, "r" );
    if ( !fp ) {
      perror ( s );
      rc = 1;
      goto cleanup;
    }
    yypush_buffer_state ( yy_create_buffer ( fp, 256 ) );
  }
cleanup:
  yylex_destroy();
  fclose ( NULL );

  return rc;
}

[westes]

Please rebase and resolve conflicts.

[happyCoder92]

Rebased.