libutil: remove the interrupt-blocking code

The interrupt-blocking code was originally introduced 20 years ago so that
trying to log an error message does not result in an interrupt exception being
thrown and then going unhandled (c8d3882cdc).
However, the logging code does not check for interrupts any more
(054be50257), so this reasoning is no longer
applicable. Delete this code so that later interrupts are unblocked again, for
example in the next line entered into the repl.

Closes: #296
Change-Id: I48253f5f4272e75001148c13046e709ef5427fbd

doc/manual/rl-next/repl-interrupt.md

@@ -0,0 +1,8 @@
+---
+synopsis: Interrupting builds in the REPL works more than once
+cls: 1097
+---
+
+Builds in the REPL can be interrupted by pressing Ctrl+C.
+Previously, this only worked once per REPL session; further attempts would be ignored.
+This issue is now fixed, so that builds can be canceled consistently.

src/libmain/shared.cc

@@ -320,16 +320,7 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
 
     std::string error = ANSI_RED "error:" ANSI_NORMAL " ";
     try {
-        try {
-            fun();
-        } catch (...) {
-            /* Subtle: we have to make sure that any `interrupted'
-               condition is discharged before we reach printMsg()
-               below, since otherwise it will throw an (uncaught)
-               exception. */
-            setInterruptThrown();
-            throw;
-        }
+        fun();
     } catch (Exit & e) {
         return e.status;
     } catch (UsageError & e) {

src/libutil/signals.cc

@@ -9,21 +9,14 @@ namespace nix {
 
 std::atomic<bool> _isInterrupted = false;
 
-static thread_local bool interruptThrown = false;
 thread_local std::function<bool()> interruptCheck;
 
-void setInterruptThrown()
-{
-    interruptThrown = true;
-}
-
 void _interrupted()
 {
     /* Block user interrupts while an exception is being handled.
        Throwing an exception while another exception is being handled
        kills the program! */
-    if (!interruptThrown && !std::uncaught_exceptions()) {
-        interruptThrown = true;
+    if (!std::uncaught_exceptions()) {
         throw Interrupted("interrupted by the user");
     }
 }

src/libutil/signals.hh

@@ -22,8 +22,6 @@ extern std::atomic<bool> _isInterrupted;
 
 extern thread_local std::function<bool()> interruptCheck;
 
-void setInterruptThrown();
-
 void _interrupted();
 
 void inline checkInterrupt()