lxd/storage/drivers/btrfs: Move config modifications into FillConfig

When creating a new storage pool ensure that the config mangling is performed beforehand in FillConfig.
This allows other procedures like the recovery to perform config validation without needing to actually create the pool.

Signed-off-by: Julian Pelizäus <julian.pelizaeus@canonical.com>

lxd/storage/drivers/driver_btrfs.go

@@ -125,15 +125,27 @@ func (d *btrfs) FillConfig() error {
 		d.config["size"] = ""
 	}
 
+	// Store the provided source as we are likely to be mangling it.
+	d.config["volatile.initial_source"] = d.config["source"]
+
+	// Set the block device's UUID in case it already has one.
+	// This allows to recover the pools configuration without actually
+	// creating the storage pool.
+	// Downstream functions should use `volatile.initial_source` to ensure
+	// they are using the path instead of the volume's UUID.
+	if shared.IsBlockdevPath(d.config["source"]) {
+		devUUID, err := fsUUID(d.config["source"])
+		if err == nil {
+			d.config["source"] = devUUID
+		}
+	}
+
 	return nil
 }
 
 // Create is called during pool creation and is effectively using an empty driver struct.
 // WARNING: The Create() function cannot rely on any of the struct attributes being set.
 func (d *btrfs) Create() error {
-	// Store the provided source as we are likely to be mangling it.
-	d.config["volatile.initial_source"] = d.config["source"]
-
 	revert := revert.New()
 	defer revert.Fail()
 
@@ -165,25 +177,29 @@ func (d *btrfs) Create() error {
 		if err != nil {
 			return fmt.Errorf("Failed to format sparse file: %w", err)
 		}
-	} else if shared.IsBlockdevPath(d.config["source"]) {
+	} else if shared.IsBlockdevPath(d.config["volatile.initial_source"]) {
+		// Make sure to use the block volumes `volatile.initial_source` here
+		// as an earlier call to the drivers FillConfig() might have set
+		// the `source` property to the block volumes UUID in case it's not empty.
+
 		// Wipe if requested.
 		if shared.IsTrue(d.config["source.wipe"]) {
-			err := wipeBlockHeaders(d.config["source"])
+			err := wipeBlockHeaders(d.config["volatile.initial_source"])
 			if err != nil {
-				return fmt.Errorf("Failed to wipe headers from disk %q: %w", d.config["source"], err)
+				return fmt.Errorf("Failed to wipe headers from disk %q: %w", d.config["volatile.initial_source"], err)
 			}
 
 			d.config["source.wipe"] = ""
 		}
 
 		// Format the block device.
-		_, err := makeFSType(d.config["source"], "btrfs", &mkfsOptions{Label: d.name})
+		_, err := makeFSType(d.config["volatile.initial_source"], "btrfs", &mkfsOptions{Label: d.name})
 		if err != nil {
 			return fmt.Errorf("Failed to format block device: %w", err)
 		}
 
 		// Record the UUID as the source.
-		devUUID, err := fsUUID(d.config["source"])
+		devUUID, err := fsUUID(d.config["volatile.initial_source"])
 		if err != nil {
 			return err
 		}