From 696271700a1308c4dff4a9c30008b4303b417d76 Mon Sep 17 00:00:00 2001
From: Michael Raitza <michael.raitza@tu-dresden.de>
Date: Wed, 17 Feb 2021 09:12:23 +0100
Subject: [PATCH] handle simultaneous writes to stdout and $3

---
 builder.nix | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/builder.nix b/builder.nix
index 1a2dd90..826271a 100644
--- a/builder.nix
+++ b/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
-        # 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 ''
-                    #!/bin/sh
-                    ${/. + builder} $d1 $d2 $out >$out
-                  '';
+      _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));
+      in if imported.success
+         then imported.value
+         else {
+           builder = ''
+             #!/bin/sh
+             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;