initLibUtil: Add exception handling self-check

src/libutil/error.cc

@@ -14,6 +14,11 @@ void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, boo
     err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });
 }
 
+void throwExceptionSelfCheck(){
+    // This is meant to be caught in initLibUtil()
+    throw SysError("C++ exception handling is broken. This would appear to be a problem with the way Nix was compiled and/or linked and/or loaded.");
+}
+
 // c++ std::exception descendants must have a 'const char* what()' function.
 // This stringifies the error and caches it for use by what(), or similarly by msg().
 const std::string & BaseError::calcWhat() const

src/libutil/error.hh

@@ -214,4 +214,8 @@ public:
     }
 };
 
+/** Throw an exception for the purpose of checking that exception handling works; see 'initLibUtil()'.
+ */
+void throwExceptionSelfCheck();
+
 }

src/libutil/util.cc

@@ -48,6 +48,23 @@ extern char * * environ __attribute__((weak));
 namespace nix {
 
 void initLibUtil() {
+    // Check that exception handling works. Exception handling has been observed
+    // not to work on darwin when the linker flags aren't quite right.
+    // In this case we don't want to expose the user to some unrelated uncaught
+    // exception, but rather tell them exactly that exception handling is
+    // broken.
+    // When exception handling fails, the message tends to be printed by the
+    // C++ runtime, followed by an abort.
+    // For example on macOS we might see an error such as
+    // libc++abi: terminating with uncaught exception of type nix::SysError: error: C++ exception handling is broken. This would appear to be a problem with the way Nix was compiled and/or linked and/or loaded.
+    bool caught = false;
+    try {
+        throwExceptionSelfCheck();
+    } catch (nix::Error _e) {
+        caught = true;
+    }
+    // This is not actually the main point of this check, but let's make sure anyway:
+    assert(caught);
 }
 
 std::optional<std::string> getEnv(const std::string & key)