Cosmos Dev Series: Cosmos-SDK-based Blockchain Upgrade - x/upgrade

module.go

func (am AppModule) BeginBlock(ctx sdk.Context, req abci.RequestBeginBlock) {
    BeginBlocker(am.keeper, ctx, req)
}

func BeginBlocker(k keeper.Keeper, ctx sdk.Context, _ abci.RequestBeginBlock) {
    plan, found := k.GetUpgradePlan(ctx)
    ...
    if !found {
        return
    }
    
    // Prepare shutdown if we don't have an upgrade handler for this upgrade name (meaning this software is out of date)
    if !k.HasHandler(plan.Name) {
    
        // Write the upgrade info to disk. The UpgradeStoreLoader uses this info to perform or skip
			  // store migrations.
        err := k.DumpUpgradeInfoWithInfoToDisk(ctx.BlockHeight(), plan.Name, plan.Info)
        if err != nil {
            panic(fmt.Errorf("unable to write upgrade info to filesystem: %s", err.Error()))
        }
        upgradeMsg := BuildUpgradeNeededMsg(plan)
        panic(upgradeMsg)
    }
    // To make sure clear upgrade is executed at the same block
    if plan.ShouldExecute(ctx) {
        ...
        k.ApplyUpgrade(ctx, plan)
        return
    }
}

// ApplyUpgrade will execute the handler associated with the Plan and mark the plan as done.
func (k Keeper) ApplyUpgrade(ctx sdk.Context, plan types.Plan) {
    handler := k.upgradeHandlers[plan.Name]
    if handler == nil {
        panic("ApplyUpgrade should never be called without first checking HasHandler")
    }
    updatedVM, err := handler(ctx, plan, k.GetModuleVersionMap(ctx))
    if err != nil {
        panic(err)
    }
    k.SetModuleVersionMap(ctx, updatedVM)
}