Merge pull request #10865 from Mic92/better-cleanup

libutil: guard Finally against invalid exception throws

src/libutil/finally.hh

@@ -2,6 +2,8 @@
 ///@file
 
 #include <utility>
+#include <cassert>
+#include <exception>
 
 /**
  * A trivial class to run a function at the end of a scope.
@@ -21,5 +23,25 @@ public:
     Finally(Finally &&other) : fun(std::move(other.fun)) {
         other.movedFrom = true;
     }
-    ~Finally() { if (!movedFrom) fun(); }
+    ~Finally() noexcept(false)
+    {
+        try {
+            if (!movedFrom)
+                fun();
+        } catch (...) {
+            // finally may only throw an exception if exception handling is not already
+            // in progress. if handling *is* in progress we have to return cleanly here
+            // but are still prohibited from doing so since eating the exception would,
+            // in almost all cases, mess up error handling even more. the only good way
+            // to handle this is to abort entirely and leave a message, so we'll assert
+            // (and rethrow anyway, just as a defense against possible NASSERT builds.)
+            if (std::uncaught_exceptions()) {
+                assert(false &&
+                    "Finally function threw an exception during exception handling. "
+                    "this is not what you want, please use some other methods (like "
+                    "std::promise or async) instead.");
+            }
+            throw;
+        }
+    }
 };