Fix Azure pricesheet billing period fetch for MCA accounts

[Copilot]

The hardcoded yyyyMM billing period format in getDownloadURL only works for EA accounts. MCA accounts use yyyyMM-1 (e.g., 202411-1), causing the Azure API to return a 404 for those users.

Changes

• pricesheetclient.go: Add GetCurrentBillingPeriod method that calls the Azure Billing Periods List API (/providers/Microsoft.Billing/billingAccounts/{id}/billingPeriods?$top=1) to retrieve the most recent billing period name, handling both EA and MCA name formats dynamically.

• pricesheetdownloader.go: Update getDownloadURL to call GetCurrentBillingPeriod instead of the hardcoded formatter. Falls back to the original yyyyMM format if the API call fails, preserving backward compatibility.

• pricesheetclient_test.go: Add validation test for GetCurrentBillingPeriod with empty billing account ID.

// Before: hardcoded, EA-only
poller, err := client.BeginDownloadByBillingPeriod(ctx, currentBillingPeriod()) // "202411"

// After: dynamically fetched, works for EA ("202411") and MCA ("202411-1")
billingPeriod, err := client.GetCurrentBillingPeriod(ctx)
if err != nil {
    log.Warnf("failed to fetch billing period from API, falling back to default format: %s", err)
    billingPeriod = currentBillingPeriod()
}

Original prompt

Problem

GitHub Issue: opencost#2996

The currentBillingPeriod() function in pkg/cloud/azure/pricesheetdownloader.go (line 263) hardcodes the billing period format as yyyyMM (e.g., 202411):

func currentBillingPeriod() string {
	return time.Now().Format("200601")
}

This works for Enterprise Agreement (EA) billing accounts, but Microsoft Customer Agreement (MCA) billing accounts use billing period names with a different format: yyyyMM-1 (e.g., 202411-1). When the hardcoded yyyyMM format is used against an MCA billing account, the Azure API returns:

RESPONSE 404: 404 Not Found
ERROR CODE: 404
{
  "error": {
    "code": "404",
    "message": "Invalid billing period 202411"
  }
}

The user confirmed that their billing periods (listed via az billing period list) are formatted as 202502-1, 202501-1, 202412-1, etc.

Required Fix

Modify getDownloadURL in pkg/cloud/azure/pricesheetdownloader.go to dynamically fetch the current billing period name from the Azure Billing Periods List API instead of using a hardcoded format. Here is the approach:

1. Add a GetCurrentBillingPeriod method to PriceSheetClient in pkg/cloud/azure/pricesheetclient.go

Add these new types and the method after the existing downloadByBillingPeriodCreateRequest function:

// billingPeriodsListTemplate is the URL template for listing billing periods.
const billingPeriodsListTemplate = "/providers/Microsoft.Billing/billingAccounts/%s/billingPeriods"

// billingPeriodsListResponse represents the response from the billing periods list API.
type billingPeriodsListResponse struct {
	Value []billingPeriodEntry `json:"value"`
}

// billingPeriodEntry represents a single billing period entry.
type billingPeriodEntry struct {
	Name string `json:"name"`
}

// GetCurrentBillingPeriod fetches the most recent billing period name from the
// Azure Billing Periods List API. This handles both EA accounts (which use
// "yyyyMM" format) and MCA accounts (which use "yyyyMM-1" format).
// See: https://learn.microsoft.com/en-us/rest/api/billing/billing-periods/list
func (client *PriceSheetClient) GetCurrentBillingPeriod(ctx context.Context) (string, error) {
	if client.billingAccountID == "" {
		return "", errors.New("parameter client.billingAccountID cannot be empty")
	}
	urlPath := fmt.Sprintf(billingPeriodsListTemplate, url.PathEscape(client.billingAccountID))
	req, err := runtime.NewRequest(ctx, http.MethodGet, runtime.JoinPaths(client.host, urlPath))
	if err != nil {
		return "", fmt.Errorf("creating billing periods list request: %w", err)
	}
	reqQP := req.Raw().URL.Query()
	reqQP.Set("api-version", "2020-05-01")
	reqQP.Set("$top", "1")
	req.Raw().URL.RawQuery = reqQP.Encode()
	req.Raw().Header["Accept"] = []string{"application/json"}

	resp, err := client.pl.Do(req)
	if err != nil {
		return "", fmt.Errorf("executing billing periods list request: %w", err)
	}
	if !runtime.HasStatusCode(resp, http.StatusOK) {
		return "", runtime.NewResponseError(resp)
	}

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return "", fmt.Errorf("reading billing periods response body: %w", err)
	}
	defer resp.Body.Close()

	var result billingPeriodsListResponse
	if err := json.Unmarshal(body, &result); err != nil {
		return "", fmt.Errorf("parsing billing periods response: %w", err)
	}

	if len(result.Value) == 0 {
		return "", errors.New("no billing periods returned from API")
	}

	return result.Value[0].Name, nil
}

You will also need to add "encoding/json" and "io" to the imports in pricesheetclient.go.

2. Update getDownloadURL in pkg/cloud/azure/pricesheetdownloader.go

Replace the existing getDownloadURL method (lines 53-73) with:

func (d *PriceSheetDownloader) getDownloadURL(ctx context.Context) (string, error) {
	cred, err := azidentity.NewClientSecretCredential(d.TenantID, d.ClientID, d.ClientSecret, nil)
	if err != nil {
		return "", fmt.Errorf("creating credential: %w", err)
	}
	client, err := NewPriceSheetClient(d.BillingAccount, cred, nil)
	if err != nil {
		return "", fmt.Errorf("creating pricesheet client: %w", err)
	}

	// Dynamically fetch the current billing period name from the API.
	// This handles both EA accounts (yyyyMM) and MCA accounts (yyyyMM-1).
	billingPeriod, err := client.GetCurrentBillingPeriod(ctx)
	if err != nil {
		// Fall back to the hardcoded format for backwards compatibility.
		log.Warnf("failed to fetch billing period from API, falling back to default format: %s", err)
		billingPeriod = currentBillingPeriod()
	}
	log.Infof("using billing period %q", billingPeriod)

	poller, err := client.BeginDownloadByBillingPeriod(ctx, billingPeriod)
	if err != nil {
		return "", fmt.Errorf("beginning pricesheet download: %w", err)
	}
	resp, err := poller.PollUntilDone(ctx, &runtime.PollUntilDoneOptions{
		Frequency: 30 * time.Second,
	})
	if err != nil {
		return "", fmt.Errorf("polling for pricesheet: %w", er...

</details>



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