expose previous backup time in backup env

and use this information to add more information to client backup log
and guide the download manifest decision.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/api2/backup.rs

@@ -311,6 +311,10 @@ pub const BACKUP_API_SUBDIRS: SubdirMap = &[
         "previous", &Router::new()
             .download(&API_METHOD_DOWNLOAD_PREVIOUS)
     ),
+    (
+        "previous_backup_time", &Router::new()
+            .get(&API_METHOD_GET_PREVIOUS_BACKUP_TIME)
+    ),
     (
         "speedtest", &Router::new()
             .upload(&API_METHOD_UPLOAD_SPEEDTEST)
@@ -694,6 +698,28 @@ fn finish_backup (
     Ok(Value::Null)
 }
 
+#[sortable]
+pub const API_METHOD_GET_PREVIOUS_BACKUP_TIME: ApiMethod = ApiMethod::new(
+    &ApiHandler::Sync(&get_previous_backup_time),
+    &ObjectSchema::new(
+        "Get previous backup time.",
+        &[],
+    )
+);
+
+fn get_previous_backup_time(
+    _param: Value,
+    _info: &ApiMethod,
+    rpcenv: &mut dyn RpcEnvironment,
+) -> Result<Value, Error> {
+
+    let env: &BackupEnvironment = rpcenv.as_ref();
+
+    let backup_time = env.last_backup.as_ref().map(|info| info.backup_dir.backup_time());
+
+    Ok(json!(backup_time))
+}
+
 #[sortable]
 pub const API_METHOD_DOWNLOAD_PREVIOUS: ApiMethod = ApiMethod::new(
     &ApiHandler::AsyncHttp(&download_previous),

src/bin/proxmox-backup-client.rs

@@ -1099,23 +1099,42 @@ async fn create_backup(
         false
     ).await?;
 
-    let previous_manifest = match client.download_previous_manifest().await {
-        Ok(previous_manifest) => {
-            match previous_manifest.check_fingerprint(crypt_config.as_ref().map(Arc::as_ref)) {
-                Ok(()) => {
-                    println!("Successfully downloaded previous manifest.");
-                    Some(Arc::new(previous_manifest))
-                },
-                Err(err) => {
-                    println!("Couldn't re-use pevious manifest - {}", err);
-                    None
-                },
+    let download_previous_manifest = match client.previous_backup_time().await {
+        Ok(Some(backup_time)) => {
+            println!(
+                "Downloading previous manifest ({})",
+                strftime_local("%c", backup_time)?
+            );
+            true
+        }
+        Ok(None) => {
+            println!("No previous manifest available.");
+            false
+        }
+        Err(_) => {
+            // Fallback for outdated server, TODO remove/bubble up with 2.0
+            true
+        }
+    };
+
+    let previous_manifest = if download_previous_manifest {
+        match client.download_previous_manifest().await {
+            Ok(previous_manifest) => {
+                match previous_manifest.check_fingerprint(crypt_config.as_ref().map(Arc::as_ref)) {
+                    Ok(()) => Some(Arc::new(previous_manifest)),
+                    Err(err) => {
+                        println!("Couldn't re-use previous manifest - {}", err);
+                        None
+                    }
+                }
             }
-        },
-        Err(err) => {
-            println!("Couldn't download pevious manifest - {}", err);
-            None
-        },
+            Err(err) => {
+                println!("Couldn't download previous manifest - {}", err);
+                None
+            }
+        }
+    } else {
+        None
     };
 
     let snapshot = BackupDir::new(backup_type, backup_id, backup_time)?;

src/client/backup_writer.rs

@@ -475,6 +475,13 @@ impl BackupWriter {
         Ok(index)
     }
 
+    /// Retrieve backup time of last backup
+    pub async fn previous_backup_time(&self) -> Result<Option<i64>, Error> {
+        let data = self.h2.get("previous_backup_time", None).await?;
+        serde_json::from_value(data)
+            .map_err(|err| format_err!("Failed to parse backup time value returned by server - {}", err))
+    }
+
     /// Download backup manifest (index.json) of last backup
     pub async fn download_previous_manifest(&self) -> Result<BackupManifest, Error> {