cmd/go: allow users to specify required fields in JSON output

For #29666

Change-Id: Ibae3d75bb2c19571c8d473cb47d6c4b3a880bba8
Reviewed-on: https://go-review.googlesource.com/c/go/+/381035
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

3 files changed, 137 insertions, 18 deletions

diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index 63e7900e02..2bd2fb6fbc 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -869,7 +869,10 @@
 // for the go/build package's Context type.
 //
 // The -json flag causes the package data to be printed in JSON format
-// instead of using the template format.
+// instead of using the template format. The JSON flag can optionally be
+// provided with a set of comma-separated required field names to be output.
+// If so, those required fields will always appear in JSON output, but
+// others may be omitted to save work in computing the JSON struct.
 //
 // The -compiled flag causes list to set CompiledGoFiles to the Go source
 // files presented to the compiler. Typically this means that it repeats
diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index 8be9211935..9cebb934bf 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -13,7 +13,9 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"reflect"
 	"sort"
+	"strconv"
 	"strings"
 	"text/template"
 
@@ -157,7 +159,10 @@ For more information about the meaning of these fields see the documentation
 for the go/build package's Context type.
 
 The -json flag causes the package data to be printed in JSON format
-instead of using the template format.
+instead of using the template format. The JSON flag can optionally be
+provided with a set of comma-separated required field names to be output.
+If so, those required fields will always appear in JSON output, but
+others may be omitted to save work in computing the JSON struct.
 
 The -compiled flag causes list to set CompiledGoFiles to the Go source
 files presented to the compiler. Typically this means that it repeats
@@ -316,29 +321,79 @@ For more about modules, see https://golang.org/ref/mod.
 func init() {
 	CmdList.Run = runList // break init cycle
 	work.AddBuildFlags(CmdList, work.DefaultBuildFlags)
+	CmdList.Flag.Var(&listJsonFields, "json", "")
 }
 
 var (
-	listCompiled  = CmdList.Flag.Bool("compiled", false, "")
-	listDeps      = CmdList.Flag.Bool("deps", false, "")
-	listE         = CmdList.Flag.Bool("e", false, "")
-	listExport    = CmdList.Flag.Bool("export", false, "")
-	listFmt       = CmdList.Flag.String("f", "", "")
-	listFind      = CmdList.Flag.Bool("find", false, "")
-	listJson      = CmdList.Flag.Bool("json", false, "")
-	listM         = CmdList.Flag.Bool("m", false, "")
-	listRetracted = CmdList.Flag.Bool("retracted", false, "")
-	listTest      = CmdList.Flag.Bool("test", false, "")
-	listU         = CmdList.Flag.Bool("u", false, "")
-	listVersions  = CmdList.Flag.Bool("versions", false, "")
+	listCompiled   = CmdList.Flag.Bool("compiled", false, "")
+	listDeps       = CmdList.Flag.Bool("deps", false, "")
+	listE          = CmdList.Flag.Bool("e", false, "")
+	listExport     = CmdList.Flag.Bool("export", false, "")
+	listFmt        = CmdList.Flag.String("f", "", "")
+	listFind       = CmdList.Flag.Bool("find", false, "")
+	listJson       bool
+	listJsonFields jsonFlag // If not empty, only output these fields.
+	listM          = CmdList.Flag.Bool("m", false, "")
+	listRetracted  = CmdList.Flag.Bool("retracted", false, "")
+	listTest       = CmdList.Flag.Bool("test", false, "")
+	listU          = CmdList.Flag.Bool("u", false, "")
+	listVersions   = CmdList.Flag.Bool("versions", false, "")
 )
 
+// A StringsFlag is a command-line flag that interprets its argument
+// as a space-separated list of possibly-quoted strings.
+type jsonFlag map[string]bool
+
+func (v *jsonFlag) Set(s string) error {
+	if v, err := strconv.ParseBool(s); err == nil {
+		listJson = v
+		return nil
+	}
+	listJson = true
+	if *v == nil {
+		*v = make(map[string]bool)
+	}
+	for _, f := range strings.Split(s, ",") {
+		(*v)[f] = true
+	}
+	return nil
+}
+
+func (v *jsonFlag) String() string {
+	var fields []string
+	for f := range *v {
+		fields = append(fields, f)
+	}
+	sort.Strings(fields)
+	return strings.Join(fields, ",")
+}
+
+func (v *jsonFlag) IsBoolFlag() bool {
+	return true
+}
+
+func (v *jsonFlag) needAll() bool {
+	return len(*v) == 0
+}
+
+func (v *jsonFlag) needAny(fields ...string) bool {
+	if v.needAll() {
+		return true
+	}
+	for _, f := range fields {
+		if (*v)[f] {
+			return true
+		}
+	}
+	return false
+}
+
 var nl = []byte{'\n'}
 
 func runList(ctx context.Context, cmd *base.Command, args []string) {
 	modload.InitWorkfile()
 
-	if *listFmt != "" && *listJson == true {
+	if *listFmt != "" && listJson == true {
 		base.Fatalf("go list -f cannot be used with -json")
 	}
 
@@ -357,9 +412,18 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 		}
 	}
 
-	var do func(any)
-	if *listJson {
+	var do func(x any)
+	if listJson {
 		do = func(x any) {
+			if !listJsonFields.needAll() {
+				v := reflect.ValueOf(x).Elem() // do is always called with a non-nil pointer.
+				// Clear all non-requested fields.
+				for i := 0; i < v.NumField(); i++ {
+					if !listJsonFields.needAny(v.Type().Field(i).Name) {
+						v.Field(i).Set(reflect.Zero(v.Type().Field(i).Type))
+					}
+				}
+			}
 			b, err := json.MarshalIndent(x, "", "\t")
 			if err != nil {
 				out.Flush()
@@ -589,7 +653,7 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 	}
 
 	// Do we need to run a build to gather information?
-	needStale := *listJson || strings.Contains(*listFmt, ".Stale")
+	needStale := (listJson && listJsonFields.needAny("Stale", "StaleReason")) || strings.Contains(*listFmt, ".Stale")
 	if needStale || *listExport || *listCompiled {
 		var b work.Builder
 		b.Init()
diff --git a/src/cmd/go/testdata/script/list_json_fields.txt b/src/cmd/go/testdata/script/list_json_fields.txt
new file mode 100644
index 0000000000..58c9efa162
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_json_fields.txt
@@ -0,0 +1,52 @@
+# Test using -json flag to specify specific fields.
+
+# Test -json produces "full" output by looking for multiple fields present.
+go list -json .
+stdout '"Name": "a"'
+stdout '"Stale": true'
+# Same thing for -json=true
+go list -json=true .
+stdout '"Name": "a"'
+stdout '"Stale": true'
+
+# Test -json=false produces non-json output.
+go list -json=false
+cmp stdout want-non-json.txt
+
+# Test -json=<field> keeps only that field.
+go list -json=Name
+cmp stdout want-json-name.txt
+
+# Test -json=<field> with multiple fields.
+go list -json=ImportPath,Name,GoFiles,Imports
+cmp stdout want-json-multiple.txt
+
+-- go.mod --
+module example.com/a
+
+go 1.18
+-- a.go --
+package a
+
+import "fmt"
+
+func F() {
+    fmt.Println("hey there")
+}
+-- want-non-json.txt --
+example.com/a
+-- want-json-name.txt --
+{
+	"Name": "a"
+}
+-- want-json-multiple.txt --
+{
+	"ImportPath": "example.com/a",
+	"Name": "a",
+	"GoFiles": [
+		"a.go"
+	],
+	"Imports": [
+		"fmt"
+	]
+}