cmd/go: skip "exclude all Go files" error in fmt

Otherwise, one can't run "go fmt" on a directory containing Go files if
none of them are buildable (e.g. because of build tags). This is
counter-intuitive, as fmt will format all Go files anyway.

If we encounter such a load error, ignore it and carry on. All other
load errors, such as when a package can't be found, should still be
shown to the user.

Add a test for the two kinds of load errors. Use fmt -n so that any
changes to the formatting of the files in testdata don't actually get
applied. The load errors still occur with -n, so the test does its job.

Fixes #22183.

Change-Id: I99d0c0cdd29015b6a3f5286a9bbff50757c78e0d
Reviewed-on: https://go-review.googlesource.com/75930
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/deps.go

@@ -167,6 +167,7 @@ var builddeps = map[string][]string{
 		"os",            // cmd/go/internal/fmtcmd
 		"path/filepath", // cmd/go/internal/fmtcmd
 		"runtime",       // cmd/go/internal/fmtcmd
+		"strings",       // cmd/go/internal/fmtcmd
 		"sync",          // cmd/go/internal/fmtcmd
 	},
 

src/cmd/go/go_test.go

@@ -4906,3 +4906,11 @@ func TestInstallDeps(t *testing.T) {
 	tg.run("install", "-i", "p2")
 	tg.mustExist(p1)
 }
+
+func TestFmtLoadErrors(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.runFail("fmt", "does-not-exist")
+	tg.run("fmt", "-n", "exclude")
+}

src/cmd/go/internal/fmtcmd/fmt.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"sync"
 
 	"cmd/go/internal/base"
@@ -55,7 +56,16 @@ func runFmt(cmd *base.Command, args []string) {
 			}
 		}()
 	}
-	for _, pkg := range load.Packages(args) {
+	for _, pkg := range load.PackagesAndErrors(args) {
+		if pkg.Error != nil {
+			if strings.HasPrefix(pkg.Error.Err, "build constraints exclude all Go files") {
+				// Skip this error, as we will format
+				// all files regardless.
+			} else {
+				base.Errorf("can't load package: %s", pkg.Error)
+				continue
+			}
+		}
 		// Use pkg.gofiles instead of pkg.Dir so that
 		// the command only applies to this package,
 		// not to packages in subdirectories.