cmd/go: skip computing BuildInfo in go list unless it's needed

The only fields of the go list output that require BuildInfo to be
computed are the Stale and StaleReason fields. If a user explicitly
requests JSON fields and does not ask for Stale or StaleReason, skip
the computation of BuildInfo.

For #29666

Change-Id: Ie77581c44babedcb5cb7f3dc7d6ed1078b56eee4
Reviewed-on: https://go-review.googlesource.com/c/go/+/402736
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/list/list.go

@@ -575,6 +575,7 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 		// might not expect those errors to stop showing up.
 		// See issue #52443.
 		SuppressDeps:      !listJsonFields.needAny("Deps", "DepsErrors"),
+		SuppressBuildInfo: !listJsonFields.needAny("Stale", "StaleReason"),
 	}
 	pkgs := load.PackagesAndErrors(ctx, pkgOpts, args)
 	if !*listE {

src/cmd/go/internal/load/pkg.go

@@ -1947,7 +1947,7 @@ func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *
 	if !opts.SuppressDeps {
 		p.collectDeps()
 	}
-	if p.Error == nil && p.Name == "main" && !p.Internal.ForceLibrary && len(p.DepsErrors) == 0 {
+	if p.Error == nil && p.Name == "main" && !p.Internal.ForceLibrary && len(p.DepsErrors) == 0 && !opts.SuppressBuildInfo {
 		// TODO(bcmills): loading VCS metadata can be fairly slow.
 		// Consider starting this as a background goroutine and retrieving the result
 		// asynchronously when we're actually ready to build the package, or when we
@@ -2688,11 +2688,15 @@ type PackageOpts struct {
 	// LoadVCS controls whether we also load version-control metadata for main packages.
 	LoadVCS bool
 
-	// NeedDepsFields is true if the caller does not need Deps and DepsErrors to be populated
+	// SuppressDeps is true if the caller does not need Deps and DepsErrors to be populated
 	// on the package. TestPackagesAndErrors examines the  Deps field to determine if the test
 	// variant has an import cycle, so SuppressDeps should not be set if TestPackagesAndErrors
 	// will be called on the package.
 	SuppressDeps bool
+
+	// SuppressBuildInfo is true if the caller does not need p.Stale, p.StaleReason, or p.Internal.BuildInfo
+	// to be populated on the package.
+	SuppressBuildInfo bool
 }
 
 // PackagesAndErrors returns the packages named by the command line arguments

src/cmd/go/testdata/script/list_json_fields.txt

@@ -26,6 +26,18 @@ go list -json=Deps
 stdout '"Deps": \['
 stdout '"errors",'
 
+[!exec:git] skip
+
+# Test -json=<field> without Stale skips computing buildinfo
+cd repo
+exec git init
+# Control case: with -json=Stale cmd/go executes git status to compute buildinfo
+go list -json=Stale -x
+stderr 'git status'
+# Test case: without -json=Stale cmd/go skips git status
+go list -json=Name -x
+! stderr 'git status'
+
 -- go.mod --
 module example.com/a
 
@@ -55,3 +67,9 @@ example.com/a
 		"fmt"
 	]
 }
+-- repo/go.mod --
+module example.com/repo
+-- repo/main.go --
+package main
+
+func main() {}