[pure-shell] making sure nix is in path in pure mode (#1143)

## Summary
This is a follow up PR from as a result of [this
discussion](#1084 (comment)).
Note, xdg dirs don't keep any nix executables as far as I know. Single
user and multi user nix installation put the nix executable in
`~/.nix-profile/bin/` and `/nix/var/nix/profiles/default/bin`
respectively.

Also added tests for devbox run pure mode. For `devbox shellenv`, pure
mode doesn't really make sense to write tests for, and for `devbox
shell` it's not possible to write testscript tests.


## How was it tested?
- in a single user nix installed env, run `./devbox shell --pure` 
- `devbox add which`
- `which nix` should point to `~/.nix-profile/bin/nix`
- in a multi-user nix, same test should point to
`/nix/var/nix/profiles/default/bin/nix`

Author: mohsenari

internal/impl/devbox.go

@@ -740,22 +740,11 @@ func (d *Devbox) computeNixEnv(ctx context.Context, usePrintDevEnvCache bool) (m
 
 	// Append variables from current env if --pure is not passed
 	currentEnv := os.Environ()
-	env := make(map[string]string, len(currentEnv))
-
-	for _, kv := range currentEnv {
-		key, val, found := strings.Cut(kv, "=")
-		if !found {
-			return nil, errors.Errorf("expected \"=\" in keyval: %s", kv)
-		}
-		if ignoreCurrentEnvVar[key] {
-			continue
-		}
-		// Passing HOME USER and DISTPLAY for pure shell to leak through
-		// otherwise devbox binary won't work - this matches nix
-		if !d.pure || key == "HOME" || key == "USER" || key == "DISPLAY" {
-			env[key] = val
-		}
+	env, err := d.convertEnvToMap(currentEnv)
+	if err != nil {
+		return nil, err
 	}
+
 	// check if contents of .envrc is old and print warning
 	if !usePrintDevEnvCache {
 		err := d.checkOldEnvrc()
@@ -765,9 +754,6 @@ func (d *Devbox) computeNixEnv(ctx context.Context, usePrintDevEnvCache bool) (m
 	}
 
 	currentEnvPath := env["PATH"]
-	if d.pure { // make nix available inside pure shell - necessary for devbox add to work
-		currentEnvPath = "/nix/var/nix/profiles/default/bin"
-	}
 	debug.Log("current environment PATH is: %s", currentEnvPath)
 	// Use the original path, if available. If not available, set it for future calls.
 	// See https://github.com/jetpack-io/devbox/issues/687
@@ -1195,3 +1181,35 @@ func (d *Devbox) addHashToEnv(env map[string]string) error {
 	}
 	return err
 }
+
+func (d *Devbox) convertEnvToMap(currentEnv []string) (map[string]string, error) {
+	env := make(map[string]string, len(currentEnv))
+	for _, kv := range currentEnv {
+		key, val, found := strings.Cut(kv, "=")
+		if !found {
+			return nil, errors.Errorf("expected \"=\" in keyval: %s", kv)
+		}
+		if ignoreCurrentEnvVar[key] {
+			continue
+		}
+		// handling special cases to for pure shell
+		// Passing HOME USER and DISPLAY for pure shell to leak through
+		// otherwise devbox binary won't work - this matches nix
+		// We include PATH to find the nix installation. It is cleaned for pure mode below
+		if !d.pure || key == "HOME" || key == "USER" || key == "DISPLAY" || key == "PATH" {
+			env[key] = val
+		}
+	}
+
+	// handling special case for PATH
+	if d.pure {
+		// Finding nix executables in path and passing it through
+		// Needed for devbox commands inside pure shell to work
+		nixInPath, err := findNixInPATH(env)
+		if err != nil {
+			return nil, err
+		}
+		env["PATH"] = nixInPath
+	}
+	return env, nil
+}

internal/impl/shell.go

@@ -411,3 +411,21 @@ func filterPathList(pathList string, keep func(string) bool) string {
 	}
 	return strings.Join(filtered, string(filepath.ListSeparator))
 }
+
+func findNixInPATH(env map[string]string) (string, error) {
+	defaultSingleUserNixBin := fmt.Sprintf("%s/.nix-profile/bin", env["HOME"])
+	defaultMultiUserNixBin := "/nix/var/nix/profiles/default/bin"
+	xdgNixBin := xdg.StateSubpath("/nix/profile/bin")
+	pathElements := strings.Split(env["PATH"], ":")
+	debug.Log("path elements: %v", pathElements)
+	for _, el := range pathElements {
+		if el == xdgNixBin ||
+			el == defaultSingleUserNixBin ||
+			el == defaultMultiUserNixBin {
+			return el, nil
+		}
+	}
+
+	// did not find nix executable in PATH, return error
+	return "", errors.New("could not find any nix executable in PATH. Make sure Nix is installed and in PATH, then try again")
+}

testscripts/run/pure.test.txt

@@ -0,0 +1,21 @@
+# Tests related to having devbox run in pure mode.
+
+env FOO=bar
+env FOO2=bar2
+
+exec devbox run --pure echo '$FOO'
+stdout 'baz'
+
+exec devbox run --pure echo '$FOO2'
+stdout ''
+
+exec devbox run --pure hello
+stdout 'Hello, world!'
+
+-- devbox.json --
+{
+  "packages": ["hello"],
+  "env": {
+    "FOO": "baz"
+  }
+}