add incremental backup support

To support incremental backups (where not all chunks are sent to the
server), a new parameter "reuse-csum" is introduced on the
"create_fixed_index" API call. When set and equal to last backups'
checksum, the backup writer clones the data from the last index of this
archive file, and only updates chunks it actually receives.

In incremental mode some checks usually done on closing an index cannot
be made, since they would be inaccurate.

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

src/api2/backup.rs

@@ -284,6 +284,8 @@ pub const API_METHOD_CREATE_FIXED_INDEX: ApiMethod = ApiMethod::new(
              .minimum(1)
              .schema()
             ),
+            ("reuse-csum", true, &StringSchema::new("If set, compare last backup's \
+                csum and reuse index for incremental backup if it matches.").schema()),
         ]),
     )
 );
@@ -298,6 +300,7 @@ fn create_fixed_index(
 
     let name = tools::required_string_param(&param, "archive-name")?.to_owned();
     let size = tools::required_integer_param(&param, "size")? as usize;
+    let reuse_csum = param["reuse-csum"].as_str();
 
     let archive_name = name.clone();
     if !archive_name.ends_with(".fidx") {
@@ -305,12 +308,49 @@ fn create_fixed_index(
     }
 
     let mut path = env.backup_dir.relative_path();
-    path.push(archive_name);
+    path.push(&archive_name);
 
     let chunk_size = 4096*1024; // todo: ??
 
-    let index = env.datastore.create_fixed_writer(&path, size, chunk_size)?;
-    let wid = env.register_fixed_writer(index, name, size, chunk_size as u32)?;
+    // do incremental backup if csum is set
+    let mut reader = None;
+    let mut incremental = false;
+    if let Some(csum) = reuse_csum {
+        incremental = true;
+        let last_backup = match &env.last_backup {
+            Some(info) => info,
+            None => {
+                bail!("cannot reuse index - no previous backup exists");
+            }
+        };
+
+        let mut last_path = last_backup.backup_dir.relative_path();
+        last_path.push(&archive_name);
+
+        let index = match env.datastore.open_fixed_reader(last_path) {
+            Ok(index) => index,
+            Err(_) => {
+                bail!("cannot reuse index - no previous backup exists for archive");
+            }
+        };
+
+        let (old_csum, _) = index.compute_csum();
+        let old_csum = proxmox::tools::digest_to_hex(&old_csum);
+        if old_csum != csum {
+            bail!("expected csum ({}) doesn't match last backup's ({}), cannot do incremental backup",
+                csum, old_csum);
+        }
+
+        reader = Some(index);
+    }
+
+    let mut writer = env.datastore.create_fixed_writer(&path, size, chunk_size)?;
+
+    if let Some(reader) = reader {
+        writer.clone_data_from(&reader)?;
+    }
+
+    let wid = env.register_fixed_writer(writer, name, size, chunk_size as u32, incremental)?;
 
     env.log(format!("created new fixed index {} ({:?})", wid, path));
 
@@ -518,15 +558,15 @@ pub const API_METHOD_CLOSE_FIXED_INDEX: ApiMethod = ApiMethod::new(
             (
                 "chunk-count",
                 false,
-                &IntegerSchema::new("Chunk count. This is used to verify that the server got all chunks.")
-                    .minimum(1)
+                &IntegerSchema::new("Chunk count. This is used to verify that the server got all chunks. Ignored for incremental backups.")
+                    .minimum(0)
                     .schema()
             ),
             (
                 "size",
                 false,
-                &IntegerSchema::new("File size. This is used to verify that the server got all data.")
-                    .minimum(1)
+                &IntegerSchema::new("File size. This is used to verify that the server got all data. Ignored for incremental backups.")
+                    .minimum(0)
                     .schema()
             ),
             ("csum", false, &StringSchema::new("Digest list checksum.").schema()),