Fleet agent policy host name format field

[dimuon]

Closes #1312

[Copilot]

Pull request overview

This PR implements support for the host_name_format field in Fleet agent policies, allowing users to control whether the host.name field uses short hostnames or fully qualified domain names (FQDNs). The implementation leverages the underlying AgentFeatures API, specifically the "fqdn" feature flag.

Key Changes:

• Added host_name_format schema attribute with validation and default value
• Implemented conversion between host_name_format field and AgentFeatures API representation
• Enhanced update logic to preserve existing agent features while managing the hostname format setting

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/fleet/agent_policy/schema.go	Adds host_name_format field definition with validators and default value
internal/fleet/agent_policy/models.go	Implements conversion logic between host_name_format and AgentFeatures, including merge logic
internal/fleet/agent_policy/models_test.go	Unit tests for conversion and merge functions
internal/fleet/agent_policy/update.go	Fetches existing policy features before update to preserve non-hostname features
internal/fleet/agent_policy/version_test.go	Updates test calls to match new toAPIUpdateModel signature
internal/fleet/agent_policy/acc_test.go	Acceptance tests covering create, update, and removal scenarios
internal/fleet/agent_policy/testdata/.../main.tf	Test configuration files for acceptance tests
internal/fleet/agent_policy/testdata/.../variables.tf	Test variable definitions for acceptance tests
examples/resources/elasticstack_fleet_agent_policy/resource.tf	Documentation example showing usage
Makefile	Makes RERUN_FAILS configurable

[Copilot]

The type alias apiAgentFeature should be a named type rather than a type alias. Using type apiAgentFeature = struct creates an alias which may cause confusion when comparing types. Change to type apiAgentFeature struct to define a proper named type.

Suggested change

	type apiAgentFeature = struct {
	type apiAgentFeature struct {

[Copilot]

This introduces an additional API call during every update operation to fetch the current policy. Consider documenting why this extra round-trip is necessary (to preserve existing agent features) and whether this could impact update performance for users with many policies.

Suggested change

	// Read current policy to get existing AgentFeatures (so we can preserve other features)
	// Read current policy to get existing AgentFeatures (so we can preserve other features).
	//
	// NOTE: This introduces an additional API call during every update operation.
	// This is necessary to fetch the current agent policy so that we can preserve agent features
	// that are not managed by Terraform. Without this, updating the policy would overwrite
	// any features set outside of Terraform, potentially breaking existing agent functionality.
	//
	// Be aware: For users with many policies, this extra round-trip may impact update performance.
	// However, it is required to ensure that unmanaged agent features are not lost during updates.

[Copilot]

The capacity allocation len(existing)+1 will always over-allocate when replacing an existing feature. Consider using len(existing) as the capacity and only growing if appending a new feature.

Suggested change

	result := make([]apiAgentFeature, 0, len(existing)+1)
	found := false
	for _, f := range existing {
	if f.Name == newFeature.Name {
	result = append(result, *newFeature)
	found = true
	// Pre-scan to determine if we are replacing or appending
	found := false
	for _, f := range existing {
	if f.Name == newFeature.Name {
	found = true
	break
	}
	}
	// Allocate with correct capacity: len(existing) if replacing, len(existing)+1 if appending
	capacity := len(existing)
	if !found {
	capacity++
	}
	result := make([]apiAgentFeature, 0, capacity)
	for _, f := range existing {
	if f.Name == newFeature.Name {
	result = append(result, *newFeature)

[tobio]

LGTM, couple of tiny nits.

[tobio]

nit:

Suggested change

	stringvalidator.OneOf("hostname", "fqdn"),
	stringvalidator.OneOf(HostNameFormatHostname, HostNameFormatFQDN),

[tobio]

Suggested change

	Default: stringdefault.StaticString("hostname"),
	Default: stringdefault.StaticString(HostNameFormatHostname),