Make nix-shell support content-addressed derivations

Resolve the derivation before trying to load its environment −
essentially reproducing what the build loop does − so that we can
effectively access our dependencies (and not just their placeholders).

Fix #4821

src/nix-build/nix-build.cc

@@ -387,6 +387,13 @@ static void main_nix_build(int argc, char * * argv)
 
         if (dryRun) return;
 
+        if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+            auto resolvedDrv = drv.tryResolve(*store);
+            if (!resolvedDrv)
+                throw Error("unable to resolve the derivation '%s'. nix-shell can’t continue", drvInfo.queryDrvPath());
+            drv = *resolvedDrv;
+        }
+
         // Set the environment.
         auto env = getEnv();
 

tests/ca/nix-shell.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+sed -i 's/experimental-features .*/& ca-derivations ca-references nix-command flakes/' "$NIX_CONF_DIR"/nix.conf
+
+CONTENT_ADDRESSED=true
+cd ..
+source ./nix-shell.sh
+

tests/local.mk

@@ -47,6 +47,7 @@ nix_tests = \
   ca/build.sh \
   ca/substitute.sh \
   ca/signatures.sh \
+  ca/nix-shell.sh \
   ca/nix-run.sh \
   ca/nix-copy.sh
   # parallel.sh

tests/nix-shell.sh

@@ -2,6 +2,12 @@ source common.sh
 
 clearStore
 
+if [[ -n ${CONTENT_ADDRESSED:-} ]]; then
+    nix-shell () {
+        command nix-shell --arg contentAddressed true "$@"
+    }
+fi
+
 # Test nix-shell -A
 export IMPURE_VAR=foo
 export SELECTED_IMPURE_VAR=baz
@@ -41,7 +47,7 @@ output=$(NIX_PATH=nixpkgs=shell.nix nix-shell --pure -p foo bar --run 'echo "$(f
 [ "$output" = "foo bar" ]
 
 # Test nix-shell shebang mode
-sed -e "s|@ENV_PROG@|$(type -p env)|" shell.shebang.sh > $TEST_ROOT/shell.shebang.sh
+sed -e "s|@ENV_PROG@|$(type -P env)|" shell.shebang.sh > $TEST_ROOT/shell.shebang.sh
 chmod a+rx $TEST_ROOT/shell.shebang.sh
 
 output=$($TEST_ROOT/shell.shebang.sh abc def)
@@ -49,7 +55,7 @@ output=$($TEST_ROOT/shell.shebang.sh abc def)
 
 # Test nix-shell shebang mode again with metacharacters in the filename.
 # First word of filename is chosen to not match any file in the test root.
-sed -e "s|@ENV_PROG@|$(type -p env)|" shell.shebang.sh > $TEST_ROOT/spaced\ \\\'\"shell.shebang.sh
+sed -e "s|@ENV_PROG@|$(type -P env)|" shell.shebang.sh > $TEST_ROOT/spaced\ \\\'\"shell.shebang.sh
 chmod a+rx $TEST_ROOT/spaced\ \\\'\"shell.shebang.sh
 
 output=$($TEST_ROOT/spaced\ \\\'\"shell.shebang.sh abc def)
@@ -58,7 +64,7 @@ output=$($TEST_ROOT/spaced\ \\\'\"shell.shebang.sh abc def)
 # Test nix-shell shebang mode for ruby
 # This uses a fake interpreter that returns the arguments passed
 # This, in turn, verifies the `rc` script is valid and the `load()` script (given using `-e`) is as expected.
-sed -e "s|@SHELL_PROG@|$(type -p nix-shell)|" shell.shebang.rb > $TEST_ROOT/shell.shebang.rb
+sed -e "s|@SHELL_PROG@|$(type -P nix-shell)|" shell.shebang.rb > $TEST_ROOT/shell.shebang.rb
 chmod a+rx $TEST_ROOT/shell.shebang.rb
 
 output=$($TEST_ROOT/shell.shebang.rb abc ruby)
@@ -66,7 +72,7 @@ output=$($TEST_ROOT/shell.shebang.rb abc ruby)
 
 # Test nix-shell shebang mode for ruby again with metacharacters in the filename.
 # Note: fake interpreter only space-separates args without adding escapes to its output.
-sed -e "s|@SHELL_PROG@|$(type -p nix-shell)|" shell.shebang.rb > $TEST_ROOT/spaced\ \\\'\"shell.shebang.rb
+sed -e "s|@SHELL_PROG@|$(type -P nix-shell)|" shell.shebang.rb > $TEST_ROOT/spaced\ \\\'\"shell.shebang.rb
 chmod a+rx $TEST_ROOT/spaced\ \\\'\"shell.shebang.rb
 
 output=$($TEST_ROOT/spaced\ \\\'\"shell.shebang.rb abc ruby)

tests/shell.nix

@@ -1,6 +1,18 @@
-{ inNixShell ? false }:
+{ inNixShell ? false, contentAddressed ? false }:
 
-with import ./config.nix;
+let cfg = import ./config.nix; in
+with cfg;
+
+let
+  mkDerivation =
+    if contentAddressed then
+      args: cfg.mkDerivation ({
+        __contentAddressed = true;
+        outputHashMode = "recursive";
+        outputHashAlgo = "sha256";
+      } // args)
+    else cfg.mkDerivation;
+in
 
 let pkgs = rec {
   setupSh = builtins.toFile "setup" ''