cmd/go: clean up installHeader action

This was confusing when I was trying to fix go build -o.
Perhaps due to that fix, this can now be simplified from
three functions to one.

Change-Id: I878a6d243b14132a631e7c62a3bb6d101bc243ea
Reviewed-on: https://go-review.googlesource.com/13027Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -909,7 +909,19 @@ func (b *builder) action1(mode buildMode, depMode buildMode, p *Package, looksha
 		a.f = (*builder).install
 		a.deps = []*action{b.action1(modeBuild, depMode, p, lookshared)}
 		a.target = a.p.target
-		a = b.maybeAddHeaderAction(a, true)
+
+		// Install header for cgo in c-archive and c-shared modes.
+		if p.usesCgo() && (buildBuildmode == "c-archive" || buildBuildmode == "c-shared") {
+			ah := &action{
+				p:      a.p,
+				deps:   []*action{a.deps[0]},
+				f:      (*builder).installHeader,
+				pkgdir: a.pkgdir,
+				objdir: a.objdir,
+				target: a.target[:len(a.target)-len(filepath.Ext(a.target))] + ".h",
+			}
+			a.deps = append(a.deps, ah)
+		}
 
 	case modeBuild:
 		a.f = (*builder).build
@@ -1034,49 +1046,6 @@ func (b *builder) libaction(libname string, pkgs []*Package, mode, depMode build
 	return a
 }
 
-// In c-archive/c-shared mode, if the package for the action uses cgo,
-// add a dependency to install the generated export header file, if
-// there is one.
-// The isInstall parameter is whether a is an install action.
-func (b *builder) maybeAddHeaderAction(a *action, isInstall bool) *action {
-	switch buildBuildmode {
-	case "c-archive", "c-shared":
-	default:
-		return a
-	}
-	if !a.p.usesCgo() {
-		return a
-	}
-
-	if isInstall {
-		// For an install action, change the action function.
-		// We can't add an action after the install action,
-		// because it deletes the working directory.
-		// Adding an action before the install action is painful,
-		// because it uses deps[0] to find the source.
-		a.f = (*builder).installWithHeader
-		return a
-	}
-
-	return &action{
-		p:      a.p,
-		deps:   []*action{a},
-		f:      (*builder).installHeader,
-		pkgdir: a.pkgdir,
-		objdir: a.objdir,
-		target: a.target[:len(a.target)-len(filepath.Ext(a.target))] + ".h",
-	}
-}
-
-// Install the library and the cgo export header if there is one.
-func (b *builder) installWithHeader(a *action) error {
-	target := a.target[:len(a.target)-len(filepath.Ext(a.target))] + ".h"
-	if err := b.doInstallHeader(a, a.objdir, target); err != nil {
-		return err
-	}
-	return b.install(a)
-}
-
 // actionList returns the list of actions in the dag rooted at root
 // as visited in a depth-first post-order traversal.
 func actionList(root *action) []*action {
@@ -1692,25 +1661,21 @@ func (b *builder) copyFile(a *action, dst, src string, perm os.FileMode, force b
 
 // Install the cgo export header file, if there is one.
 func (b *builder) installHeader(a *action) error {
-	return b.doInstallHeader(a, a.objdir, a.target)
-}
-
-func (b *builder) doInstallHeader(a *action, objdir, target string) error {
-	src := objdir + "_cgo_install.h"
+	src := a.objdir + "_cgo_install.h"
 	if _, err := os.Stat(src); os.IsNotExist(err) {
 		// If the file does not exist, there are no exported
 		// functions, and we do not install anything.
 		return nil
 	}
 
-	dir, _ := filepath.Split(target)
+	dir, _ := filepath.Split(a.target)
 	if dir != "" {
 		if err := b.mkdir(dir); err != nil {
 			return err
 		}
 	}
 
-	return b.moveOrCopyFile(a, target, src, 0644, true)
+	return b.moveOrCopyFile(a, a.target, src, 0644, true)
 }
 
 // cover runs, in effect,