Merge ~mwhudson/curtin:default-first-lba into curtin:master

[Michael Hudson-Doyle]

block_meta_v2: set first-lba if needed when creating a new GPT

sfdisk sets first-lba to 2048 by default, which fails if one of
the partitions has an start smaller than that. Having a
partition start so early in the disk is a bit strange but it
works if you are preserving a partition (because we preserve
first-lba) so it should probably work when creating one too.

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

PASSED: Continuous integration, rev:68d084399c46ad57aa25b1f44cd3f06444100f98
https://jenkins.canonical.com/server-team/job/curtin-ci/55/
Executed test runs:
    SUCCESS: https://jenkins.canonical.com/server-team/job/curtin-ci/nodes=metal-amd64/55/
    SUCCESS: https://jenkins.canonical.com/server-team/job/curtin-ci/nodes=metal-arm64/55/
    SUCCESS: https://jenkins.canonical.com/server-team/job/curtin-ci/nodes=metal-ppc64el/55/
    SUCCESS: https://jenkins.canonical.com/server-team/job/curtin-ci/nodes=metal-s390x/55/

Click here to trigger a rebuild:
https://jenkins.canonical.com/server-team/job/curtin-ci/55//rebuild

review: Approve (continuous-integration)

[Dan Bungert (dbungert)]

This looks great. Would you add a test covering this case?
I did see sub-2048 values when playing with gpt preservation so this is quite worth doing.

02bc28d... by Michael Hudson-Doyle on 2022-11-28

add integration test

[Michael Hudson-Doyle (mwhudson)]

Good point, added.

[Dan Bungert (dbungert)]

review: Approve

diff --git a/curtin/commands/block_meta_v2.py b/curtin/commands/block_meta_v2.py
index 348bf89..3c6e1a5 100644
--- a/curtin/commands/block_meta_v2.py
+++ b/curtin/commands/block_meta_v2.py
@@ -227,8 +227,14 @@ class GPTPartTable(SFDiskPartTable):
 
     def _headers(self):
         r = []
-        if self.first_lba is not None:
-            r.extend(['first-lba: ' + str(self.first_lba)])
+        first_lba = self.first_lba
+        if first_lba is None:
+            min_start = min(
+                [entry.start for entry in self.entries], default=2048)
+            if min_start < 2048:
+                first_lba = min_start
+        if first_lba is not None:
+            r.extend(['first-lba: ' + str(first_lba)])
         if self.last_lba is not None:
             r.extend(['last-lba: ' + str(self.last_lba)])
         if self.table_length is not None:
diff --git a/tests/integration/test_block_meta.py b/tests/integration/test_block_meta.py
index 5f7434f..ff3877a 100644
--- a/tests/integration/test_block_meta.py
+++ b/tests/integration/test_block_meta.py
@@ -350,6 +350,27 @@ class TestBlockMeta(IntegrationTestCase):
     def test_specified_offsets_msdos_v2(self):
         self._test_specified_offsets('msdos', 2)
 
+    def test_minimum_gpt_offset_v2(self):
+        # The default first-lba for a GPT is 2048 (i.e. one megabyte
+        # into the disk) but it is possible to create a GPT with a
+        # value as low as 34 and curtin shouldn't get in the way of
+        # you doing that.
+        psize = 20 << 20
+        offset = 34*512
+        img = self.tmp_path('image.img')
+        config = StorageConfigBuilder(version=2)
+        config.add_image(path=img, size='100M', ptable='gpt')
+        config.add_part(size=psize, number=1, offset=offset)
+        self.run_bm(config.render())
+
+        with loop_dev(img) as dev:
+            self.assertEqual(
+                summarize_partitions(dev), [
+                    PartData(number=1, offset=offset, size=psize),
+                ])
+        config.set_preserve()
+        self.run_bm(config.render())
+
     def _test_non_default_numbering(self, ptable, version):
         psize = 40 << 20
         img = self.tmp_path('image.img')