Removes special treatment of SystemExit

Previously, SystemExit exceptions were being caught at the module level,
presumably to not crash an interpreter which did not expect this
exception. However, this had several problems:

  1. Importing a module which calls exit() in its body would not
     correctly exit the importing module.

  2. The additional try-catch logic required for the special treatment
     of SystemExit had a noticeable performance penalty.

  3. Special casing SystemExit on a module level is inconsistent with
     Python's behaviour, which treats it like any other exception, and
     where it is up to the interpreter (not the module) to catch this
     exception and deal with it as desired.

  4. Additionally, SystemExit was incorrectly set to inherit from
     Exception (instead of BaseException), which meant that except
     blocks catching Exception would also catch SystemExit

This patch fixes the above issues by:

  1. Removing the special check for SystemExit

  2. Changing SystemExit to inherit from BaseException (and, while I was
     at it, change other exceptions to inherit from SystemError)

  3. Updating the tests to special case SystemExit as required

  4. Adding new-style unit tests for testing the above issues

Author: LeszekSwirski

src/compile.js

@@ -927,7 +927,7 @@ Compiler.prototype.outputSuspensionHelpers = function (unit) {
         }
     }
 
-    output +=  "try { $ret=susp.child.resume(); } catch(err) { if (!(err instanceof Sk.builtin.Exception)) { err = new Sk.builtin.ExternalError(err); } err.traceback.push({lineno: currLineNo, colno: currColNo, filename: '"+this.filename+"'}); if($exc.length>0) { $err=err; $blk=$exc.pop(); } else { throw err; } }" +
+    output +=  "try { $ret=susp.child.resume(); } catch(err) { if (!(err instanceof Sk.builtin.BaseException)) { err = new Sk.builtin.ExternalError(err); } err.traceback.push({lineno: currLineNo, colno: currColNo, filename: '"+this.filename+"'}); if($exc.length>0) { $err=err; $blk=$exc.pop(); } else { throw err; } }" +
                 "};";
 
     output += "var $saveSuspension = function(child, filename, lineno, colno) {" +
@@ -1586,7 +1586,7 @@ Compiler.prototype.buildcodeobj = function (n, coname, decorator_list, args, cal
     this.u.switchCode = "while(true){try{"
     this.u.switchCode += this.outputInterruptTest();
     this.u.switchCode += "switch($blk){";
-    this.u.suffixCode = "} }catch(err){ if (!(err instanceof Sk.builtin.Exception)) { err = new Sk.builtin.ExternalError(err); } err.traceback.push({lineno: currLineNo, colno: currColNo, filename: '"+this.filename+"'}); if ($exc.length>0) { $err = err; $blk=$exc.pop(); continue; } else { throw err; }} }});";
+    this.u.suffixCode = "} }catch(err){ if (!(err instanceof Sk.builtin.BaseException)) { err = new Sk.builtin.ExternalError(err); } err.traceback.push({lineno: currLineNo, colno: currColNo, filename: '"+this.filename+"'}); if ($exc.length>0) { $err = err; $blk=$exc.pop(); continue; } else { throw err; }} }});";
 
     //
     // jump back to the handler so it can do the main actual work of the
@@ -2225,8 +2225,6 @@ Compiler.prototype.cmod = function (mod) {
         this.u.varDeclsCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = new Date()}";
     }
 
-    this.u.varDeclsCode += "try {";
-
     this.u.varDeclsCode += "if ("+modf+".wakingSuspension!==undefined) { $wakeFromSuspension(); }" +
         "if (Sk.retainGlobals) {" +
         "    if (Sk.globals) { $gbl = Sk.globals; Sk.globals = $gbl; $loc = $gbl; }" +
@@ -2245,8 +2243,8 @@ Compiler.prototype.cmod = function (mod) {
     this.u.switchCode = "while(true){try{";
     this.u.switchCode += this.outputInterruptTest();
     this.u.switchCode += "switch($blk){";
-    this.u.suffixCode = "} }catch(err){ if (!(err instanceof Sk.builtin.Exception)) { err = new Sk.builtin.ExternalError(err); } err.traceback.push({lineno: currLineNo, colno: currColNo, filename: '"+this.filename+"'}); if ($exc.length>0) { $err = err; $blk=$exc.pop(); continue; } else { throw err; }} }" +
-        " }catch(err){ if (err instanceof Sk.builtin.SystemExit && !Sk.throwSystemExit) { Sk.misceval.print_(err.toString() + '\\n'); return $loc; } else { throw err; } } });";
+    this.u.suffixCode = "}"
+    this.u.suffixCode += "}catch(err){ if (!(err instanceof Sk.builtin.BaseException)) { err = new Sk.builtin.ExternalError(err); } err.traceback.push({lineno: currLineNo, colno: currColNo, filename: '"+this.filename+"'}); if ($exc.length>0) { $err = err; $blk=$exc.pop(); continue; } else { throw err; }} } });";
 
     // Note - this change may need to be adjusted for all the other instances of
     // switchCode and suffixCode in this file.  Not knowing how to test those

src/env.js

@@ -46,11 +46,8 @@ Sk.configure = function (options) {
     Sk.inputfun = options["inputfun"] || Sk.inputfun;
     goog.asserts.assert(typeof Sk.inputfun === "function");
 
-    Sk.throwSystemExit = options["systemexit"] || false;
-    goog.asserts.assert(typeof Sk.throwSystemExit === "boolean");
-
     Sk.retainGlobals = options["retainglobals"] || false;
-    goog.asserts.assert(typeof Sk.throwSystemExit === "boolean");
+    goog.asserts.assert(typeof Sk.retainGlobals === "boolean");
 
     Sk.debugging = options["debugging"] || false;
     goog.asserts.assert(typeof Sk.debugging === "boolean");