[CCV-4000] [datadog_cost_budget] Fix update creating duplicate instead of updating existing

[zeinageb]

What

Fix datadog_cost_budget resource Update operation creating duplicate budgets instead of updating existing ones

Why

When users run terraform apply to update an existing budget, the provider was creating a new budget with a different ID instead of modifying the existing one. This issue was reported by Indeed customer

Root Cause

The Update() function in resource_datadog_cost_budget.go was only reading from req.Plan, which contains the user's desired changes from their .tf file. However, Terraform doesn't include the resource ID in the plan (since users don't specify it in their config, it's marked as Optional: true in the schema)

The ID is only available in req.State (from terraform.tfstate file). without the ID, the UpsertBudget API call creates a new resource instead of updating the existing one.

The Fix

Added code to:

1. Read the current state to get the existing ID
2. Copy the ID from state into the plan
3. Build the API request with the ID included

Testing

!! Please read the testing process below to understand how this was verified!!

before anything, wanted to test if problem from API or Terraform so:
Tested the API directly with curl to confirm it handles updates correctly when ID is provided:

curl -X PUT -H "DD-API-KEY: $DD_API_KEY" -H "DD-APPLICATION-KEY: $DD_APP_KEY" \
  -H "Content-Type: application/vnd.api+json" \
  https://api.datadoghq.com/api/v2/cost/budget \
  -d '{"data":{"id":"...","type":"budget","attributes":{...}}}'

Returns same ID (API works correctly)This confirmed the bug was in the Terraform provider, not the API!

Test 1: Verify Update Fix (Create -> Update Workflow)

This test confirms the update bug is fixed

Commands:
cd terraform_examples

Step 1: Create main.tf

cat > main.tf << 'EOF'
terraform {
  required_providers {
    datadog = { source = "datadog/datadog" }
  }
}
provider "datadog" {}

resource "datadog_cost_budget" "test" {
  name          = "Terraform Test Budget Zeina"
  metrics_query = "sum:aws.cost.amortized{*} by {service}"
  start_month   = 202501
  end_month     = 202503
  entries {
    month  = 202501
    amount = 100
    tag_filters {
      tag_key   = "service"
      tag_value = "ec2"
    }
  }
  entries {
    month  = 202502
    amount = 100
    tag_filters {
      tag_key   = "service"
      tag_value = "ec2"
    }
  }
  entries {
    month  = 202503
    amount = 100
    tag_filters {
      tag_key   = "service"
      tag_value = "ec2"
    }
  }
}
EOF

Step 2: Create budget

terraform init
terraform apply -auto-approve
# Created with ID: ...

Step 3: Modify main.tf (change name and amounts to 150)

... edit file ...

Step 4: Update budget

terraform apply -auto-approve

ID stayed same: so fix is verified

Result:

• Before fix: ID changed on update (new budget created)

• After fix: ID stays the same (existing budget updated)

Test 2: Check for Schema Discrepancies (Import -> Plan)

This test confirms no other discrepancies exist between UI-created budgets and Terraform schema

Commands:

Step 1: Import a UI-created budget (has tag filters, multiple entries so we check if problem with tags)

cat > import_test.tf << 'EOF'
terraform {
  required_providers {
    datadog = { source = "datadog/datadog" }
  }
}
provider "datadog" {}
resource "datadog_cost_budget" "ui_created" {}
EOF

terraform import datadog_cost_budget.ui_created ... (type its id)

Import successful

Step 2: See what was imported

terraform show > imported_budget.txt

Shows 12 monthly entries, each with tag_filters

Step 3: Copy imported config to import_test.tf

Step 4: Check for drift

terraform plan
Output: "No changes. Your infrastructure matches the configuration"

Result:

• Tag filters work correctly (nested in entries as expected)
• No schema mismatches between UI and Terraform
• No drift detected