db: backfill zero-time node expiry to NULL

Versions before 0.28 persisted a pointer to a zero time.Time as
'0001-01-01 00:00:00+00:00' in nodes.expiry rather than NULL. 0.29
reports those rows as expired. #3197 and #3199 stopped new writes;
this normalises the existing rows so the column once again means
"no expiry" when unset.

Fixes: #3284

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hscontrol/db/db_test.go

@@ -144,6 +144,60 @@ func TestSQLiteMigrationAndDataValidation(t *testing.T) {
 				assert.NotContains(t, node7.Tags, "tag:forbidden", "node7 should NOT have tag:forbidden (unauthorized)")
 			},
 		},
+		// Test for the zero-time node expiry migration
+		// (202605221435-clear-zero-time-node-expiry). Pre-0.28 versions
+		// stored a zero time.Time as '0001-01-01 00:00:00+00:00' rather
+		// than NULL, which caused 0.29 to report those nodes as expired.
+		// Fixes: https://github.com/juanfont/headscale/issues/3284
+		{
+			dbPath: "testdata/sqlite/zero_time_expiry_migration_test.sql",
+			wantFunc: func(t *testing.T, hsdb *HSDatabase) {
+				t.Helper()
+
+				nodes, err := Read(hsdb.DB, func(rx *gorm.DB) (types.Nodes, error) {
+					return ListNodes(rx)
+				})
+				require.NoError(t, err)
+				require.Len(t, nodes, 5, "should have all 5 nodes")
+
+				byHostname := make(map[string]*types.Node, len(nodes))
+				for _, n := range nodes {
+					byHostname[n.Hostname] = n
+				}
+
+				// Node 1 had a zero-time expiry; should be cleared.
+				node1 := byHostname["node1"]
+				require.NotNil(t, node1, "node1 should exist")
+				assert.Nil(t, node1.Expiry, "node1 zero-time expiry should be cleared to NULL")
+				assert.False(t, node1.IsExpired(), "node1 should not be reported as expired")
+
+				// Node 2 already had NULL expiry; should still be NULL.
+				node2 := byHostname["node2"]
+				require.NotNil(t, node2, "node2 should exist")
+				assert.Nil(t, node2.Expiry, "node2 NULL expiry should be preserved")
+				assert.False(t, node2.IsExpired(), "node2 should not be reported as expired")
+
+				// Node 3 had a real future expiry; should be preserved.
+				node3 := byHostname["node3"]
+				require.NotNil(t, node3, "node3 should exist")
+				require.NotNil(t, node3.Expiry, "node3 future expiry should be preserved")
+				assert.Equal(t, 2099, node3.Expiry.UTC().Year(), "node3 expiry year should be 2099")
+				assert.False(t, node3.IsExpired(), "node3 with future expiry should not be expired")
+
+				// Node 4 had a real past expiry; should be preserved.
+				node4 := byHostname["node4"]
+				require.NotNil(t, node4, "node4 should exist")
+				require.NotNil(t, node4.Expiry, "node4 past expiry should be preserved")
+				assert.Equal(t, 2020, node4.Expiry.UTC().Year(), "node4 expiry year should be 2020")
+				assert.True(t, node4.IsExpired(), "node4 with past expiry should still be expired")
+
+				// Node 5 also had a zero-time expiry; should be cleared.
+				node5 := byHostname["node5"]
+				require.NotNil(t, node5, "node5 should exist")
+				assert.Nil(t, node5.Expiry, "node5 zero-time expiry should be cleared to NULL")
+				assert.False(t, node5.IsExpired(), "node5 should not be reported as expired")
+			},
+		},
 	}
 
 	for _, tt := range tests {