fix(nix): detect incompatible Lix and print a clear hint (#2946)

internal/nix/install.go

@@ -53,6 +53,22 @@ func EnsureNixInstalled(ctx context.Context, writer io.Writer, withDaemonFunc fu
 			)
 			return
 		}
+
+		// Lix 2.95 removed builtins.fetchClosure, which Devbox relies on to
+		// install packages from a binary cache. Detect it early and print a
+		// clear compatibility hint instead of a cryptic
+		// "attribute 'fetchClosure' missing" error later on.
+		if info, infoErr := nix.Default.Info(); infoErr == nil && !info.SupportsFetchClosure() {
+			err = usererr.New(
+				"Devbox is not compatible with your Nix installation. Lix %s "+
+					"removed `builtins.fetchClosure`, which Devbox relies on to "+
+					"install packages. Please switch to Nix "+
+					"(https://nixos.org/download), or downgrade to Lix < %s.\n",
+				nix.Version(),
+				nix.LixVersionWithoutFetchClosure,
+			)
+			return
+		}
 	}()
 
 	if BinaryInstalled() {

nix/nix.go

@@ -175,12 +175,19 @@ const (
 	MinVersion = Version2_18
 )
 
+// LixVersionWithoutFetchClosure is the first Lix version that removed
+// builtins.fetchClosure, which Devbox relies on to install packages from a
+// binary cache. Devbox is not compatible with Lix at or above this version.
+//
+// See https://lix.systems/blog/2026-03-25-lix-2.95-release/.
+const LixVersionWithoutFetchClosure = "2.95.0"
+
 // versionRegexp matches the first line of "nix --version" output.
 //
 // The semantic component is sourced from <https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string>.
 // It's been modified to tolerate Nix prerelease versions, which don't have a
 // hyphen before the prerelease component and contain underscores.
-var versionRegexp = regexp.MustCompile(`^(.+) \(.+\) ((?P<major>0|[1-9]\d*)\.(?P<minor>0|[1-9]\d*)\.(?P<patch>0|[1-9]\d*)(?:(?:-|pre)(?P<prerelease>(?:0|[1-9]\d*|\d*[_a-zA-Z-][_0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[_a-zA-Z-][_0-9a-zA-Z-]*))*))?(?:\+(?P<buildmetadata>[0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?)$`)
+var versionRegexp = regexp.MustCompile(`^(.+) \((.+)\) ((?P<major>0|[1-9]\d*)\.(?P<minor>0|[1-9]\d*)\.(?P<patch>0|[1-9]\d*)(?:(?:-|pre)(?P<prerelease>(?:0|[1-9]\d*|\d*[_a-zA-Z-][_0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[_a-zA-Z-][_0-9a-zA-Z-]*))*))?(?:\+(?P<buildmetadata>[0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?)$`)
 
 // preReleaseRegexp matches Nix prerelease version strings, which are not valid
 // semvers.
@@ -192,6 +199,11 @@ type Info struct {
 	// also be a fork like "lix".
 	Name string
 
+	// Implementation is the parenthetical descriptor from the first line of
+	// "nix --version" output. It is "Nix" for upstream Nix and something
+	// like "Lix, like Nix" for the Lix fork.
+	Implementation string
+
 	// Version is the semantic Nix version string.
 	Version string
 
@@ -251,11 +263,12 @@ func parseInfo(data []byte) (Info, error) {
 
 	lines := strings.Split(string(data), "\n")
 	matches := versionRegexp.FindStringSubmatch(lines[0])
-	if len(matches) < 3 {
+	if len(matches) < 4 {
 		return info, redact.Errorf("parse nix version: %s", redact.Safe(lines[0]))
 	}
 	info.Name = matches[1]
-	info.Version = matches[2]
+	info.Implementation = matches[2]
+	info.Version = matches[3]
 	for _, line := range lines {
 		name, value, found := strings.Cut(line, ": ")
 		if !found {
@@ -306,6 +319,25 @@ func (i Info) AtLeast(version string) bool {
 	return semver.Compare("v"+prerelease, version) >= 0
 }
 
+// IsLix reports whether the Nix installation is the Lix fork, which identifies
+// itself as "Lix, like Nix" in the parenthetical of its "nix --version" output.
+func (i Info) IsLix() bool {
+	return strings.Contains(strings.ToLower(i.Implementation), "lix")
+}
+
+// SupportsFetchClosure reports whether the Nix installation provides
+// builtins.fetchClosure, which Devbox relies on to install packages from a
+// binary cache. The Lix fork removed fetchClosure in version 2.95, so Devbox is
+// not compatible with it (see LixVersionWithoutFetchClosure). When the version
+// cannot be determined, this returns true to avoid blocking on a false
+// positive.
+func (i Info) SupportsFetchClosure() bool {
+	if i.IsLix() {
+		return !i.AtLeast(LixVersionWithoutFetchClosure)
+	}
+	return true
+}
+
 // sourceProfileMutex guards against multiple goroutines attempting to source
 // the Nix profile scripts concurrently.
 var sourceProfileMutex sync.Mutex

nix/nix_test.go

@@ -25,6 +25,9 @@ Data directory: /nix/store/m0ns07v8by0458yp6k30rfq1rs3kaz6g-nix-2.21.2/share
 	if got, want := info.Name, "nix"; got != want {
 		t.Errorf("got Name = %q, want %q", got, want)
 	}
+	if got, want := info.Implementation, "Nix"; got != want {
+		t.Errorf("got Implementation = %q, want %q", got, want)
+	}
 	if got, want := info.Version, "2.21.2"; got != want {
 		t.Errorf("got Version = %q, want %q", got, want)
 	}
@@ -73,6 +76,9 @@ Data directory: /nix/store/12asl5a17ffj78njcy2fj31v59rdmanx-lix-2.90-beta.1/shar
 	if got, want := info.Name, "nix"; got != want {
 		t.Errorf("got Name = %q, want %q", got, want)
 	}
+	if got, want := info.Implementation, "Lix, like Nix"; got != want {
+		t.Errorf("got Implementation = %q, want %q", got, want)
+	}
 	if got, want := info.Version, "2.90.0-beta.1"; got != want {
 		t.Errorf("got Version = %q, want %q", got, want)
 	}
@@ -204,3 +210,70 @@ func TestVersionInfoAtLeast(t *testing.T) {
 		info.AtLeast(v)
 	})
 }
+
+func TestInfoIsLix(t *testing.T) {
+	cases := []struct {
+		implementation string
+		want           bool
+	}{
+		{"Nix", false},
+		{"Lix, like Nix", true},
+		{"lix", true},
+		{"", false},
+	}
+	for _, tt := range cases {
+		t.Run(tt.implementation, func(t *testing.T) {
+			info := Info{Implementation: tt.implementation}
+			if got := info.IsLix(); got != tt.want {
+				t.Errorf("Info{Implementation:%q}.IsLix() = %v, want %v", tt.implementation, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestInfoSupportsFetchClosure(t *testing.T) {
+	cases := []struct {
+		name string
+		info Info
+		want bool
+	}{
+		{
+			name: "nix",
+			info: Info{Implementation: "Nix", Version: "2.34.6"},
+			want: true,
+		},
+		{
+			name: "lix before 2.95",
+			info: Info{Implementation: "Lix, like Nix", Version: "2.94.0"},
+			want: true,
+		},
+		{
+			name: "lix prerelease before 2.95",
+			info: Info{Implementation: "Lix, like Nix", Version: "2.90.0-beta.1"},
+			want: true,
+		},
+		{
+			name: "lix 2.95",
+			info: Info{Implementation: "Lix, like Nix", Version: "2.95.0"},
+			want: false,
+		},
+		{
+			name: "lix after 2.95",
+			info: Info{Implementation: "Lix, like Nix", Version: "2.95.2"},
+			want: false,
+		},
+		{
+			// Unknown version: assume support to avoid a false positive.
+			name: "lix unknown version",
+			info: Info{Implementation: "Lix, like Nix"},
+			want: true,
+		},
+	}
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.info.SupportsFetchClosure(); got != tt.want {
+				t.Errorf("SupportsFetchClosure() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}