fix: use nixos-version --json for configuration revision

- Use `nixos-version --json` command instead of reading files directly
- Add nixpkgs_rev and nixos_version labels to nixos_flake_info metric
- Show "unknown" for current_rev when system.configurationRevision not set
- Document configurationRevision setup in README

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -80,9 +80,35 @@ services.prometheus.exporters.nixos = {
 |--------|------|--------|-------------|
 | `nixos_flake_input_age_seconds` | Gauge | `input` | Age of flake input in seconds |
 | `nixos_flake_input_info` | Gauge | `input`, `rev`, `type` | Info gauge with revision and type labels |
-| `nixos_flake_info` | Gauge | `current_rev`, `remote_rev` | Info gauge with current and remote flake revisions |
+| `nixos_flake_info` | Gauge | `current_rev`, `remote_rev`, `nixpkgs_rev`, `nixos_version` | Info gauge with system version details |
 | `nixos_flake_revision_behind` | Gauge | | 1 if current system revision differs from remote latest |
 
+### Setting Configuration Revision
+
+For `current_rev` to show the flake's git revision, you must set `system.configurationRevision` in your flake:
+
+```nix
+{
+  outputs = { self, nixpkgs, ... }: {
+    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
+      modules = [
+        {
+          system.configurationRevision = self.rev or self.dirtyRev or "dirty";
+        }
+        ./configuration.nix
+      ];
+    };
+  };
+}
+```
+
+This sets the revision to:
+- `self.rev` - Git commit hash (only available when tree is clean and committed)
+- `self.dirtyRev` - Dirty revision (e.g., `abc1234-dirty`) when uncommitted changes exist
+- `"dirty"` - Fallback when neither is available
+
+Without this setting, `current_rev` will be `"unknown"` and `nixos_flake_revision_behind` will always be 0.
+
 ## Example Prometheus Alerts
 
 ```yaml
@@ -130,13 +156,9 @@ groups:
 
 ## Known Limitations
 
-- The `nixos_flake_info` and `nixos_flake_revision_behind` metrics rely on parsing the git hash from `/run/current-system/nixos-version`. The format of this file varies depending on NixOS configuration:
-  - Standard format: `25.11.20260203.e576e3c`
-  - Custom format: `1994-294a625`
+- Flake input metrics (`nixos_flake_input_age_seconds`, `nixos_flake_input_info`) reflect the remote flake state, not the currently deployed system. If the deployed system is behind, these will show newer data than what's actually deployed.
 
-  If your system uses a non-standard format that doesn't end with a git hash, the revision comparison may not work correctly.
-
-- Flake input ages reflect the remote flake state. If the deployed system is behind, these will show newer timestamps than what's actually deployed.
+- The `nixos_flake_revision_behind` metric requires `system.configurationRevision` to be set. Without it, the metric will always be 0 since there's no local revision to compare against.
 
 ## License
 

collector/flake.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"log/slog"
-	"os"
 	"os/exec"
 	"strings"
 	"sync"
@@ -14,7 +13,12 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-const configRevisionPath = "/run/current-system/configuration-revision"
+// nixosVersionInfo holds the parsed output of nixos-version --json
+type nixosVersionInfo struct {
+	ConfigurationRevision string `json:"configurationRevision"`
+	NixosVersion          string `json:"nixosVersion"`
+	NixpkgsRevision       string `json:"nixpkgsRevision"`
+}
 
 type FlakeCollector struct {
 	flakeURL      string
@@ -74,7 +78,7 @@ func NewFlakeCollector(flakeURL string, checkInterval time.Duration) *FlakeColle
 		flakeInfo: prometheus.NewDesc(
 			"nixos_flake_info",
 			"Info gauge with current and remote flake revisions",
-			[]string{"current_rev", "remote_rev"}, nil,
+			[]string{"current_rev", "remote_rev", "nixpkgs_rev", "nixos_version"}, nil,
 		),
 		revisionBehind: prometheus.NewDesc(
 			"nixos_flake_revision_behind",
@@ -164,22 +168,35 @@ func (c *FlakeCollector) collectInputMetrics(ch chan<- prometheus.Metric, data *
 }
 
 func (c *FlakeCollector) collectRevisionBehind(ch chan<- prometheus.Metric, data *flakeMetadata) {
-	currentRev, err := getCurrentSystemRevision()
+	versionInfo, err := getNixosVersionInfo()
 	if err != nil {
-		slog.Error("Failed to get current system revision", "error", err)
+		slog.Error("Failed to get NixOS version info", "error", err)
 		return
 	}
 
+	currentRev := versionInfo.ConfigurationRevision
+	if currentRev == "" {
+		currentRev = "unknown"
+	} else if len(currentRev) > 7 {
+		currentRev = currentRev[:7]
+	}
+
 	remoteRev := data.Revision
 	if len(remoteRev) > 7 {
 		remoteRev = remoteRev[:7]
 	}
 
+	nixpkgsRev := versionInfo.NixpkgsRevision
+	if len(nixpkgsRev) > 7 {
+		nixpkgsRev = nixpkgsRev[:7]
+	}
+
 	// Emit flake info metric with revisions
-	ch <- prometheus.MustNewConstMetric(c.flakeInfo, prometheus.GaugeValue, 1, currentRev, remoteRev)
+	ch <- prometheus.MustNewConstMetric(c.flakeInfo, prometheus.GaugeValue, 1,
+		currentRev, remoteRev, nixpkgsRev, versionInfo.NixosVersion)
 
 	behind := 0.0
-	if currentRev != "" && data.Revision != "" {
+	if currentRev != "unknown" && data.Revision != "" {
 		if currentRev != remoteRev && !strings.HasPrefix(data.Revision, currentRev) {
 			behind = 1.0
 		}
@@ -209,19 +226,23 @@ func fetchFlakeMetadata(flakeURL string) (*flakeMetadata, error) {
 	return &data, nil
 }
 
-func getCurrentSystemRevision() (string, error) {
-	data, err := os.ReadFile(configRevisionPath)
+func getNixosVersionInfo() (*nixosVersionInfo, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, "nixos-version", "--json")
+	output, err := cmd.Output()
 	if err != nil {
-		if os.IsNotExist(err) {
-			// configuration-revision doesn't exist; user hasn't set system.configurationRevision
-			return "", nil
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			return nil, fmt.Errorf("nixos-version failed: %s", strings.TrimSpace(string(exitErr.Stderr)))
 		}
-		return "", err
+		return nil, fmt.Errorf("nixos-version failed: %w", err)
 	}
 
-	rev := strings.TrimSpace(string(data))
-	if len(rev) > 7 {
-		rev = rev[:7]
+	var info nixosVersionInfo
+	if err := json.Unmarshal(output, &info); err != nil {
+		return nil, fmt.Errorf("failed to parse nixos-version output: %w", err)
 	}
-	return rev, nil
+
+	return &info, nil
 }

collector/flake_test.go

@@ -2,23 +2,46 @@ package collector
 
 import (
 	"encoding/json"
-	"os"
+	"os/exec"
 	"testing"
 )
 
-func TestGetCurrentSystemRevision(t *testing.T) {
-	// Skip if not on NixOS with system.configurationRevision set
-	if _, err := os.Stat(configRevisionPath); os.IsNotExist(err) {
-		t.Skip("not running on NixOS with system.configurationRevision set")
+func TestGetNixosVersionInfo(t *testing.T) {
+	// Skip if nixos-version command is not available
+	if _, err := exec.LookPath("nixos-version"); err != nil {
+		t.Skip("nixos-version command not available")
 	}
 
-	rev, err := getCurrentSystemRevision()
+	info, err := getNixosVersionInfo()
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	// Just check it returns something reasonable
-	t.Logf("current system revision: %s", rev)
+	t.Logf("NixOS version info: configurationRevision=%s, nixosVersion=%s, nixpkgsRevision=%s",
+		info.ConfigurationRevision, info.NixosVersion, info.NixpkgsRevision)
+}
+
+func TestNixosVersionInfoUnmarshal(t *testing.T) {
+	jsonData := `{
+		"configurationRevision": "138aeb9e450890efd7ca3be6159b1eb22256c2e3",
+		"nixosVersion": "25.11.20260203.e576e3c",
+		"nixpkgsRevision": "e576e3c9cf9bad747afcddd9e34f51d18c855b4e"
+	}`
+
+	var info nixosVersionInfo
+	if err := json.Unmarshal([]byte(jsonData), &info); err != nil {
+		t.Fatal(err)
+	}
+
+	if info.ConfigurationRevision != "138aeb9e450890efd7ca3be6159b1eb22256c2e3" {
+		t.Errorf("expected configurationRevision 138aeb9e450890efd7ca3be6159b1eb22256c2e3, got %s", info.ConfigurationRevision)
+	}
+	if info.NixosVersion != "25.11.20260203.e576e3c" {
+		t.Errorf("expected nixosVersion 25.11.20260203.e576e3c, got %s", info.NixosVersion)
+	}
+	if info.NixpkgsRevision != "e576e3c9cf9bad747afcddd9e34f51d18c855b4e" {
+		t.Errorf("expected nixpkgsRevision e576e3c9cf9bad747afcddd9e34f51d18c855b4e, got %s", info.NixpkgsRevision)
+	}
 }
 
 func TestFlakeLocksUnmarshal(t *testing.T) {

main.go

@@ -15,7 +15,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const version = "0.2.1"
+const version = "0.2.2"
 
 func main() {
 	cfg, err := config.Parse()