Content written to response writer shows erratic behaviour

File: main.go

import (
  "github.com/gorilla/mux"
  )
  
func setupRouting(pBus *bus.Bus) *mux.Router {

	log.Debug("Setting up routing")

	r := mux.NewRouter()

	pInvoiceHandler := handlers.NewInvoiceHandler(pBus)

	postRouter.HandleFunc("/invoice", pInvoiceHandler.HandleInvoiceGenerationRequest)


	return r
}

main() {
	pRouter := setupRouting(pBus)
	
	s := &http.Server{
		Addr:         address,
		Handler:      pRouter,
		IdleTimeout:  time.Duration(idlTimeout) * time.Second,
		ReadTimeout:  time.Duration(rdTimeout) * time.Second,
		WriteTimeout: time.Duration(wrtTimeout) * time.Second,
	}
}

File: InvoiceGenerationHandler.go

func (ih *InvoiceHandler) HandleInvoiceGenerationRequest(rw http.ResponseWriter, pReq *http.Request) {

	log.Info("Handling invoice generation request")

	query := pReq.URL.Query()
	resourceGrp := query.Get("rGrp")

	if resourceGrp == "" {
		http.Error(rw, "Invalid URL, rGrp parameter should be present in the URL ", http.StatusBadRequest)
		return
	}

	pSliceCostDetailsCtx, err := ih.pApiLoadTesting.CalculateCost(resourceGrp)

	if err != nil {
		log.Error("Could not calculate cost :", err.Error())
		http.Error(rw, err.Error(), http.StatusInternalServerError)
		return
	}

	pResponseJson, err := json.Marshal(pSliceCostDetailsCtx)

	if err != nil {
		log.Error("Could not convert price context slice to json: ", err.Error())
		http.Error(rw, "Could not generate cost response", http.StatusInternalServerError)
		log.Error(pResponseJson)
		return
	}

	rw.Write(pResponseJson)
}

File: apiloadtestpricing.go

type ApiLoadTestPricing struct {
	mpHttpClient *http.Client
}

func NewApiLoadTesting() *ApiLoadTestPricing {
	var pHttpClient = &http.Client{
		Timeout: time.Second * 120,
	}
	return &ApiLoadTestPricing{pHttpClient}
}

func getBearerToken(pHttpClient *http.Client, microsoftTokenEndpoint string) (string, error) {

	v := url.Values{}
	v.Set("grant_type", "client_credentials")
	v.Set("client_id", "******************************")
	v.Set("client_secret", "**************************")

	v.Set("resource", "https://management.core.windows.net")

	params := strings.NewReader(v.Encode())

	req, err := http.NewRequest(http.MethodPost, microsoftTokenEndpoint, params)

	if err != nil {
		log.Error("Could not create a new request")
		log.Error("Encountered error: ", err.Error())
		return "", err
	}

	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Set("Host", "login.microsoftonline.com")

	resp, err := pHttpClient.Do(req)

	if err != nil {
		log.Error("Could not get token from microsoft token endpoint ", microsoftTokenEndpoint)
		log.Error("Encountered error: ", err.Error())
		return "", err
	}
	
	/* More Code here not shown */
}

func getCostFromAzureCostManagement(pHttpClient *http.Client, bearerToken string, resourcegroup string) (*[]models.CostDetailsContext, error) {

	log.Info("Getting Cost from Azure")

	url := "https://management.azure.com/subscriptions/6a877c8f-4ca3-4135-9a0c-8e92db2384b4/resourceGroups/" + resourcegroup + "/providers/Microsoft.CostManagement/query?api-version=2021-10-01"
	postBody := `{
			    "type": "Usage",
			    "timeframe": "MonthToDate",
			    "dataset": {
			      "granularity": "Daily",
			      "aggregation": {
			       "totalCost": {
			          "name": "PreTaxCost",
			          "function": "Sum"
			        }
			      },
			     "grouping": [
			        {
			         "type": "Dimension",
			          "name": "resourceGroup"
			        }
			      ]
			    }
			  }`

	req, err := http.NewRequest(http.MethodPost, url, strings.NewReader(postBody))
	if err != nil {
		panic(err)
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Authorization", bearerToken)

	resp, err := pHttpClient.Do(req)
	
	/* More Code here not shown */
}

func (pPrice *ApiLoadTestPricing) CalculateCost(rGrp string) (*[]models.CostDetailsContext, error) {

	log.Info("Calculating cost from azure for resource group: ", rGrp)

	token, err := getBearerToken(pPrice.mpHttpClient, "https://login.microsoftonline.com/7a9376d4-7c43-480f-82ba-a090647f651d/oauth2/token")

	if err != nil {
		log.Error("Cost calculation has failed because proper bearer token was not present")
		return nil, err
	}

	pSliceCostDetailsContext, err := getCostFromAzureCostManagement(pPrice.mpHttpClient, "Bearer "+token, rGrp)

	return pSliceCostDetailsContext, err
}

Problem Statement:

I have written a small API in golang which does two things:
1. Get Bearer token from azure active directory
2. Call the Microsoft cost management API to get the resource group cost

Issue:
Case 1:
When getBearerToken and getCostFromAzureCostManagement get called. The response written to the response writer in InvoiceGenerationHandler.go sometimes reach and sometimes doesn't to the client.

Case 2:
When only getBearerToken is called and the token string is returned it reaches the client on every write.

Note:

1. Both getBearerToken and getCostFromAzureCostManagement use the same http client for the
   request
2. gorilla mux is used for routing

What have I done ?

1. Tried commenting out the CalculateCost code - response writer works.
2. Tried commenting out getCostFromAzureCostManagement - responsewriter works
3. Tried with both of them uncommented - reponsewriter doesnt work
4. Read about the possible issue of shared httpclient and tried the follwing code
   https://github.com/hashicorp/go-cleanhttp - Responsewriter doesnt work.

I am stuck with this and have run out of ideas can someone please help me debug this ?

Try increasing the timeouts.

IdleTimeout:  60 * time.Second,
ReadTimeout:  60* time.Second,
WriteTimeout: 60* time.Second,