[PATCH v1 proxmox-backup 3/3] api: backup: disallow incremental backups with different index length

[Robert Obkircher <r.obkircher@proxmox.com>]

Restore the previous restrictions because fixed index files are only
reused for qemu backups where the total size never changes.

Signed-off-by: Robert Obkircher <r.obkircher@proxmox.com>
---
 src/api2/backup/mod.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/api2/backup/mod.rs b/src/api2/backup/mod.rs
index 3c044c78..4a37947b 100644
--- a/src/api2/backup/mod.rs
+++ b/src/api2/backup/mod.rs
@@ -498,6 +498,11 @@ fn create_fixed_index(
     let mut incremental = false;
     if let Some(csum) = reuse_csum {
         incremental = true;
+
+        if size.is_none() {
+            bail!("size is required for incremental backups");
+        }
+
         let last_backup = match &env.last_backup {
             Some(info) => info,
             None => {
@@ -531,6 +536,14 @@ fn create_fixed_index(
     let mut writer = env.datastore.create_fixed_writer(&path, size, chunk_size)?;
 
     if let Some(reader) = reader {
+        // compares chunk count instead of size for backwards compatability
+        if writer.index_length() != reader.index_count() {
+            bail!(
+                "cannot reuse index - index sizes not equal ({} != {})",
+                writer.index_length(),
+                reader.index_count()
+            );
+        }
         writer.clone_data_from(&reader)?;
     }
 
-- 
2.47.3