Add custom decoder to Linux struct for optional fields (#455)

Allows decoding of minimal OCI specs with empty linux objects by
providing default values for missing fields. Also added unit test to
ensure that empty Linux struct {} works correctly with the fix.

Author: yibozhuang

Sources/ContainerizationOCI/Spec.swift

@@ -426,6 +426,22 @@ public struct Linux: Codable, Sendable {
     public var mountLabel: String
     public var personality: LinuxPersonality?
 
+    public enum CodingKeys: String, CodingKey {
+        case uidMappings
+        case gidMappings
+        case sysctl
+        case resources
+        case cgroupsPath
+        case namespaces
+        case devices
+        case seccomp
+        case rootfsPropagation
+        case maskedPaths
+        case readonlyPaths
+        case mountLabel
+        case personality
+    }
+
     public init(
         uidMappings: [LinuxIDMapping] = [],
         gidMappings: [LinuxIDMapping] = [],
@@ -455,6 +471,43 @@ public struct Linux: Codable, Sendable {
         self.mountLabel = mountLabel
         self.personality = personality
     }
+
+    public init(from decoder: Decoder) throws {
+        self.init()
+
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        if let uidMappings = try container.decodeIfPresent([LinuxIDMapping].self, forKey: .uidMappings) {
+            self.uidMappings = uidMappings
+        }
+        if let gidMappings = try container.decodeIfPresent([LinuxIDMapping].self, forKey: .gidMappings) {
+            self.gidMappings = gidMappings
+        }
+        self.sysctl = try container.decodeIfPresent([String: String].self, forKey: .sysctl)
+        self.resources = try container.decodeIfPresent(LinuxResources.self, forKey: .resources)
+        if let cgroupsPath = try container.decodeIfPresent(String.self, forKey: .cgroupsPath) {
+            self.cgroupsPath = cgroupsPath
+        }
+        if let namespaces = try container.decodeIfPresent([LinuxNamespace].self, forKey: .namespaces) {
+            self.namespaces = namespaces
+        }
+        if let devices = try container.decodeIfPresent([LinuxDevice].self, forKey: .devices) {
+            self.devices = devices
+        }
+        self.seccomp = try container.decodeIfPresent(LinuxSeccomp.self, forKey: .seccomp)
+        if let rootfsPropagation = try container.decodeIfPresent(String.self, forKey: .rootfsPropagation) {
+            self.rootfsPropagation = rootfsPropagation
+        }
+        if let maskedPaths = try container.decodeIfPresent([String].self, forKey: .maskedPaths) {
+            self.maskedPaths = maskedPaths
+        }
+        if let readonlyPaths = try container.decodeIfPresent([String].self, forKey: .readonlyPaths) {
+            self.readonlyPaths = readonlyPaths
+        }
+        if let mountLabel = try container.decodeIfPresent(String.self, forKey: .mountLabel) {
+            self.mountLabel = mountLabel
+        }
+        self.personality = try container.decodeIfPresent(LinuxPersonality.self, forKey: .personality)
+    }
 }
 
 public struct LinuxNamespace: Codable, Sendable {

Tests/ContainerizationOCITests/OCISpecTests.swift

@@ -143,4 +143,26 @@ struct OCISpecTests {
         #expect(decodedSpec.uidMappings == nil)
         #expect(decodedSpec.gidMappings == nil)
     }
+
+    @Test func minimalCapabilitiesDecode() throws {
+        let minCapabilitiesSpec =
+            """
+                {
+                    "ociVersion": "1.1.0",
+                    "capabilities": {
+                        "permitted": [
+                            "CAP_SYS_ADMIN"
+                        ]
+                    },
+                    "linux": {}
+                }
+            """
+
+        guard let data = minCapabilitiesSpec.data(using: .utf8) else {
+            Issue.record("test capabilities spec is not valid: \(minCapabilitiesSpec)")
+            return
+        }
+
+        let _ = try JSONDecoder().decode(ContainerizationOCI.Spec.self, from: data)
+    }
 }