Fix nil pointer dereference in pdbminavailable check (#1041) (#1043)

The pdbminavailable check crashed with a panic when it encountered a
PodDisruptionBudget that has spec.minAvailable defined but is missing
the required spec.selector field.

The code attempted to access pdb.Spec.Selector.MatchLabels and
pdb.Spec.Selector.MatchExpressions without checking if Selector was nil,
causing a nil pointer dereference.

This fix adds a nil check for pdb.Spec.Selector before accessing its
fields. If the selector is missing, the check now returns a clear
diagnostic message with a link to Kubernetes documentation instead of
panicking.

Also added a unit test TestPDBWithMinAvailableButNoSelector to verify
the fix handles this edge case correctly.

Fixes #1041

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: janisz

pkg/templates/pdbminavailable/template.go

@@ -76,6 +76,15 @@ func minAvailableCheck(lintCtx lintcontext.LintContext, object lintcontext.Objec
 		}
 	}
 
+	// Check if selector is present - it's a required field for PDB
+	if pdb.Spec.Selector == nil {
+		return []diagnostic.Diagnostic{
+			{
+				Message: "PDB is missing required selector field: https://kubernetes.io/docs/tasks/run-application/configure-pdb/#specifying-a-poddisruptionbudget",
+			},
+		}
+	}
+
 	// Build the label selector for the PDB to use for comparison
 	labelSelector, err := metaV1.LabelSelectorAsSelector(&metaV1.LabelSelector{
 		MatchLabels:      pdb.Spec.Selector.MatchLabels,

pkg/templates/pdbminavailable/template_test.go

@@ -296,3 +296,25 @@ func (p *PDBTestSuite) TestPDBWithMinAvailableAndKedaScaledObjectHasMinReplicas(
 		},
 	})
 }
+
+// test that the check handles PDB with minAvailable but no selector (should not panic)
+func (p *PDBTestSuite) TestPDBWithMinAvailableButNoSelector() {
+	p.ctx.AddMockPodDisruptionBudget(p.T(), "test-pdb")
+	p.ctx.ModifyPodDisruptionBudget(p.T(), "test-pdb", func(pdb *v1.PodDisruptionBudget) {
+		pdb.Namespace = "test"
+		pdb.Spec.Selector = nil // Missing required selector field
+		pdb.Spec.MinAvailable = &intstr.IntOrString{StrVal: "40%", Type: intstr.String}
+	})
+
+	p.Validate(p.ctx, []templates.TestCase{
+		{
+			Param: params.Params{},
+			Diagnostics: map[string][]diagnostic.Diagnostic{
+				"test-pdb": {
+					{Message: "PDB is missing required selector field: https://kubernetes.io/docs/tasks/run-application/configure-pdb/#specifying-a-poddisruptionbudget"},
+				},
+			},
+			ExpectInstantiationError: false,
+		},
+	})
+}