Finally exorcise Darwin Demons causing flakey functional tests

macos-builder02 in particular has been having problems for a while now
that no one could reproduce. We believe we have finally found the cause:
missing shebangs, not just in the tests themselves but *also* in the
inline scripts `nix-shell` itself writes.

Fixes #1042.
I believe this will also fix #1093.

See also: https://github.com/NixOS/nix/pull/14778

Change-Id: I54d04a770b8e78484815db88a8bc88776a6a6964

lix/legacy/nix-build.cc

@@ -503,11 +503,13 @@ static int main_nix_build(AsyncIoRoot & aio, std::string programName, Strings ar
         auto rcfile = (Path) tmpDir + "/rc";
         auto tz = getEnv("TZ");
         std::string rc =
-            fmt("%1%"
+            fmt("#!%1%\n"
+                "%2%"
                 // always clear PATH.
                 // when nix-shell is run impure, we rehydrate it with the `p=$PATH` above
                 "unset PATH;"
                 "dontAddDisableDepTrack=1;\n",
+                *shell,
                 (pure ? "" : "[ -n \"$PS1\" ] && [ -e ~/.bashrc ] && source ~/.bashrc; p=$PATH; "));
         rc += structuredAttrsRC;
         rc += fmt(

tests/functional/nix-shell/shell.nix

@@ -63,14 +63,16 @@ let pkgs = rec {
 
   foo = runCommand "foo" {} ''
     mkdir -p $out/bin
-    echo 'echo ${fooContents}' > $out/bin/foo
+    echo '#!${shell}' >> $out/bin/foo
+    echo 'echo ${fooContents}' >> $out/bin/foo
     chmod a+rx $out/bin/foo
     ln -s ${shell} $out/bin/bash
   '';
 
   bar = runCommand "bar" {} ''
     mkdir -p $out/bin
-    echo 'echo bar' > $out/bin/bar
+    echo '#!${shell}' > $out/bin/bar
+    echo 'echo bar' >> $out/bin/bar
     chmod a+rx $out/bin/bar
   '';
 
@@ -83,7 +85,8 @@ let pkgs = rec {
   # ruby "interpreter" that outputs "$@"
   ruby = runCommand "ruby" {} ''
     mkdir -p $out/bin
-    echo 'printf %s "$*"' > $out/bin/ruby
+    echo '#!${shell}' > $out/bin/ruby
+    echo 'printf %s "$*"' >> $out/bin/ruby
     chmod a+rx $out/bin/ruby
   '';
 

tests/functional/user-envs.builder.sh

@@ -1,3 +1,4 @@
+#!/usr/bin/env bash
 mkdir $out
 mkdir $out/bin
 echo "#! $shell" > $out/bin/$progName