fix(generator): error-only methods in response handling

Signed-off-by: Deluan <deluan@navidrome.org>

plugins/cmd/hostgen/internal/generator.go

@@ -22,6 +22,7 @@ func GenerateService(svc Service, pkgName string) ([]byte, error) {
 		"needsJSON":        NeedsJSON,
 		"needsRequestType": func(m Method) bool { return m.NeedsRequestType() },
 		"needsRespType":    func(m Method) bool { return m.NeedsResponseType() },
+		"isErrorOnly":      func(m Method) bool { return m.IsErrorOnly() },
 		"hasErrFromRead":   hasErrorFromRead,
 		"readParam":        generateReadParam,
 		"writeReturn":      generateWriteReturn,
@@ -292,14 +293,24 @@ func new{{$.Service.Name}}{{.Name}}HostFunction(service {{$.Service.Interface}})
 {{- end}}
 {{- if .HasError}}
 			if err != nil {
-{{- if needsRespType .}}
+{{- if isErrorOnly .}}
+				// Write error string to plugin memory
+				if ptr, err := p.WriteString(err.Error()); err == nil {
+					stack[0] = ptr
+				}
+{{- else if needsRespType .}}
 				{{$.Service.Name | lower}}WriteError(p, stack, err)
 {{- end}}
 				return
 			}
 {{- end}}
 
-{{- if needsRespType .}}
+{{- if isErrorOnly .}}
+			// Write empty string to indicate success
+			if ptr, err := p.WriteString(""); err == nil {
+				stack[0] = ptr
+			}
+{{- else if needsRespType .}}
 			// Write JSON response to plugin memory
 			resp := {{responseType .}}{
 {{- range .Returns}}
@@ -319,7 +330,7 @@ func new{{$.Service.Name}}{{.Name}}HostFunction(service {{$.Service.Interface}})
 {{- else}}
 		[]extism.ValueType{ {{- range $i, $p := .Params}}{{if $i}}, {{end}}{{valueType $p.Type}}{{end}}{{if not .HasParams}}{{end}} },
 {{- end}}
-{{- if needsRespType .}}
+{{- if or (needsRespType .) (isErrorOnly .)}}
 		[]extism.ValueType{extism.ValueTypePTR},
 {{- else}}
 		[]extism.ValueType{ {{- range $i, $r := .Returns}}{{if $i}}, {{end}}{{valueType $r.Type}}{{end}}{{if not .HasReturns}}{{end}} },

plugins/cmd/hostgen/internal/types.go

@@ -176,14 +176,15 @@ func (m Method) NeedsRequestType() bool {
 }
 
 // NeedsResponseType returns true if a response struct is needed.
-// Needed when we have complex returns that require JSON, or error handling.
+// Needed when we have complex returns that require JSON (but not for error-only methods).
 func (m Method) NeedsResponseType() bool {
-	// If there's an error, we need a response type to serialize it
+	// Error-only methods return a simple string, not JSON
-	if m.HasError {
+	if m.IsErrorOnly() {
-		// But only if there are also returns, or if returns need JSON
+		return false
-		if m.HasReturns() {
+	}
-			return true
+	// If there's an error with other returns, we need a response type
-		}
+	if m.HasError && m.HasReturns() {
+		return true
 	}
 	for _, r := range m.Returns {
 		if NeedsJSON(r.Type) {
@@ -193,6 +194,11 @@ func (m Method) NeedsResponseType() bool {
 	return false
 }
 
+// IsErrorOnly returns true if the method only returns an error (no other return values).
+func (m Method) IsErrorOnly() bool {
+	return m.HasError && !m.HasReturns()
+}
+
 // toJSONName converts a Go identifier to camelCase JSON field name.
 func toJSONName(name string) string {
 	if name == "" {

plugins/cmd/hostgen/testdata/meta_expected.go

@@ -75,11 +75,19 @@ func newMetaSetHostFunction(service MetaService) extism.HostFunction {
 			// Call the service method
 			err = service.Set(ctx, req.Data)
 			if err != nil {
+				// Write error string to plugin memory
+				if ptr, err := p.WriteString(err.Error()); err == nil {
+					stack[0] = ptr
+				}
 				return
 			}
+			// Write empty string to indicate success
+			if ptr, err := p.WriteString(""); err == nil {
+				stack[0] = ptr
+			}
 		},
 		[]extism.ValueType{extism.ValueTypePTR},
-		[]extism.ValueType{},
+		[]extism.ValueType{extism.ValueTypePTR},
 	)
 }
 

plugins/cmd/hostgen/testdata/ping_expected.go

@@ -24,10 +24,18 @@ func newPingPingHostFunction(service PingService) extism.HostFunction {
 			// Call the service method
 			err := service.Ping(ctx)
 			if err != nil {
+				// Write error string to plugin memory
+				if ptr, err := p.WriteString(err.Error()); err == nil {
+					stack[0] = ptr
+				}
 				return
 			}
+			// Write empty string to indicate success
+			if ptr, err := p.WriteString(""); err == nil {
+				stack[0] = ptr
+			}
 		},
 		[]extism.ValueType{},
-		[]extism.ValueType{},
+		[]extism.ValueType{extism.ValueTypePTR},
 	)
 }