flake: Enable UBSAN for checks

Doing this makes catching non-obvious bugs easier. GHA CI workload is
already a concern and there isn't much benefit in running the tests with
and without sanitizers at the same time, so UBSAN is enabled for default
checks.

This change doesn't affect production builds in any way, but is rather a
step in the direction of improving automated testing during development.

Relates to #10969.

Author: xokdvium

flake.nix

@@ -263,18 +263,46 @@
           flatMapAttrs
             (
               {
-                "" = nixpkgsFor.${system}.native;
+                # Run all tests with UBSAN enabled. Running both with ubsan and
+                # without doesn't seem to have much immediate benefit for doubling
+                # the GHA CI workaround.
+                #
+                # TODO: Work toward enabling "address,undefined" if it seems feasible.
+                # This would maybe require dropping Boost coroutines and ignoring intentional
+                # memory leaks with detect_leaks=0.
+                "" = rec {
+                  nixpkgs = nixpkgsFor.${system}.native;
+                  nixComponents = nixpkgs.nixComponents.overrideScope (
+                    nixCompFinal: nixCompPrev: {
+                      mesonComponentOverrides = _finalAttrs: prevAttrs: {
+                        mesonFlags =
+                          (prevAttrs.mesonFlags or [ ])
+                          # TODO: Macos builds instrumented with ubsan take very long
+                          # to run functional tests.
+                          ++ lib.optionals (!nixpkgs.stdenv.hostPlatform.isDarwin) [
+                            (lib.mesonOption "b_sanitize" "undefined")
+                          ];
+                      };
+                    }
+                  );
+                };
               }
               // lib.optionalAttrs (!nixpkgsFor.${system}.native.stdenv.hostPlatform.isDarwin) {
                 # TODO: enable static builds for darwin, blocked on:
                 #       https://github.com/NixOS/nixpkgs/issues/320448
                 # TODO: disabled to speed up GHA CI.
-                #"static-" = nixpkgsFor.${system}.native.pkgsStatic;
+                # "static-" = {
+                #   nixpkgs = nixpkgsFor.${system}.native.pkgsStatic;
+                # };
               }
             )
             (
-              nixpkgsPrefix: nixpkgs:
-              flatMapAttrs nixpkgs.nixComponents (
+              nixpkgsPrefix:
+              {
+                nixpkgs,
+                nixComponents ? nixpkgs.nixComponents,
+              }:
+              flatMapAttrs nixComponents (
                 pkgName: pkg:
                 flatMapAttrs pkg.tests or { } (
                   testName: test: {
@@ -283,7 +311,7 @@
                 )
               )
               // lib.optionalAttrs (nixpkgs.stdenv.hostPlatform == nixpkgs.stdenv.buildPlatform) {
-                "${nixpkgsPrefix}nix-functional-tests" = nixpkgs.nixComponents.nix-functional-tests;
+                "${nixpkgsPrefix}nix-functional-tests" = nixComponents.nix-functional-tests;
               }
             )
         // devFlake.checks.${system} or { }

src/libutil/strings.cc

@@ -17,8 +17,10 @@ struct view_stringbuf : public std::stringbuf
     }
 };
 
-std::string_view toView(const std::ostringstream & os)
+__attribute__((no_sanitize("undefined"))) std::string_view toView(const std::ostringstream & os)
 {
+    /* Downcasting like this is very much undefined behavior, so we disable
+       UBSAN for this function. */
     auto buf = static_cast<view_stringbuf *>(os.rdbuf());
     return buf->toView();
 }