storage: Robustify btrfs polling

It is only a warning when one of the "btrfs" invocations terminates
unsuccessfully, and the loop continues with the next volume.

Some failures are expected when polling happens concurrently with
removing subvolumes or volumes.

pkg/storaged/client.js

@@ -243,7 +243,6 @@ export async function btrfs_poll() {
         }).map(block_fsys => block_fsys.MountPoints).reduce((accum, current) => accum.concat(current));
         const mp = MountPoints[0];
         if (mp) {
-            let output;
             const mount_point = utils.decode_filename(mp);
             try {
                 // HACK: UDisks GetSubvolumes method uses `subvolume list -p` which
@@ -258,50 +257,47 @@ export async function btrfs_poll() {
                 // ID 256 gen 7 parent 5 top level 5 path <FS_TREE>/one
                 // ID 257 gen 7 parent 256 top level 256 path one/two
                 // ID 258 gen 7 parent 257 top level 257 path <FS_TREE>/one/two/three/four
-                output = await cockpit.spawn(["btrfs", "subvolume", "list", "-ap", mount_point], { superuser: true, err: "message" });
+                const output = await cockpit.spawn(["btrfs", "subvolume", "list", "-ap", mount_point], { superuser: true, err: "message" });
+                const subvols = [{ pathname: "/", id: 5 }];
+                for (const line of output.split("\n")) {
+                    const m = line.match(/ID (\d+).*parent (\d+).*path (<FS_TREE>\/)?(.*)/);
+                    if (m)
+                        subvols.push({ pathname: m[4], id: Number(m[1]) });
+                }
+                uuids_subvols[uuid] = subvols;
             } catch (err) {
-                console.error(`unable to obtain subvolumes for mount point ${mount_point}`, err);
-                return;
-            }
-            const subvols = [{ pathname: "/", id: 5 }];
-            for (const line of output.split("\n")) {
-                const m = line.match(/ID (\d+).*parent (\d+).*path (<FS_TREE>\/)?(.*)/);
-                if (m)
-                    subvols.push({ pathname: m[4], id: Number(m[1]) });
+                console.warn(`unable to obtain subvolumes for mount point ${mount_point}`, err);
             }
-            uuids_subvols[uuid] = subvols;
 
             // HACK: Obtain the default subvolume, required for mounts in which do not specify a subvol and subvolid.
             // In the future can be obtained via UDisks, it requires the btrfs partition to be mounted somewhere.
             // https://github.com/storaged-project/udisks/commit/b6966b7076cd837f9d307eef64beedf01bc863ae
             try {
-                output = await cockpit.spawn(["btrfs", "subvolume", "get-default", mount_point], { superuser: true, err: "message" });
+                const output = await cockpit.spawn(["btrfs", "subvolume", "get-default", mount_point], { superuser: true, err: "message" });
                 const id_match = output.match(/ID (\d+).*/);
                 if (id_match)
                     btrfs_default_subvol[uuid] = Number(id_match[1]);
             } catch (err) {
-                console.error(`unable to obtain default subvolume for mount point ${mount_point}`, err);
+                console.warn(`unable to obtain default subvolume for mount point ${mount_point}`, err);
             }
 
             // HACK: UDisks should expose a better btrfs API with btrfs device information
             // https://github.com/storaged-project/udisks/issues/1232
             // TODO: optimise into just parsing one `btrfs filesystem show`?
-            const usages = {};
-            let usage_output;
             try {
-                usage_output = await cockpit.spawn(["btrfs", "filesystem", "show", "--raw", uuid], { superuser: true, err: "message" });
-            } catch (err) {
-                console.error(`btrfs filesystem show ${uuid}`, err);
-                return;
-            }
-            for (const line of usage_output.split("\n")) {
-                const match = usage_regex.exec(line);
-                if (match) {
-                    const { used, device } = match.groups;
-                    usages[device] = used;
+                const usage_output = await cockpit.spawn(["btrfs", "filesystem", "show", "--raw", uuid], { superuser: true, err: "message" });
+                const usages = {};
+                for (const line of usage_output.split("\n")) {
+                    const match = usage_regex.exec(line);
+                    if (match) {
+                        const { used, device } = match.groups;
+                        usages[device] = used;
+                    }
                 }
+                uuids_usage[uuid] = usages;
+            } catch (err) {
+                console.warn(`btrfs filesystem show ${uuid}`, err);
             }
-            uuids_usage[uuid] = usages;
         } else {
             uuids_subvols[uuid] = null;
             uuids_usage[uuid] = null;