service_splitter: nil pointer dereference when RequestHeaders and ResponseHeaders are null

[charlie-collard]

The consul_config_entry_service_splitter resource crashes with a nil pointer dereference when the service splitter is updated outside of Terraform (e.g., via Consul CLI/API) in a way that removes the RequestHeaders and ResponseHeaders fields.

These fields are set to {} on creation by the provider, but they are optional and may be null if the resource is modified by non-Terraform sources.

I have reproduced this on the latest version v2.23.0

Reproduction

Terraform config

terraform {
  required_providers {
    consul = {
      source  = "hashicorp/consul"
      version = "2.23.0"
    }
  }
}

provider "consul" {
  address = "https://your-consul-server.example.com"
}

resource "consul_config_entry_service_defaults" "main" {
  name     = "bug-repro"
  protocol = "http"
  expose { checks = false }
}

resource "consul_config_entry_service_defaults" "canary" {
  name     = "bug-repro-canary"
  protocol = "http"
  expose { checks = false }
}

resource "consul_config_entry_service_splitter" "test" {
  name = "bug-repro"

  splits {
    weight  = 100
    service = "bug-repro"
  }

  splits {
    weight  = 0
    service = "bug-repro-canary"
  }

  depends_on = [
    consul_config_entry_service_defaults.main,
    consul_config_entry_service_defaults.canary
  ]
}

Steps to reproduce

# Create splitter via Terraform
terraform apply -auto-approve

# Verify RequestHeaders and ResponseHeaders are non-null in Consul
consul config read -kind service-splitter -name bug-repro

# Update via Consul CLI (common for adjusting canary weights)
consul config write - <<EOF
Kind = "service-splitter"
Name = "bug-repro"
Splits = [
  { Weight = 90, Service = "bug-repro" },
  { Weight = 10, Service = "bug-repro-canary" }
]
EOF

# Verify RequestHeaders and ResponseHeaders are now null in Consul
consul config read -kind service-splitter -name bug-repro

# Trigger refresh - crashes
terraform plan

Expected Behavior

Terraform should handle null header fields gracefully during refresh.

Actual Behavior

Provider crashes with nil pointer dereference:

  Stack trace from the terraform-provider-consul_v2.23.0_x4 plugin:
11:16:16   
11:16:16   panic: runtime error: invalid memory address or nil pointer dereference
11:16:16   [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb4d4a4]
11:16:16   
11:16:16   goroutine 1822 [running]:
11:16:16   github.com/hashicorp/terraform-provider-consul/consul.(*serviceSplitter).Write.func1(0x0, {0xee9ddb, 0xf})
11:16:16     github.com/hashicorp/terraform-provider-consul/consul/resource_consul_config_entry_service_splitter.go:239 +0x144
11:16:16   github.com/hashicorp/terraform-provider-consul/consul.(*serviceSplitter).Write(0x40001a2a10?, {0x11d9e50?, 0x40001a2e70?}, 0x40001a2a10, 0x40009d2f60)
11:16:16     github.com/hashicorp/terraform-provider-consul/consul/resource_consul_config_entry_service_splitter.go:263 +0x4a8
11:16:16   github.com/hashicorp/terraform-provider-consul/consul.resourceFromConfigEntryImplementation.configEntryImplementationRead.func4(0x40001a2a10, {0xce79c0?, 0x40004b60e0?})
11:16:16     github.com/hashicorp/terraform-provider-consul/consul/resource_consul_config_entry_concrete.go:111 +0x110
11:16:16   github.com/hashicorp/terraform-plugin-sdk/helper/schema.(*Resource).RefreshWithoutUpgrade(0x4000452320, 0x4000a38500, {0xce79c0, 0x40004b60e0})
11:16:16     github.com/hashicorp/terraform-plugin-sdk@v1.17.2/helper/schema/resource.go:470 +0x224
11:16:16   github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin.(*GRPCProviderServer).ReadResource(0x400007a200, {0x40005943c0?, 0x4000994a38?}, 0x40005943c0)
11:16:16     github.com/hashicorp/terraform-plugin-sdk@v1.17.2/internal/helper/plugin/grpc_provider.go:535 +0x2b4
11:16:16   github.com/hashicorp/terraform-plugin-sdk/internal/tfplugin5._Provider_ReadResource_Handler({0xe90120, 0x400007a200}, {0x11d5158, 0x4000a033e0}, 0x40001a24d0, 0x0)
11:16:16     github.com/hashicorp/terraform-plugin-sdk@v1.17.2/internal/tfplugin5/tfplugin5.pb.go:3269 +0x1b8
11:16:16   google.golang.org/grpc.(*Server).processUnaryRPC(0x4000194b40, {0x11dc2c0, 0x400045c9c0}, 0x4000a33200, 0x4000459470, 0x1b98c90, 0x0)
11:16:16     google.golang.org/grpc@v1.56.3/server.go:1335 +0xaf4
11:16:16   google.golang.org/grpc.(*Server).handleStream(0x4000194b40, {0x11dc2c0, 0x400045c9c0}, 0x4000a33200, 0x0)
11:16:16     google.golang.org/grpc@v1.56.3/server.go:1712 +0x720
11:16:16   google.golang.org/grpc.(*Server).serveStreams.func1.1()
11:16:16     google.golang.org/grpc@v1.56.3/server.go:947 +0xa0
11:16:16   created by google.golang.org/grpc.(*Server).serveStreams.func1 in goroutine 26
11:16:16     google.golang.org/grpc@v1.56.3/server.go:958 +0x12c
11:16:16   
11:16:16   Error: The terraform-provider-consul_v2.23.0_x4 plugin crashed!