cmd/go/internal/work: copy vet tool's stdout to our stdout

The go command connects both the stdout and stderr files of
its child commands (cmd/compile, cmd/vet, etc) to the go
command's own stderr. If the child command is supposed to
produce structure output on stderr, as is the case for
go vet -json or go fix -diff, it will be merged with the
error stream, making it useless.

This change to the go vet <-> unitchecker protocol specifies
the name of a file into which the vet tool should write its
stdout. On success, the go command will then copy the entire
content of that file to its own stdout, under a lock.
This ensures that partial writes to stdout in case of failure,
concurrent writes to stdout by parallel vet tasks, or other
junk on stderr, cannot interfere with the integrity of the
go command's structure output on stdout.

CL 702835 is the corresponding change on the x/tools side.

For #75432

Change-Id: Ib4db25b6b0095d359152d7543bd9bf692551bbfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/702815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

src/cmd/go/internal/work/exec.go

@@ -1184,6 +1184,7 @@ type vetConfig struct {
 	PackageVetx   map[string]string // map package path to vetx data from earlier vet run
 	VetxOnly      bool              // only compute vetx data; don't report detected problems
 	VetxOutput    string            // write vetx data to this output file
+	Stdout        string            // write stdout (JSON, unified diff) to this output file
 	GoVersion     string            // Go version for package
 
 	SucceedOnTypecheckFailure bool // awful hack; see #18395 and below
@@ -1297,6 +1298,7 @@ func (b *Builder) vet(ctx context.Context, a *Action) error {
 
 	vcfg.VetxOnly = a.VetxOnly
 	vcfg.VetxOutput = a.Objdir + "vet.out"
+	vcfg.Stdout = a.Objdir + "vet.stdout"
 	vcfg.PackageVetx = make(map[string]string)
 
 	h := cache.NewHash("vet " + a.Package.ImportPath)
@@ -1392,13 +1394,25 @@ func (b *Builder) vet(ctx context.Context, a *Action) error {
 	// If vet wrote export data, save it for input to future vets.
 	if f, err := os.Open(vcfg.VetxOutput); err == nil {
 		a.built = vcfg.VetxOutput
-		cache.Default().Put(key, f)
-		f.Close()
+		cache.Default().Put(key, f) // ignore error
+		f.Close()                   // ignore error
+	}
+
+	// If vet wrote to stdout, copy it to go's stdout, atomically.
+	if f, err := os.Open(vcfg.Stdout); err == nil {
+		stdoutMu.Lock()
+		if _, err := io.Copy(os.Stdout, f); err != nil && runErr == nil {
+			runErr = fmt.Errorf("copying vet tool stdout: %w", err)
+		}
+		f.Close() // ignore error
+		stdoutMu.Unlock()
 	}
 
 	return runErr
 }
 
+var stdoutMu sync.Mutex // serializes concurrent writes (e.g. JSON values) to stdout
+
 // linkActionID computes the action ID for a link action.
 func (b *Builder) linkActionID(a *Action) cache.ActionID {
 	p := a.Package

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

@@ -14,12 +14,13 @@ stderr '2		MOVW'
 stderr '3		RET'
 stderr '4'
 
-# -json causes success, even with diagnostics and errors.
+# -json causes success, even with diagnostics and errors,
+# and writes to stdout.
 go vet -json -asmdecl a
-stderr '"a": {'
-stderr   '"asmdecl":'
-stderr     '"posn": ".*asm.s:2:1",'
-stderr     '"message": ".*invalid MOVW.*"'
+stdout '"a": {'
+stdout   '"asmdecl":'
+stdout     '"posn": ".*asm.s:2:1",'
+stdout     '"message": ".*invalid MOVW.*"'
 
 -- a/a.go --
 package a