fix panic due to empty DestinationRule LoadBalancerPolicy

[Abirdcfly]

What type of PR is this?
/kind bug

What this PR does / why we need it:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x1d8f337]

goroutine 303 [running]:
k8s.io/apimachinery/pkg/util/runtime.HandleCrash({0x0, 0x0, 0x1?})
        /code/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:55 +0xd7
panic({0x20a2ca0, 0x3c91910})
        /usr/local/go/src/runtime/panic.go:884 +0x212
github.com/kubeedge/edgemesh/pkg/loadbalancer.(*LoadBalancer).OnDestinationRuleUpdate(0xc000293130, 0xc0000bdc00?, 0xc002ba2000)
        /code/pkg/loadbalancer/loadbalancer.go:778 +0x497
github.com/kubeedge/edgemesh/pkg/loadbalancer.(*LoadBalancer).handleUpdateDestinationRule(0xc000483040?, {0x23d50c0?, 0xc00021d4d8?}, {0x23d50c0?, 0xc002ba2000?})
        /code/pkg/loadbalancer/loadbalancer.go:376 +0x65
k8s.io/client-go/tools/cache.ResourceEventHandlerFuncs.OnUpdate(...)
        /code/vendor/k8s.io/client-go/tools/cache/controller.go:238
k8s.io/client-go/tools/cache.(*processorListener).run.func1()
        /code/vendor/k8s.io/client-go/tools/cache/shared_informer.go:785 +0xf7
k8s.io/apimachinery/pkg/util/wait.BackoffUntil.func1(0x0?)
        /code/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:155 +0x3e
k8s.io/apimachinery/pkg/util/wait.BackoffUntil(0xc00361ff38?, {0x29de7a0, 0xc0031fc1e0}, 0x1, 0xc0007372c0)
        /code/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:156 +0xb6
k8s.io/apimachinery/pkg/util/wait.JitterUntil(0x0?, 0x3b9aca00, 0x0, 0x0?, 0xc00361ff88?)
        /code/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:133 +0x89
k8s.io/apimachinery/pkg/util/wait.Until(...)
        /code/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:90
k8s.io/client-go/tools/cache.(*processorListener).run(0xc002ee6400?)
        /code/vendor/k8s.io/client-go/tools/cache/shared_informer.go:781 +0x6b
k8s.io/apimachinery/pkg/util/wait.(*Group).Start.func1()
        /code/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:73 +0x5a
created by k8s.io/apimachinery/pkg/util/wait.(*Group).Start
        /code/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:71 +0x85

due to DestinationRule has no LoadBalancerPolicy, note this DestinationRule may not relate with kubeedge or edge computing.

apiVersion: networking.istio.io/v1beta1
kind: DestinationRule
metadata:
  name: productpage
  namespace: default
spec:
  host: productpage
  subsets:
  - labels:
      version: v1
    name: v1

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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[gemini-code-assist]

Summary of Changes

Hello @Abirdcfly, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug that caused a runtime panic in the load balancer component when processing DestinationRule objects without an explicitly defined LoadBalancerPolicy. The fix introduces a safeguard to verify the existence of the policy in the internal map before attempting to update it, significantly enhancing the stability and robustness of the system. Additionally, minor formatting changes were applied to improve code readability.

Highlights

• Panic Prevention: Implemented a nil check in the OnDestinationRuleUpdate function to prevent a runtime panic. This panic occurred when a DestinationRule lacked a LoadBalancerPolicy, leading to an attempt to dereference a nil pointer during map access.
• Code Readability: Improved the readability of several function signatures (tryPickEndpoint, nextEndpointWithConn, TryConnectEndpoints) by reformatting their parameters to span multiple lines.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a panic that occurs when a DestinationRule without a LoadBalancerPolicy is updated. The added check prevents a nil pointer dereference, and the fix is effective. The other changes are stylistic formatting improvements.

While reviewing, I noticed a couple of areas for potential improvement in related code:

• There appears to be a similar potential for a panic in OnDestinationRuleDelete. If a DestinationRule without a policy is deleted, lb.policyMap[svcPort] could be nil, leading to a panic on lb.policyMap[svcPort].Release(). It would be beneficial to address this in a follow-up PR to make the DestinationRule handling more robust.
• The logic in OnDestinationRuleUpdate seems to recreate the load balancer policy object on every update (by calling setLoadBalancerPolicy unconditionally), even when the policy type hasn't changed. This can be inefficient, especially for ConsistentHashPolicy which rebuilds its hash ring. Refactoring this to only update the existing policy object when the type is the same would improve performance. This could also be addressed in a follow-up.

Overall, the change in this PR is a good and necessary fix.