feat(platform): detect distro, WSL, and architecture

internal/platform/platform.go

@@ -0,0 +1,63 @@
+// Package platform detects the host OS/distro/WSL status.
+package platform
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"runtime"
+	"strings"
+)
+
+// Distro is a narrow enum of distros we care about.
+type Distro string
+
+const (
+	DistroArch    Distro = "arch"
+	DistroUnknown Distro = "unknown"
+)
+
+// Platform holds detected host information.
+type Platform struct {
+	Distro Distro
+	IsWSL  bool
+	Arch   string // e.g. "amd64", "arm64"
+}
+
+// Detect reads the live system. Returns DistroUnknown rather than an error
+// when /etc/os-release is missing or unrecognized; callers decide what to do.
+func Detect() (Platform, error) {
+	f, err := os.Open("/etc/os-release")
+	distro := DistroUnknown
+	if err == nil {
+		distro = parseDistro(f)
+		_ = f.Close()
+	} else if !os.IsNotExist(err) {
+		return Platform{}, fmt.Errorf("open /etc/os-release: %w", err)
+	}
+
+	wsl := false
+	if data, err := os.ReadFile("/proc/sys/kernel/osrelease"); err == nil {
+		lower := strings.ToLower(string(data))
+		wsl = strings.Contains(lower, "microsoft") || strings.Contains(lower, "wsl")
+	}
+
+	return Platform{Distro: distro, IsWSL: wsl, Arch: runtime.GOARCH}, nil
+}
+
+func parseDistro(r io.Reader) Distro {
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		line := sc.Text()
+		if !strings.HasPrefix(line, "ID=") {
+			continue
+		}
+		id := strings.Trim(strings.TrimPrefix(line, "ID="), `"`)
+		if id == "arch" {
+			return DistroArch
+		}
+		return DistroUnknown
+	}
+	return DistroUnknown
+}