snapshot: make memory path optional in snapshot creation

There are cases where a user might want to snapshot the memoyr of a VM
externally. In these cases, we can ask Firecracker to avoid serializing
the memory file to disk when we create a snapshot.

Signed-off-by: Babis Chalios <babis.chalios@e2b.dev>

Author: bchalios

src/firecracker/src/api_server/mod.rs

@@ -275,7 +275,7 @@ mod tests {
             Box::new(VmmAction::CreateSnapshot(CreateSnapshotParams {
                 snapshot_type: SnapshotType::Diff,
                 snapshot_path: PathBuf::new(),
-                mem_file_path: PathBuf::new(),
+                mem_file_path: Some(PathBuf::new()),
             })),
             start_time_us,
         );
@@ -288,7 +288,7 @@ mod tests {
             Box::new(VmmAction::CreateSnapshot(CreateSnapshotParams {
                 snapshot_type: SnapshotType::Diff,
                 snapshot_path: PathBuf::new(),
-                mem_file_path: PathBuf::new(),
+                mem_file_path: Some(PathBuf::new()),
             })),
             start_time_us,
         );

src/firecracker/src/api_server/request/snapshot.rs

@@ -144,7 +144,7 @@ mod tests {
         let expected_config = CreateSnapshotParams {
             snapshot_type: SnapshotType::Diff,
             snapshot_path: PathBuf::from("foo"),
-            mem_file_path: PathBuf::from("bar"),
+            mem_file_path: Some(PathBuf::from("bar")),
         };
         assert_eq!(
             vmm_action_from_request(parse_put_snapshot(&Body::new(body), Some("create")).unwrap()),
@@ -158,7 +158,7 @@ mod tests {
         let expected_config = CreateSnapshotParams {
             snapshot_type: SnapshotType::Full,
             snapshot_path: PathBuf::from("foo"),
-            mem_file_path: PathBuf::from("bar"),
+            mem_file_path: Some(PathBuf::from("bar")),
         };
         assert_eq!(
             vmm_action_from_request(parse_put_snapshot(&Body::new(body), Some("create")).unwrap()),

src/firecracker/swagger/firecracker.yaml

@@ -1665,12 +1665,14 @@ definitions:
   SnapshotCreateParams:
     type: object
     required:
-      - mem_file_path
       - snapshot_path
     properties:
       mem_file_path:
         type: string
-        description: Path to the file that will contain the guest memory.
+        description:
+          Path to the file that will contain the guest memory. It is optional.
+          In case that a user doesn't provide a path, they are responsible to
+          ensure they store the microVM's memory state via external means.
       snapshot_path:
         type: string
         description: Path to the file that will contain the microVM state.

src/vmm/src/persist.rs

@@ -161,8 +161,10 @@ pub fn create_snapshot(
 
     snapshot_state_to_file(&microvm_state, &params.snapshot_path)?;
 
+    if let Some(mem_file_path) = params.mem_file_path.as_ref() {
     vmm.vm
-        .snapshot_memory_to_file(&params.mem_file_path, params.snapshot_type)?;
+        .snapshot_memory_to_file(mem_file_path, params.snapshot_type)?;
+    }
 
     // We need to mark queues as dirty again for all activated devices. The reason we
     // do it here is that we don't mark pages as dirty during runtime

src/vmm/src/rpc_interface.rs

@@ -1243,7 +1243,7 @@ mod tests {
             CreateSnapshotParams {
                 snapshot_type: SnapshotType::Full,
                 snapshot_path: PathBuf::new(),
-                mem_file_path: PathBuf::new(),
+                mem_file_path: Some(PathBuf::new()),
             },
         )));
         #[cfg(target_arch = "x86_64")]

src/vmm/src/vmm_config/snapshot.rs

@@ -44,7 +44,7 @@ pub struct CreateSnapshotParams {
     /// Path to the file that will contain the microVM state.
     pub snapshot_path: PathBuf,
     /// Path to the file that will contain the guest memory.
-    pub mem_file_path: PathBuf,
+    pub mem_file_path: Option<PathBuf>,
 }
 
 /// Allows for changing the mapping between tap devices and host devices

src/vmm/tests/integration_tests.rs

@@ -235,7 +235,7 @@ fn verify_create_snapshot(
     let snapshot_params = CreateSnapshotParams {
         snapshot_type,
         snapshot_path: snapshot_file.as_path().to_path_buf(),
-        mem_file_path: memory_file.as_path().to_path_buf(),
+        mem_file_path: Some(memory_file.as_path().to_path_buf()),
     };
 
     controller