fix(nix): detect nix at known locations, not just $PATH (#2787)

internal/nix/install.go

@@ -16,13 +16,18 @@ import (
 	"github.com/mattn/go-isatty"
 
 	"go.jetify.com/devbox/internal/boxcli/usererr"
-	"go.jetify.com/devbox/internal/cmdutil"
 	"go.jetify.com/devbox/internal/fileutil"
 	"go.jetify.com/devbox/nix"
 )
 
 func BinaryInstalled() bool {
-	return cmdutil.Exists("nix")
+	// Use the same resolver that Devbox uses to run nix commands (searching
+	// $PATH and well-known install locations), rather than a bare $PATH lookup.
+	// Otherwise Devbox can wrongly report nix as missing when it exists at a
+	// known location but isn't on $PATH — e.g. on NixOS, or when a non-POSIX
+	// login shell such as fish hasn't sourced the Nix profile (issue #2787).
+	_, err := nix.Default.LookPath()
+	return err == nil
 }
 
 func dirExistsAndIsNotEmpty(dir string) bool {

nix/nix.go

@@ -34,6 +34,11 @@ func System() string {
 	return Default.System()
 }
 
+// LookPath calls [Nix.LookPath] on the default Nix installation.
+func LookPath() (string, error) {
+	return Default.LookPath()
+}
+
 // Version calls [Nix.Version] on the default Nix installation.
 func Version() string {
 	return Default.Version()
@@ -88,24 +93,46 @@ func (n *Nix) resolvePath() (string, error) {
 		return path, nil
 	}
 
-	try := []string{
-		"/nix/var/nix/profiles/default/bin/nix",
-		"/run/current-system/sw/bin",
-	}
-	for _, path := range try {
+	for _, path := range nixBinaryFallbackPaths() {
 		stat, err := os.Stat(path)
-		if err == nil {
-			// Is it executable and not a directory?
-			m := stat.Mode()
-			if !m.IsDir() && m.Perm()&0o111 != 0 {
-				n.lookPath.Store(&path)
-				return path, nil
-			}
+		if err != nil {
+			continue
+		}
+		// Is it an executable file (and not a directory)?
+		m := stat.Mode()
+		if !m.IsDir() && m.Perm()&0o111 != 0 {
+			n.lookPath.Store(&path)
+			return path, nil
 		}
 	}
 	return "", pathErr
 }
 
+// LookPath returns the absolute path to the nix executable. It searches $PATH
+// (after attempting to source the Nix profile) and, failing that, the
+// well-known installation locations in [nixBinaryFallbackPaths]. It returns an
+// error if nix cannot be found.
+//
+// Because Devbox invokes nix by absolute path, a non-error result here means
+// nix commands will run even when nix is not on $PATH — which happens, for
+// example, when the login shell has not sourced the Nix profile (common with
+// non-POSIX shells such as fish).
+func (n *Nix) LookPath() (string, error) {
+	return n.resolvePath()
+}
+
+// nixBinaryFallbackPaths returns well-known absolute paths to the nix
+// executable, searched in order when nix is not found on $PATH. Each entry must
+// point at the nix binary itself, not the directory that contains it.
+func nixBinaryFallbackPaths() []string {
+	return []string{
+		"/nix/var/nix/profiles/default/bin/nix",
+		// On NixOS, nix is provided through the current system profile rather
+		// than /nix/var/nix/profiles/default.
+		"/run/current-system/sw/bin/nix",
+	}
+}
+
 func (n *Nix) logger() *slog.Logger {
 	if n.Logger == nil {
 		return slog.Default()

nix/nix_test.go

@@ -2,6 +2,7 @@
 package nix
 
 import (
+	"path/filepath"
 	"slices"
 	"testing"
 )
@@ -204,3 +205,25 @@ func TestVersionInfoAtLeast(t *testing.T) {
 		info.AtLeast(v)
 	})
 }
+
+func TestNixBinaryFallbackPaths(t *testing.T) {
+	paths := nixBinaryFallbackPaths()
+	if len(paths) == 0 {
+		t.Fatal("expected at least one fallback path")
+	}
+	// Every fallback must point at the nix binary itself, not a directory that
+	// contains it. resolvePath rejects directories, so a bare bin dir here
+	// would silently never match (regression guard for issue #2787).
+	for _, p := range paths {
+		if filepath.Base(p) != "nix" {
+			t.Errorf("fallback path %q must end in the nix binary, got base %q", p, filepath.Base(p))
+		}
+		if !filepath.IsAbs(p) {
+			t.Errorf("fallback path %q must be absolute", p)
+		}
+	}
+	// The NixOS system-profile location must be covered.
+	if !slices.Contains(paths, "/run/current-system/sw/bin/nix") {
+		t.Errorf("expected NixOS system-profile nix path in fallbacks, got %v", paths)
+	}
+}