[pbs-devel] [PATCH proxmox-backup v2] api: config: use guard for unmounting on failed datastore creation

[Hannes Laimer]

Currently if any `?`/`bail!` happens between mounting and completing
the creation process unmounting will be skipped. Adding this guard
solves that problem and makes it easier to add things in the future
without having to worry about a disk not being unmounted in case of a
failed creation.

Signed-off-by: Hannes Laimer <h.laimer at proxmox.com>
Reported-by: Wolfgang Bumiller <w.bumiller at proxmox.com>
---
v2, thanks @Wolfgang:
 - replace `should_unmount` flag with wrapping path in an Option

 src/api2/config/datastore.rs | 82 +++++++++++++++++++++---------------
 1 file changed, 48 insertions(+), 34 deletions(-)

diff --git a/src/api2/config/datastore.rs b/src/api2/config/datastore.rs
index fe3260f6d..58acaa861 100644
--- a/src/api2/config/datastore.rs
+++ b/src/api2/config/datastore.rs
@@ -1,7 +1,7 @@
 use std::path::{Path, PathBuf};
 
 use ::serde::{Deserialize, Serialize};
-use anyhow::{bail, format_err, Error};
+use anyhow::{bail, Error};
 use hex::FromHex;
 use serde_json::Value;
 use tracing::warn;
@@ -70,6 +70,29 @@ pub fn list_datastores(
     Ok(list.into_iter().filter(filter_by_privs).collect())
 }
 
+struct UnmountGuard {
+    path: Option<PathBuf>,
+}
+
+impl UnmountGuard {
+    fn new(path: Option<PathBuf>) -> Self {
+        UnmountGuard { path }
+    }
+    fn disable(mut self) {
+        self.path = None;
+    }
+}
+
+impl Drop for UnmountGuard {
+    fn drop(&mut self) {
+        if let Some(path) = &self.path {
+            if let Err(e) = unmount_by_mountpoint(path) {
+                warn!("could not unmount device: {e}");
+            }
+        }
+    }
+}
+
 pub(crate) fn do_create_datastore(
     _lock: BackupLockGuard,
     mut config: SectionConfigData,
@@ -87,59 +110,50 @@ pub(crate) fn do_create_datastore(
         param_bail!("path", err);
     }
 
-    let need_unmount = datastore.backing_device.is_some();
-    if need_unmount {
-        do_mount_device(datastore.clone())?;
-    };
-
     let tuning: DatastoreTuning = serde_json::from_value(
         DatastoreTuning::API_SCHEMA
             .parse_property_string(datastore.tuning.as_deref().unwrap_or(""))?,
     )?;
 
-    let res = if reuse_datastore {
-        ChunkStore::verify_chunkstore(&path)
+    let unmount_guard = if datastore.backing_device.is_some() {
+        do_mount_device(datastore.clone())?;
+        UnmountGuard::new(Some(path.clone()))
+    } else {
+        UnmountGuard::new(None)
+    };
+
+    if reuse_datastore {
+        ChunkStore::verify_chunkstore(&path)?;
     } else {
-        let mut is_empty = true;
         if let Ok(dir) = std::fs::read_dir(&path) {
             for file in dir {
                 let name = file?.file_name();
                 let name = name.to_str();
                 if !name.is_some_and(|name| name.starts_with('.') || name == "lost+found") {
-                    is_empty = false;
-                    break;
+                    bail!("datastore path not empty");
                 }
             }
         }
-        if is_empty {
-            let backup_user = pbs_config::backup_user()?;
-            ChunkStore::create(
-                &datastore.name,
-                path.clone(),
-                backup_user.uid,
-                backup_user.gid,
-                tuning.sync_level.unwrap_or_default(),
-            )
-            .map(|_| ())
-        } else {
-            Err(format_err!("datastore path not empty"))
-        }
+        let backup_user = pbs_config::backup_user()?;
+        ChunkStore::create(
+            &datastore.name,
+            path.clone(),
+            backup_user.uid,
+            backup_user.gid,
+            tuning.sync_level.unwrap_or_default(),
+        )
+        .map(|_| ())?;
     };
 
-    if res.is_err() {
-        if need_unmount {
-            if let Err(e) = unmount_by_mountpoint(&path) {
-                warn!("could not unmount device: {e}");
-            }
-        }
-        return res;
-    }
-
     config.set_data(&datastore.name, "datastore", &datastore)?;
 
     pbs_config::datastore::save_config(&config)?;
 
-    jobstate::create_state_file("garbage_collection", &datastore.name)
+    jobstate::create_state_file("garbage_collection", &datastore.name)?;
+
+    unmount_guard.disable();
+
+    Ok(())
 }
 
 #[api(
-- 
2.39.5