nix3-develop: export NIX_LOG_FD

As far as I can tell, there's no harm to let stdenv know that they can
write to file descriptor 2 which is stderr inside of a nix3-develop
shell.

Contributes towards #336.

Change-Id: I51dcbcbc19a1698a0d1255f9d943d9ebd15d115d
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/nix/develop.cc

@@ -364,6 +364,9 @@ struct Common : InstallableCommand, MixProfile
             out << fmt("%s=\"$%s${nix_saved_%s:+:$nix_saved_%s}\"\n", var, var, var, var);
 
         out << "export NIX_BUILD_TOP=\"$(mktemp -d -t nix-shell.XXXXXX)\"\n";
+        // We re-export similarly to what occurs inside of a derivation goal `NIX_LOG_FD` to stderr.
+        // So that stdenv hooks that logs information can be observed inside this debugging tool.
+        out << "export NIX_LOG_FD=2\n";
         for (auto & i : {"TMP", "TMPDIR", "TEMP", "TEMPDIR"})
             out << fmt("export %s=\"$NIX_BUILD_TOP\"\n", i);
 

tests/functional/nix-shell/basic.sh

@@ -173,6 +173,9 @@ shellDrv=$(nix-instantiate "$shellDotNix" -A shellDrv.out)
 
 nix develop $shellDrv -c bash -c '[[ -n $stdenv ]]'
 
+# Test $NIX_LOG_FD
+expectStderr 0 nix develop $shellDrv -c bash -c 'echo hello > $NIX_LOG_FD' |& grepQuiet 'hello'
+
 nix print-dev-env $shellDrv > $TEST_ROOT/dev-env2.sh
 nix print-dev-env $shellDrv --json > $TEST_ROOT/dev-env2.json