handle simultaneous writes to stdout and $3

builder.nix

@@ -14,15 +14,27 @@ let
       # Funktionalise pkgs; our main entry point to resolve default*nix build scripts.
       funcPkgs = { __functor = self: arg: resolve arg; } // pkgs;
 
-      _builder = if (tryEval (let
+      _builder = let
+        imported = (tryEval (let
           # Needs to be done as nix chokes on empty files rn.
           content = readFile builder;
-      in assert content != "" && (! (hasPrefix "#!" content)); import builder)).success
-                  then import builder
-                  else ''
+        in assert content != "" && (! (hasPrefix "#!" content)); import builder));
+      in if imported.success
+         then imported.value
+         else {
+           builder = ''
              #!/bin/sh
-                    ${/. + builder} $d1 $d2 $out >$out
+             set -e
+             pout=$(mktemp -u -p $PWD)
+             ${/. + builder} $d1 $d2 $out >$pout
+             if [ -e $out -a -s $pout ]; then
+               printf "Error: %s wrote to stdout and created \$3\n" "$d1" >&2
+               exit 207
+             elif [ -s $pout ]; then
+               mv $pout $out
+             fi
            '';
+         };
       __builder = if isFunction _builder then _builder funcPkgs
                   else _builder;