Configure reentrant bison/flex (#789)

Make the parser reentrant, working towards #769 and the ability to load other packages cleanly.

executable_semantics/syntax/parse.cpp

@@ -7,36 +7,49 @@
 #include "common/check.h"
 #include "executable_semantics/common/error.h"
 #include "executable_semantics/common/tracing_flag.h"
+#include "executable_semantics/syntax/lexer.h"
 #include "executable_semantics/syntax/parse_and_lex_context.h"
 #include "executable_semantics/syntax/parser.h"
 
-extern FILE* yyin;
-
 namespace Carbon {
 
 // Returns an abstract representation of the program contained in the
 // well-formed input file, or if the file was malformed, a description of the
 // problem.
-auto parse(const std::string& input_file_name)
+auto Parse(const std::string& input_file_name)
     -> std::variant<AST, SyntaxErrorCode> {
-  yyin = fopen(input_file_name.c_str(), "r");
-  if (yyin == nullptr) {
+  FILE* input_file = fopen(input_file_name.c_str(), "r");
+  if (input_file == nullptr) {
     FATAL_PROGRAM_ERROR_NO_LINE() << "Error opening '" << input_file_name
                                   << "': " << std::strerror(errno);
   }
 
+  // Prepare the lexer.
+  yyscan_t scanner;
+  yylex_init(&scanner);
+  yyset_in(input_file, scanner);
+
+  // Prepare other parser arguments.
   std::optional<AST> parsed_input = std::nullopt;
   ParseAndLexContext context(input_file_name);
 
-  auto parser = Parser(parsed_input, context);
+  // Do the parse.
+  auto parser = Parser(parsed_input, scanner, context);
   if (tracing_output) {
     parser.set_debug_level(1);
   }
   auto syntax_error_code = parser();
+
+  // Clean up the lexer.
+  fclose(input_file);
+  yylex_destroy(scanner);
+
+  // Return an error if appropriate.
   if (syntax_error_code != 0) {
     return syntax_error_code;
   }
 
+  // Return parse results.
   CHECK(parsed_input != std::nullopt)
       << "parser validated syntax yet didn't produce an AST.";
   return *parsed_input;