add state serializing and loading functions

For dirty-bitmap migration, QEMU also needs to move the static state of
the library to the target. proxmox_{import,export}_state provide a means
of accessing said data in a serialized fashion.

QEMU treats the state as some unknown quantity of bytes and the result
does not need to be human-readable, so we encode it with 'bincode',
which is based on serde.

Since the quantity is only known *after* serialization, we have to
allocate the buffer ourselves. This is handled by Box::leak-ing a Rust
allocated buffer and cleaning up via the explicit
proxmox_free_state_buf function.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

Cargo.toml

@@ -32,3 +32,4 @@ proxmox-backup = { git = "git://git.proxmox.com/git/proxmox-backup.git", tag = "
 #proxmox-backup = { path = "../proxmox-backup" }
 serde_json = "1.0"
 tokio = { version = "0.2.9", features = [ "blocking", "fs", "io-util", "macros", "rt-threaded", "signal", "stream", "tcp", "time", "uds" ] }
+bincode = "1.0"

current-api.h

@@ -263,6 +263,26 @@ void proxmox_backup_write_data_async(ProxmoxBackupHandle *handle,
                                      int *result,
                                      char **error);
 
+/**
+ * Serialize all state data into a byte buffer. Can be loaded again with
+ * proxmox_import_state. Use for migration for example.
+ *
+ * Length of the returned buffer is written to buf_size. Returned buffer must
+ * be freed with proxmox_free_state_buf.
+ */
+uint8_t *proxmox_export_state(uintptr_t *buf_size);
+
+/**
+ * Free a buffer acquired from proxmox_export_state.
+ */
+void proxmox_free_state_buf(uint8_t *buf);
+
+/**
+ * Load state serialized by proxmox_export_state. If loading fails, a message
+ * will be logged to stderr, but the function will not fail.
+ */
+void proxmox_import_state(const uint8_t *buf, uintptr_t buf_size);
+
 /**
  * Open connection to the backup server (sync)
  *

src/commands.rs

@@ -16,6 +16,9 @@ use crate::upload_queue::*;
 use lazy_static::lazy_static;
 
 lazy_static!{
+    // Note: Any state stored here that needs to be sent along with migration
+    // needs to be specified in (de)serialize_state as well!
+
     static ref PREVIOUS_CSUMS: Mutex<HashMap<String, [u8;32]>> = {
         Mutex::new(HashMap::new())
     };
@@ -35,6 +38,22 @@ pub struct ImageUploadInfo {
 }
 
 
+pub(crate) fn serialize_state() -> Vec<u8> {
+    let prev_csums = &*PREVIOUS_CSUMS.lock().unwrap();
+    let prev_crypt_digest = &*PREVIOUS_CRYPT_CONFIG_DIGEST.lock().unwrap();
+    bincode::serialize(&(prev_csums, prev_crypt_digest)).unwrap()
+}
+
+pub(crate) fn deserialize_state(data: &[u8]) -> Result<(), Error> {
+    let (prev_csums, prev_crypt_digest) = bincode::deserialize(data)?;
+    let mut prev_csums_guard = PREVIOUS_CSUMS.lock().unwrap();
+    let mut prev_crypt_digest_guard = PREVIOUS_CRYPT_CONFIG_DIGEST.lock().unwrap();
+    *prev_csums_guard = prev_csums;
+    *prev_crypt_digest_guard = prev_crypt_digest;
+    Ok(())
+}
+
+
 // Note: We alway register/upload a chunk containing zeros
 async fn register_zero_chunk(
     client: Arc<BackupWriter>,

src/lib.rs

@@ -967,3 +967,37 @@ pub extern "C" fn proxmox_restore_read_image_at_async(
         callback_info.send_result(result);
     });
 }
+
+/// Serialize all state data into a byte buffer. Can be loaded again with
+/// proxmox_import_state. Use for migration for example.
+///
+/// Length of the returned buffer is written to buf_size. Returned buffer must
+/// be freed with proxmox_free_state_buf.
+#[no_mangle]
+#[allow(clippy::not_unsafe_ptr_arg_deref)]
+pub extern "C" fn proxmox_export_state(buf_size: *mut usize) -> *mut u8 {
+    let data = commands::serialize_state().into_boxed_slice();
+    unsafe { *buf_size = data.len(); }
+    Box::leak(data).as_mut_ptr()
+}
+
+/// Load state serialized by proxmox_export_state. If loading fails, a message
+/// will be logged to stderr, but the function will not fail.
+#[no_mangle]
+#[allow(clippy::not_unsafe_ptr_arg_deref)]
+pub extern "C" fn proxmox_import_state(buf: *const u8, buf_size: usize) {
+    let data = unsafe { std::slice::from_raw_parts(buf, buf_size) };
+    // ignore errors, just log what happened
+    if let Err(err) = commands::deserialize_state(data) {
+        eprintln!("error deserializing PBS state - {}", err);
+    }
+}
+
+/// Free a buffer acquired from proxmox_export_state.
+#[no_mangle]
+#[allow(clippy::not_unsafe_ptr_arg_deref)]
+pub extern "C" fn proxmox_free_state_buf(buf: *mut u8) {
+    if !buf.is_null() {
+        unsafe { Box::from_raw(buf); }
+    }
+}