* Catch SIGINT to terminate cleanly when the user tries to interrupt

Nix.  This is to prevent Berkeley DB from becoming wedged.

  Unfortunately it is not possible to throw C++ exceptions from a
  signal handler.  In fact, you can't do much of anything except
  change variables of type `volatile sig_atomic_t'.  So we set an
  interrupt flag in the signal handler and check it at various
  strategic locations in the code (by calling checkInterrupt()).
  Since this is unlikely to cover all cases (e.g., (semi-)infinite
  loops), sometimes SIGTERM may now be required to kill Nix.

src/libexpr/eval.cc

@@ -227,6 +227,8 @@ Expr evalExpr2(EvalState & state, Expr e)
 
 Expr evalExpr(EvalState & state, Expr e)
 {
+    checkInterrupt();
+    
     startNest(nest, lvlVomit,
         format("evaluating expression: %1%") % e);
 

src/libexpr/nixexpr.cc

@@ -87,6 +87,8 @@ string aterm2String(ATerm t)
 
 ATerm bottomupRewrite(TermFun & f, ATerm e)
 {
+    checkInterrupt();
+
     if (ATgetType(e) == AT_APPL) {
         AFun fun = ATgetAFun(e);
         int arity = ATgetArity(fun);
@@ -149,6 +151,8 @@ Expr makeAttrs(const ATermMap & attrs)
 
 Expr substitute(const ATermMap & subs, Expr e)
 {
+    checkInterrupt();
+
     ATMatcher m;
     string s;
 

src/libexpr/parser.cc

@@ -26,6 +26,8 @@ struct Cleanup : TermFun
 
     virtual ATerm operator () (ATerm e)
     {
+        checkInterrupt();
+        
         ATMatcher m;
         string s;