vmm: omit unset Option fields from API responses

VmConfig and its nested configuration structs, the VmInfoResponse
wrapper and DeviceNode serialize their Option<T> fields as JSON null
when unset. The OpenAPI specification types these fields as
non-nullable, so strict client generators (for example ogen for Go)
reject /vm.info responses and cannot generate a working API client.

Apply serde_with's skip_serializing_none to the affected structs so
that unset optional fields are omitted from the serialized JSON instead
of being emitted as null. API responses now validate against the
existing specification unchanged; no nullable annotations are required.

Fixes: #7775

Signed-off-by: Max Makarov <maxpain@linux.com>
Assisted-by: Claude:claude-opus-4-8 [Claude Code]

Cargo.lock

@@ -2737,6 +2737,7 @@ dependencies = [
  "seccompiler",
  "serde",
  "serde_json",
+ "serde_with",
  "serial_buffer",
  "sha2",
  "signal-hook",

vmm/Cargo.toml

@@ -80,6 +80,7 @@ rate_limiter = { path = "../rate_limiter" }
 seccompiler = { workspace = true }
 serde = { workspace = true, features = ["derive", "rc"] }
 serde_json = { workspace = true }
+serde_with = { workspace = true, features = ["macros"] }
 serial_buffer = { path = "../serial_buffer" }
 sha2 = { workspace = true }
 signal-hook = { workspace = true }

vmm/src/api/mod.rs

@@ -213,6 +213,7 @@ pub enum ApiError {
 }
 pub type ApiResult<T> = Result<T, ApiError>;
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Deserialize, Serialize)]
 pub struct VmInfoResponse {
     pub config: Box<VmConfig>,

vmm/src/device_tree.rs

@@ -12,6 +12,7 @@ use vm_migration::Migratable;
 
 use crate::device_manager::PciDeviceHandle;
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Serialize, Deserialize)]
 pub struct DeviceNode {
     pub id: String,

vmm/src/vm_config.rs

@@ -67,6 +67,7 @@ pub fn default_cpuconfig_max_phys_bits() -> u8 {
     DEFAULT_MAX_PHYS_BITS
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct CpusConfig {
     pub boot_vcpus: u32,
@@ -123,6 +124,7 @@ pub fn default_platformconfig_vfio_p2p_dma() -> bool {
     true
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct PlatformConfig {
     #[serde(default = "default_platformconfig_num_pci_segments")]
@@ -221,6 +223,7 @@ pub struct PciSegmentConfig {
     pub mmio64_aperture_weight: u32,
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, Default, PartialEq, Eq, Deserialize, Serialize)]
 pub struct MemoryZoneConfig {
     pub id: String,
@@ -269,6 +272,7 @@ fn default_cpusconfig_nested() -> bool {
     true
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct MemoryConfig {
     pub size: u64,
@@ -335,6 +339,7 @@ pub struct VirtQueueAffinity {
     pub host_cpus: Box<[usize]>,
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize, Default)]
 pub struct PciDeviceCommonConfig {
     #[serde(default)]
@@ -347,6 +352,7 @@ pub struct PciDeviceCommonConfig {
     pub pci_device_id: Option<u8>,
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct DiskConfig {
     #[serde(flatten)]
@@ -412,6 +418,7 @@ pub fn default_diskconfig_sparse() -> bool {
     true
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct NetConfig {
     #[serde(flatten)]
@@ -584,6 +591,7 @@ impl ApplyLandlock for GenericVhostUserConfig {
     }
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct PmemConfig {
     #[serde(flatten)]
@@ -616,6 +624,7 @@ pub enum ConsoleOutputMode {
 /// Common configuration for plain console configs.
 ///
 /// Independent of PCI or legacy devices.
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct CommonConsoleConfig {
     #[serde(default)]
@@ -703,6 +712,7 @@ impl ApplyLandlock for ConsoleConfig {
 }
 
 #[cfg(target_arch = "x86_64")]
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct DebugConsoleConfig {
     #[serde(default)]
@@ -736,6 +746,7 @@ impl ApplyLandlock for DebugConsoleConfig {
     }
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct DeviceConfig {
     #[serde(flatten)]
@@ -846,6 +857,7 @@ pub struct NumaDistance {
     pub distance: u8,
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct NumaConfig {
     pub guest_numa_id: u32,
@@ -898,6 +910,7 @@ pub enum PayloadConfigError {
     FwCfgInvalidItem(String),
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
 pub struct PayloadConfig {
     #[serde(default)]
@@ -919,6 +932,7 @@ pub struct PayloadConfig {
 }
 
 #[cfg(feature = "fw_cfg")]
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
 pub struct FwCfgConfig {
     pub e820: bool,
@@ -951,6 +965,7 @@ pub struct FwCfgItemList {
 }
 
 #[cfg(feature = "fw_cfg")]
+#[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
 pub struct FwCfgItem {
     #[serde(default)]
@@ -1081,6 +1096,7 @@ impl ApplyLandlock for LandlockConfig {
     }
 }
 
+#[serde_with::skip_serializing_none]
 #[derive(Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct VmConfig {
     #[serde(default)]