Make tests use the psueod fs and call detectHostLinuxDistribution directly.

Tests were reimplementing the parsing, which is not
of much value.  Call the detectHostLinuxDistribution()
function directly to exercise the parsing logic.

Add a way to pass in a filesystem so we can pass
the mocked filesystem.

Author: kcieplak

Sources/SWBGenericUnixPlatform/Plugin.swift

@@ -129,15 +129,17 @@ struct GenericUnixSDKRegistryExtension: SDKRegistryExtension {
             let shouldUseLLD = {
                 switch operatingSystem {
                 case .freebsd:
-                    return true // FreeBSD is always LLVM-based.
+                    // FreeBSD is always LLVM-based.
+                    return true
                 case .linux:
                     // Amazon Linux 2 has a gold linker bug see: https://sourceware.org/bugzilla/show_bug.cgi?id=23016.
                     guard let distribution = operatingSystem.distribution else {
                         return false
                     }
                     return distribution.kind == .amazon && distribution.version == "2"
                 default:
-                    return operatingSystem != context.hostOperatingSystem // Cross-compiling.
+                    // Cross-compiling.
+                    return operatingSystem != context.hostOperatingSystem
                 }
             }()
 

Sources/SWBUtil/ProcessInfo.swift

@@ -228,35 +228,44 @@ public enum OperatingSystem: Hashable, Sendable {
         }
     }
 
-    /// Detects the Linux distribution by examining system files
+    private func detectHostLinuxDistribution() -> LinuxDistribution? {
+        return detectHostLinuxDistribution(fs: localFS)
+    }
+
+    /// Detects the Linux distribution by examining system files with an injected filesystem
     /// Start with the "generic" /etc/os-release then fallback
     /// to various distribution named files.
-    private func detectHostLinuxDistribution() -> LinuxDistribution? {
-        #if os(Linux)
-            // Try /etc/os-release first (standard)
-            if let osRelease = try? String(contentsOfFile: "/etc/os-release") {
+    public func detectHostLinuxDistribution(fs: any FSProxy) -> LinuxDistribution? {
+        // Try /etc/os-release first (standard)
+        let osReleasePath = Path("/etc/os-release")
+        if fs.exists(osReleasePath) {
+            if let osReleaseData = try? fs.read(osReleasePath),
+               let osRelease = String(data: Data(osReleaseData.bytes), encoding: .utf8) {
                 if let distribution = parseOSRelease(osRelease) {
                     return distribution
                 }
             }
-            // Fallback to distribution-specific files
-            let distributionFiles: [(String, LinuxDistribution.Kind)] = [
-                ("/etc/ubuntu-release", .ubuntu),
-                ("/etc/debian_version", .debian),
-                ("/etc/amazon-release", .amazon),
-                ("/etc/centos-release", .centos),
-                ("/etc/redhat-release", .rhel),
-                ("/etc/fedora-release", .fedora),
-                ("/etc/SuSE-release", .suse),
-                ("/etc/alpine-release", .alpine),
-                ("/etc/arch-release", .arch),
-          ]
-           for (file, kind) in distributionFiles {
-                if FileManager.default.fileExists(atPath: file) {
-                    return LinuxDistribution(kind: kind)
-                }
+        }
+
+        // Fallback to distribution-specific files
+        let distributionFiles: [(String, LinuxDistribution.Kind)] = [
+            ("/etc/ubuntu-release", .ubuntu),
+            ("/etc/debian_version", .debian),
+            ("/etc/amazon-release", .amazon),
+            ("/etc/centos-release", .centos),
+            ("/etc/redhat-release", .rhel),
+            ("/etc/fedora-release", .fedora),
+            ("/etc/SuSE-release", .suse),
+            ("/etc/alpine-release", .alpine),
+            ("/etc/arch-release", .arch),
+        ]
+
+        for (file, kind) in distributionFiles {
+            if fs.exists(Path(file)) {
+                return LinuxDistribution(kind: kind)
             }
-        #endif
+        }
+        
         return nil
     }