cmd/go: ignore dot and underscore files in fmt, fix, and get -fix

No test because as far as I can tell, there aren't existing tests for
these.

Fixes #18383

Change-Id: I06eaef05777a1474886167e3797c5bcd93189d1b
Reviewed-on: https://go-review.googlesource.com/45156
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/base/path.go

@@ -44,6 +44,28 @@ func RelPaths(paths []string) []string {
 	return out
 }
 
+// FilterDotUnderscoreFiles returns a slice containing all elements
+// of path whose base name doesn't begin with "." or "_".
+func FilterDotUnderscoreFiles(path []string) []string {
+	var out []string // lazily initialized
+	for i, p := range path {
+		base := filepath.Base(p)
+		if strings.HasPrefix(base, ".") || strings.HasPrefix(base, "_") {
+			if out == nil {
+				out = append(make([]string, 0, len(path)), path[:i]...)
+			}
+			continue
+		}
+		if out != nil {
+			out = append(out, p)
+		}
+	}
+	if out == nil {
+		return path
+	}
+	return out
+}
+
 // IsTestFile reports whether the source file is a set of tests and should therefore
 // be excluded from coverage analysis.
 func IsTestFile(file string) bool {

src/cmd/go/internal/fix/fix.go

@@ -33,6 +33,7 @@ func runFix(cmd *base.Command, args []string) {
 		// Use pkg.gofiles instead of pkg.Dir so that
 		// the command only applies to this package,
 		// not to packages in subdirectories.
-		base.Run(str.StringList(cfg.BuildToolexec, base.Tool("fix"), base.RelPaths(pkg.Internal.AllGoFiles)))
+		files := base.FilterDotUnderscoreFiles(base.RelPaths(pkg.Internal.AllGoFiles))
+		base.Run(str.StringList(cfg.BuildToolexec, base.Tool("fix"), files))
 	}
 }

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

@@ -45,7 +45,8 @@ func runFmt(cmd *base.Command, args []string) {
 		// Use pkg.gofiles instead of pkg.Dir so that
 		// the command only applies to this package,
 		// not to packages in subdirectories.
-		base.Run(str.StringList(gofmt, "-l", "-w", base.RelPaths(pkg.Internal.AllGoFiles)))
+		files := base.FilterDotUnderscoreFiles(base.RelPaths(pkg.Internal.AllGoFiles))
+		base.Run(str.StringList(gofmt, "-l", "-w", files))
 	}
 }
 

src/cmd/go/internal/get/get.go

@@ -298,7 +298,8 @@ func download(arg string, parent *load.Package, stk *load.ImportStack, mode int)
 	// due to wildcard expansion.
 	for _, p := range pkgs {
 		if *getFix {
-			base.Run(cfg.BuildToolexec, str.StringList(base.Tool("fix"), base.RelPaths(p.Internal.AllGoFiles)))
+			files := base.FilterDotUnderscoreFiles(base.RelPaths(p.Internal.AllGoFiles))
+			base.Run(cfg.BuildToolexec, str.StringList(base.Tool("fix"), files))
 
 			// The imports might have changed, so reload again.
 			p = load.ReloadPackage(arg, stk)