nix-shell: Add temp roots for outputs

doc/manual/src/release-notes/rl-next.md

@@ -5,3 +5,7 @@
   arguments will be ignored and the resulting derivation will have
   `__impure` set to `true`, making it an impure derivation.
 
+* `nix-shell` now spawns a child process instead of `exec`ing immediately.
+
+* `nix-shell` creates temp roots for as long as the executed shell
+  process is alive.

src/nix-build/nix-build.cc

@@ -571,9 +571,28 @@ static void main_nix_build(int argc, char * * argv)
 
         logger->stop();
 
-        execvp(shell->c_str(), argPtrs.data());
+        pid_t pid = fork();
 
-        throw SysError("executing shell '%s'", *shell);
+        if (pid == -1)
+            throw SysError("forking shell");
+
+        if (pid == 0) {
+            for (auto & [outputName, output] : drv.outputsAndOptPaths(*store))
+                if (output.second) store->addTempRoot(*output.second);
+
+            execvp(shell->c_str(), argPtrs.data());
+
+            throw SysError("executing shell '%s'", *shell);
+        }
+
+        siginfo_t shellStatus;
+
+        auto shellExited = waitid(P_PID, pid, &shellStatus, WEXITED);
+
+        if (shellExited == -1)
+            throw SysError("waiting on shell");
+
+        throw Exit(shellStatus.si_status);
     }
 
     else {

tests/nix-shell.sh

@@ -84,6 +84,9 @@ chmod a+rx $TEST_ROOT/spaced\ \\\'\"shell.shebang.rb
 output=$($TEST_ROOT/spaced\ \\\'\"shell.shebang.rb abc ruby)
 [ "$output" = '-e load(ARGV.shift) -- '"$TEST_ROOT"'/spaced \'\''"shell.shebang.rb abc ruby' ]
 
+# Text nix-shell --run preserves exit code
+test ! $(nix-shell -p foo --run 'exit 1')
+
 # Test 'nix develop'.
 nix develop -f "$shellDotNix" shellDrv -c bash -c '[[ -n $stdenv ]]'