tape: add magic number to identify media catalog files

src/tape/media_catalog.rs

@@ -1,6 +1,7 @@
 use std::convert::TryFrom;
 use std::fs::File;
-use std::io::{Write, Read, BufReader};
+use std::io::{Write, Read, BufReader, Seek, SeekFrom};
+use std::os::unix::io::AsRawFd;
 use std::path::Path;
 use std::collections::HashMap;
 
@@ -25,6 +26,9 @@ use crate::{
     tape::drive::MediaLabelInfo,
 };
 
+// openssl::sha::sha256(b"Proxmox Backup Media Catalog v1.0")[0..8]
+pub const PROXMOX_BACKUP_MEDIA_CATALOG_MAGIC_1_0: [u8; 8] = [221, 29, 164, 1, 59, 69, 19, 40];
+
 /// The Media Catalog
 ///
 /// Stores what chunks and snapshots are stored on a specific media,
@@ -109,12 +113,9 @@ impl MediaCatalog {
                 .create(create)
                 .open(&path)?;
 
-            use std::os::unix::io::AsRawFd;
-
             let backup_user = crate::backup::backup_user()?;
-            if let Err(err) = fchown(file.as_raw_fd(), Some(backup_user.uid), Some(backup_user.gid)) {
-                bail!("fchown on media catalog {:?} failed - {}", path, err);
-            }
+            fchown(file.as_raw_fd(), Some(backup_user.uid), Some(backup_user.gid))
+                .map_err(|err| format_err!("fchown failed - {}", err))?;
 
             let mut me = Self {
                 uuid: uuid.clone(),
@@ -127,7 +128,11 @@ impl MediaCatalog {
                 pending: Vec::new(),
             };
 
-            me.load_catalog(&mut file)?;
+            let found_magic_number = me.load_catalog(&mut file)?;
+
+            if !found_magic_number {
+                me.pending.extend(&PROXMOX_BACKUP_MEDIA_CATALOG_MAGIC_1_0);
+            }
 
             if write {
                 me.file = Some(file);
@@ -147,15 +152,16 @@ impl MediaCatalog {
         log_to_stdout: bool,
     ) -> Result<Self, Error> {
 
-
-        Self::create_basedir(base_path)?;
-
         let uuid = &label_info.label.uuid;
 
         let mut tmp_path = base_path.to_owned();
         tmp_path.push(uuid.to_string());
         tmp_path.set_extension("tmp");
 
+        let me = proxmox::try_block!({
+
+            Self::create_basedir(base_path)?;
+
             let file = std::fs::OpenOptions::new()
                 .read(true)
                 .write(true)
@@ -163,12 +169,9 @@ impl MediaCatalog {
                 .truncate(true)
                 .open(&tmp_path)?;
 
-        use std::os::unix::io::AsRawFd;
-
             let backup_user = crate::backup::backup_user()?;
-        if let Err(err) = fchown(file.as_raw_fd(), Some(backup_user.uid), Some(backup_user.gid)) {
-            bail!("fchown on media catalog {:?} failed - {}", tmp_path, err);
-        }
+            fchown(file.as_raw_fd(), Some(backup_user.uid), Some(backup_user.gid))
+                .map_err(|err| format_err!("fchown failed - {}", err))?;
 
             let mut me = Self {
                 uuid: uuid.clone(),
@@ -189,8 +192,14 @@ impl MediaCatalog {
                 me.register_label(&content_uuid, 1)?;
             }
 
+            me.pending.extend(&PROXMOX_BACKUP_MEDIA_CATALOG_MAGIC_1_0);
             me.commit()?;
 
+            Ok(me)
+        }).map_err(|err: Error| {
+            format_err!("unable to create temporary media catalog {:?} - {}", tmp_path, err)
+        })?;
+
         Ok(me)
     }
 
@@ -525,11 +534,28 @@ impl MediaCatalog {
         Ok(())
     }
 
-    fn load_catalog(&mut self, file: &mut File) -> Result<(), Error> {
+    fn load_catalog(&mut self, file: &mut File) -> Result<bool, Error> {
 
         let mut file = BufReader::new(file);
+        let mut found_magic_number = false;
 
         loop {
+            let pos = file.seek(SeekFrom::Current(0))?;
+
+            if pos == 0 { // read/check magic number
+                let mut magic = [0u8; 8];
+                match file.read_exact_or_eof(&mut magic) {
+                    Ok(false) => { /* EOF */ break; }
+                    Ok(true) => { /* OK */ }
+                    Err(err) => bail!("read failed - {}", err),
+                }
+                if magic != PROXMOX_BACKUP_MEDIA_CATALOG_MAGIC_1_0 {
+                    bail!("wrong magic number");
+                }
+                found_magic_number = true;
+                continue;
+            }
+
             let mut entry_type = [0u8; 1];
             match file.read_exact_or_eof(&mut entry_type) {
                 Ok(false) => { /* EOF */ break; }
@@ -597,7 +623,7 @@ impl MediaCatalog {
 
         }
 
-        Ok(())
+        Ok(found_magic_number)
     }
 }