navidrome/plugins/cmd/hostgen

hostgen

A code generator for Navidrome's plugin host functions. It reads Go interface definitions with special annotations and generates Extism host function wrappers.

Usage

hostgen -input <dir> -output <dir> -package <name> [-v] [-dry-run]

Flags

Flag	Description	Default
-input	Directory containing Go source files with annotated interfaces	Required
-output	Directory where generated files will be written	Required
-package	Package name for generated files	Required
-v	Verbose output	false
-dry-run	Parse and validate without writing files	false

Example

go run ./plugins/cmd/hostgen \
  -input ./plugins/host \
  -output ./plugins/host \
  -package host

Or via go generate (recommended):

//go:generate go run ../cmd/hostgen -input . -output . -package host
package host

Annotations

//nd:hostservice

Marks an interface as a host service that will have wrappers generated.

//nd:hostservice name=<ServiceName> permission=<permission>
type MyService interface { ... }

Parameter	Description	Required
name	Service name used in generated type names and function prefixes	Yes
permission	Permission required by plugins to use this service	Yes

//nd:hostfunc

Marks a method within a host service interface for export to plugins.

//nd:hostfunc [name=<export_name>]
MethodName(ctx context.Context, ...) (result Type, err error)

Parameter	Description	Required
name	Custom export name (default: <servicename>_<methodname> in lowercase)	No

Input Format

Host service interfaces must follow these conventions:

1. First parameter must be context.Context - Required for all methods
2. Last return value should be error - For proper error handling
3. Annotations must be on consecutive lines - No blank comment lines between doc and annotation

Example Interface

package host

import "context"

// SubsonicAPIService provides access to Navidrome's Subsonic API.
// This documentation becomes part of the generated code.
//nd:hostservice name=SubsonicAPI permission=subsonicapi
type SubsonicAPIService interface {
    // Call executes a Subsonic API request and returns the response.
    //nd:hostfunc
    Call(ctx context.Context, uri string) (response string, err error)
}

Generated Output

For each annotated interface, hostgen generates:

Request/Response Types

// SubsonicAPICallRequest is the request type for SubsonicAPI.Call.
type SubsonicAPICallRequest struct {
    Uri string `json:"uri"`
}

// SubsonicAPICallResponse is the response type for SubsonicAPI.Call.
type SubsonicAPICallResponse struct {
    Response string `json:"response,omitempty"`
    Error    string `json:"error,omitempty"`
}

Registration Function

// RegisterSubsonicAPIHostFunctions registers SubsonicAPI service host functions.
func RegisterSubsonicAPIHostFunctions(service SubsonicAPIService) []extism.HostFunction {
    return []extism.HostFunction{
        newSubsonicAPICallHostFunction(service),
    }
}

Host Function Wrappers

Each method gets a wrapper that:

1. Reads JSON request from plugin memory
2. Unmarshals to the request type
3. Calls the service method
4. Marshals the response
5. Writes JSON response to plugin memory

Supported Types

hostgen supports these Go types in method signatures:

Type	JSON Representation
string, int, bool, etc.	Native JSON types
[]T (slices)	JSON arrays
map[K]V (maps)	JSON objects
*T (pointers)	Nullable fields
interface{} / any	Converts to any
Custom structs	JSON objects (must be JSON-serializable)

Multiple Return Values

Methods can return multiple values (plus error):

//nd:hostfunc
Search(ctx context.Context, query string) (results []string, total int, hasMore bool, err error)

Generates:

type ServiceSearchResponse struct {
    Results []string `json:"results,omitempty"`
    Total   int      `json:"total,omitempty"`
    HasMore bool     `json:"hasMore,omitempty"`
    Error   string   `json:"error,omitempty"`
}

Output Files

Generated files are named <servicename>_gen.go (lowercase). Each file includes:

• // Code generated by hostgen. DO NOT EDIT. header
• Required imports (context, encoding/json, extism)
• Request/response struct types
• Registration function
• Host function wrappers
• Helper functions (writeResponse, writeErrorResponse)

Troubleshooting

Annotations Not Detected

Ensure annotations are on consecutive lines with no blank // lines:

// ✅ Correct
// Documentation for the service.
//nd:hostservice name=Test permission=test

// ❌ Wrong - blank comment line breaks detection
// Documentation for the service.
//
//nd:hostservice name=Test permission=test

Methods Not Exported

Methods without //nd:hostfunc annotation are skipped. Ensure the annotation is directly above the method:

// ✅ Correct
// Method documentation.
//nd:hostfunc
MyMethod(ctx context.Context) error

// ❌ Wrong - annotation not directly above method
//nd:hostfunc

MyMethod(ctx context.Context) error

Generated Files Skipped

Files ending in _gen.go are automatically skipped during parsing to avoid processing previously generated code.

Development

Run tests:

go test -v ./plugins/cmd/hostgen/...

The test suite includes:

• CLI integration tests
• Complex type handling (structs, slices, maps, pointers)
• Multiple return value scenarios
• Error cases and edge conditions