Merge ~raharper/curtin:fix/clear-holders-bcache-partitions-lp1811117-v2 into curtin:master

[Ryan Harper]

bcache: ensure partitions on bcache devices are detected as partition

In some cases, when curtin is discovering existing storage devices, a
bcache device with partitions created on top of it, may be recognized
as a bcache device, rather than a partition. If curtin treats a
bcache partition as a bcache device, it will attempt to write to
non-existent files on /sys which results in a failure after timing
out. Resolve this issue by detecting if the target device is a
partition inside the bcache identity function.

[Server Team CI bot (server-team-bot)]

PASSED: Continuous integration, rev:0723e0da24e859d78b893dd14b8f4e96efa78915
https://jenkins.ubuntu.com/server/job/curtin-ci/1168/
Executed test runs:
    SUCCESS: https://jenkins.ubuntu.com/server/job/curtin-ci/nodes=metal-arm64/1168
    SUCCESS: https://jenkins.ubuntu.com/server/job/curtin-ci/nodes=metal-ppc64el/1168
    SUCCESS: https://jenkins.ubuntu.com/server/job/curtin-ci/nodes=metal-s390x/1168
    SUCCESS: https://jenkins.ubuntu.com/server/job/curtin-ci/nodes=torkoal/1168

Click here to trigger a rebuild:
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review: Approve (continuous-integration)

[Chad Smith (chad.smith)]

+1 LGTM!

review: Approve

diff --git a/curtin/block/clear_holders.py b/curtin/block/clear_holders.py
index 05d756c..d722669 100644
--- a/curtin/block/clear_holders.py
+++ b/curtin/block/clear_holders.py
@@ -410,7 +410,10 @@ def identify_bcache(device):
     """
     determine if specified device is a bcache device
     """
-    return block.path_to_kname(device).startswith('bcache')
+    # bcache devices can be partitioned and the partitions are *not*
+    # bcache devices with a sysfs 'slaves' subdirectory
+    partition = identify_partition(device)
+    return block.path_to_kname(device).startswith('bcache') and not partition
 
 
 def identify_partition(device):