Annotated the Update function in Kubernetes Store.go to explain how version conflicts are handled.

store.go

const (
	OptimisticLockErrorMsg        = "the object has been modified; please apply your changes to the latest version and try again"
	resourceCountPollPeriodJitter = 1.2
)

// Update performs an atomic update and set of the object. Returns the result of the update
// or an error. If the registry allows create-on-update, the create flow will be executed.
// A bool is returned along with the object and any errors, to indicate object creation.
func (e *Store) Update(ctx context.Context, name string, objInfo rest.UpdatedObjectInfo, createValidation rest.ValidateObjectFunc, updateValidation rest.ValidateObjectUpdateFunc, forceAllowCreate bool, options *metav1.UpdateOptions) (runtime.Object, bool, error) {
	key, err := e.KeyFunc(ctx, name)
	if err != nil {
		return nil, false, err
	}

	var (
		creatingObj runtime.Object
		creating    = false
	)

	qualifiedResource := e.qualifiedResourceFromContext(ctx)
	storagePreconditions := &storage.Preconditions{}
	if preconditions := objInfo.Preconditions(); preconditions != nil {
		storagePreconditions.UID = preconditions.UID
		storagePreconditions.ResourceVersion = preconditions.ResourceVersion
	}

	out := e.NewFunc()
	// deleteObj is only used in case a deletion is carried out
	var deleteObj runtime.Object
	err = e.Storage.GuaranteedUpdate(ctx, key, out, true, storagePreconditions, func(existing runtime.Object, res storage.ResponseMeta) (runtime.Object, *uint64, error) {
		// 下面的逻辑都在一个函数变量 tryUpdate 里，作为变量传递给 GuaranteedUpdate 函数
		// GuaranteedUpdate 会根据 key 在 etcd 中查找到最新版本的 obj（existing 变量），然后作为函数变量传递给 tryUpdate 函数，也就是当前函数

		// 获取当前资源存储在 etcd 中的 mod_revision，直接调用函数返回的，没有从 etcd 中再次获取
		existingResourceVersion, err := e.Storage.Versioner().ObjectResourceVersion(existing)
		if err != nil {
			return nil, nil, err
		}
		if existingResourceVersion == 0 {
			if !e.UpdateStrategy.AllowCreateOnUpdate() && !forceAllowCreate {
				return nil, nil, apierrors.NewNotFound(qualifiedResource, name)
			}
		}

		// Given the existing object, get the new object
		// objInfo 是用户传递的更新对象
		// UpdatedObject 函数需要传递一个 context 和一个 oldObj，其中 oldObj 是当前资源存储在 etcd 中的最新版本
		// 返回的是一个新的 obj，和一个 error
		// 其实可以理解为是一个 merge 函数，将 oldObj 和 objInfo 中的 obj 合并为一个新的 obj
		obj, err := objInfo.UpdatedObject(ctx, existing)
		if err != nil {
			return nil, nil, err
		}

		// If AllowUnconditionalUpdate() is true and the object specified by
		// the user does not have a resource version, then we populate it with
		// the latest version. Else, we check that the version specified by
		// the user matches the version of latest storage object.
		// obj 是 UpdatedObject 函数返回的新的资源对象
		// 它的 ResourceVersion 会使用我们传递 objInfo 中的版本号
		newResourceVersion, err := e.Storage.Versioner().ObjectResourceVersion(obj)
		if err != nil {
			return nil, nil, err
		}

		doUnconditionalUpdate := newResourceVersion == 0 && e.UpdateStrategy.AllowUnconditionalUpdate()

		if existingResourceVersion == 0 {
			// Init metadata as early as possible.
			if objectMeta, err := meta.Accessor(obj); err != nil {
				return nil, nil, err
			} else {
				rest.FillObjectMetaSystemFields(objectMeta)
			}

			var finishCreate FinishFunc = finishNothing

			if e.BeginCreate != nil {
				fn, err := e.BeginCreate(ctx, obj, newCreateOptionsFromUpdateOptions(options))
				if err != nil {
					return nil, nil, err
				}
				finishCreate = fn
				defer func() {
					finishCreate(ctx, false)
				}()
			}

			creating = true
			creatingObj = obj
			if err := rest.BeforeCreate(e.CreateStrategy, ctx, obj); err != nil {
				return nil, nil, err
			}
			// at this point we have a fully formed object.  It is time to call the validators that the apiserver
			// handling chain wants to enforce.
			if createValidation != nil {
				if err := createValidation(ctx, obj.DeepCopyObject()); err != nil {
					return nil, nil, err
				}
			}
			ttl, err := e.calculateTTL(obj, 0, false)
			if err != nil {
				return nil, nil, err
			}

			// The operation has succeeded.  Call the finish function if there is one,
			// and then make sure the defer doesn't call it again.
			fn := finishCreate
			finishCreate = finishNothing
			fn(ctx, true)

			return obj, &ttl, nil
		}

		creating = false
		creatingObj = nil
		if doUnconditionalUpdate {
			// Update the object's resource version to match the latest
			// storage object's resource version.
			err = e.Storage.Versioner().UpdateObject(obj, res.ResourceVersion)
			if err != nil {
				return nil, nil, err
			}
		} else {
			// Check if the object's resource version matches the latest
			// resource version.
			if newResourceVersion == 0 {
				// TODO: The Invalid error should have a field for Resource.
				// After that field is added, we should fill the Resource and
				// leave the Kind field empty. See the discussion in #18526.
				qualifiedKind := schema.GroupKind{Group: qualifiedResource.Group, Kind: qualifiedResource.Resource}
				fieldErrList := field.ErrorList{field.Invalid(field.NewPath("metadata").Child("resourceVersion"), newResourceVersion, "must be specified for an update")}
				return nil, nil, apierrors.NewInvalid(qualifiedKind, name, fieldErrList)
			}
			// 如果资源版本不匹配，则返回冲突错误
			if newResourceVersion != existingResourceVersion {
				return nil, nil, apierrors.NewConflict(qualifiedResource, name, fmt.Errorf(OptimisticLockErrorMsg))
			}
		}

		var finishUpdate FinishFunc = finishNothing

		if e.BeginUpdate != nil {
			fn, err := e.BeginUpdate(ctx, obj, existing, options)
			if err != nil {
				return nil, nil, err
			}
			finishUpdate = fn
			defer func() {
				finishUpdate(ctx, false)
			}()
		}

		if err := rest.BeforeUpdate(e.UpdateStrategy, ctx, obj, existing); err != nil {
			return nil, nil, err
		}

		// Ignore changes that only affect managed fields timestamps.
		// FieldManager can't know about changes like normalized fields, defaulted
		// fields and other mutations.
		obj, err = fieldmanager.IgnoreManagedFieldsTimestampsTransformer(ctx, obj, existing)
		if err != nil {
			return nil, nil, err
		}

		// at this point we have a fully formed object.  It is time to call the validators that the apiserver
		// handling chain wants to enforce.
		if updateValidation != nil {
			if err := updateValidation(ctx, obj.DeepCopyObject(), existing.DeepCopyObject()); err != nil {
				return nil, nil, err
			}
		}
		// Check the default delete-during-update conditions, and store-specific conditions if provided
		if ShouldDeleteDuringUpdate(ctx, key, obj, existing) &&
			(e.ShouldDeleteDuringUpdate == nil || e.ShouldDeleteDuringUpdate(ctx, key, obj, existing)) {
			deleteObj = obj
			return nil, nil, errEmptiedFinalizers
		}
		ttl, err := e.calculateTTL(obj, res.TTL, true)
		if err != nil {
			return nil, nil, err
		}

		// The operation has succeeded.  Call the finish function if there is one,
		// and then make sure the defer doesn't call it again.
		fn := finishUpdate
		finishUpdate = finishNothing
		fn(ctx, true)

		if int64(ttl) != res.TTL {
			return obj, &ttl, nil
		}
		return obj, nil, nil
	}, dryrun.IsDryRun(options.DryRun), nil)

	if err != nil {
		// delete the object
		if err == errEmptiedFinalizers {
			return e.deleteWithoutFinalizers(ctx, name, key, deleteObj, storagePreconditions, newDeleteOptionsFromUpdateOptions(options))
		}
		if creating {
			err = storeerr.InterpretCreateError(err, qualifiedResource, name)
			err = rest.CheckGeneratedNameError(ctx, e.CreateStrategy, err, creatingObj)
		} else {
			err = storeerr.InterpretUpdateError(err, qualifiedResource, name)
		}
		return nil, false, err
	}

	if creating {
		if e.AfterCreate != nil {
			e.AfterCreate(out, newCreateOptionsFromUpdateOptions(options))
		}
	} else {
		if e.AfterUpdate != nil {
			e.AfterUpdate(out, options)
		}
	}
	if e.Decorator != nil {
		e.Decorator(out)
	}
	return out, creating, nil
}

SourceCode: registry/store.go