Fix selfdescribe ast cache (#703)

This brings down selfdescribe.json generation from 6s to 1.2s.

Author: Jay Camp

cmd/agent/main.go

@@ -85,7 +85,7 @@ func doStatus() {
 // Print out agent self-description of config/metadata
 func doSelfDescribe() {
 	log.SetOutput(os.Stderr)
-	fmt.Print(selfdescribe.JSON())
+	selfdescribe.JSON(os.Stdout)
 }
 
 // glog is a transitive dependency of the agent and puts a bunch of flags in

internal/selfdescribe/selfdescribe.go

@@ -7,6 +7,7 @@ package selfdescribe
 
 import (
 	"encoding/json"
+	"io"
 	"reflect"
 
 	"github.com/signalfx/signalfx-agent/internal/core/config"
@@ -17,7 +18,7 @@ import (
 // various components in the agent.  It is meant to be used as an intermediate
 // form which serves as a data source for automatically generating docs about
 // the agent.
-func JSON() string {
+func JSON(writer io.Writer) {
 	out, err := json.MarshalIndent(map[string]interface{}{
 		"TopConfig":             getStructMetadata(reflect.TypeOf(config.Config{})),
 		"GenericMonitorConfig":  getStructMetadata(reflect.TypeOf(config.MonitorConfig{})),
@@ -29,6 +30,5 @@ func JSON() string {
 	if err != nil {
 		panic(err)
 	}
-
-	return string(out)
+	writer.Write(out)
 }

internal/selfdescribe/sourcedocs.go

@@ -12,10 +12,12 @@ import (
 	"strings"
 )
 
-var astCache = make(map[string]struct {
+type astCacheEntry struct {
 	fset *token.FileSet
 	pkgs map[string]*ast.Package
-})
+}
+
+var astCache = map[string]astCacheEntry{}
 
 // Returns the ast node of the struct itself and the comment group on the
 // struct type.
@@ -34,6 +36,7 @@ func structNodes(packageDir, structName string) (*ast.TypeSpec, *ast.CommentGrou
 		if err != nil {
 			panic(err)
 		}
+		astCache[packageDir] = astCacheEntry{fset: fset, pkgs: pkgs}
 	}
 
 	for _, p := range pkgs {
@@ -112,7 +115,7 @@ func notesFromDocs(docs []*doc.Package, noteType string) []*doc.Note {
 
 func structFieldDocs(packageDir, structName string) map[string]string {
 	configStruct, _ := structNodes(packageDir, structName)
-	fieldDocs := make(map[string]string)
+	fieldDocs := map[string]string{}
 	for _, field := range configStruct.Type.(*ast.StructType).Fields.List {
 		if field.Names != nil {
 			fieldDocs[field.Names[0].Name] = commentTextToParagraphs(field.Doc.Text())