cmd/cgo: use objdir consistently, create it as needed

Previously we added a slash to the end of objdir only after
processing input files. The effect was that the temporary gcc
output files were placed in /tmp, using objdir as a prefix.
Those output files were not removed by anything.

Now we consistently use objdir as a directory, not a prefix.
We only create it when needed; there is already a test for that in
cmd/go/testdata/script/build_cwd_newline.txt.

Change-Id: Ie66d9c04ecc3c0f5950fc1111c74e1d01c67304c
Reviewed-on: https://go-review.googlesource.com/c/go/+/740742
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/gcc.go

@@ -24,6 +24,7 @@ import (
 	"math"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"slices"
 	"strconv"
 	"strings"
@@ -1795,7 +1796,7 @@ var n atomic.Int64
 
 func gccTmp() string {
 	c := strconv.Itoa(int(n.Add(1)))
-	return *objDir + "_cgo_" + c + ".o"
+	return filepath.Join(outputDir(), "_cgo_"+c+".o")
 }
 
 // gccCmd returns the gcc command line to use for compiling

src/cmd/cgo/main.go

@@ -320,6 +320,10 @@ func main() {
 		conf.Mode &^= printer.SourcePos
 	}
 
+	if *objDir == "" {
+		*objDir = "_obj"
+	}
+
 	args := flag.Args()
 	if len(args) < 1 {
 		usage()
@@ -446,14 +450,6 @@ func main() {
 
 	cPrefix = fmt.Sprintf("_%x", h.Sum(nil)[0:6])
 
-	if *objDir == "" {
-		*objDir = "_obj"
-	}
-	// make sure that `objDir` directory exists, so that we can write
-	// all the output files there.
-	os.MkdirAll(*objDir, 0o700)
-	*objDir += string(filepath.Separator)
-
 	for i, input := range goFiles {
 		f := fs[i]
 		p.Translate(f)

src/cmd/cgo/out.go

@@ -33,15 +33,15 @@ var (
 // writeDefs creates output files to be compiled by gc and gcc.
 func (p *Package) writeDefs() {
 	var fgo2, fc io.Writer
-	f := creat(*objDir + "_cgo_gotypes.go")
+	f := creat("_cgo_gotypes.go")
 	defer f.Close()
 	fgo2 = f
 	if *gccgo {
-		f := creat(*objDir + "_cgo_defun.c")
+		f := creat("_cgo_defun.c")
 		defer f.Close()
 		fc = f
 	}
-	fm := creat(*objDir + "_cgo_main.c")
+	fm := creat("_cgo_main.c")
 
 	var gccgoInit strings.Builder
 
@@ -50,7 +50,7 @@ func (p *Package) writeDefs() {
 			fmt.Fprintf(fgo2, "//go:cgo_ldflag %q\n", arg)
 		}
 	} else {
-		fflg := creat(*objDir + "_cgo_flags")
+		fflg := creat("_cgo_flags")
 		for _, arg := range p.LdFlags {
 			fmt.Fprintf(fflg, "_CGO_LDFLAGS=%s\n", arg)
 		}
@@ -242,8 +242,8 @@ func (p *Package) writeDefs() {
 		}
 	}
 
-	fgcc := creat(*objDir + "_cgo_export.c")
-	fgcch := creat(*objDir + "_cgo_export.h")
+	fgcc := creat("_cgo_export.c")
+	fgcch := creat("_cgo_export.h")
 	if *gccgo {
 		p.writeGccgoExports(fgo2, fm, fgcc, fgcch)
 	} else {
@@ -263,8 +263,11 @@ func (p *Package) writeDefs() {
 	}
 
 	if *exportHeader != "" && len(p.ExpFunc) > 0 {
-		fexp := creat(*exportHeader)
-		fgcch, err := os.Open(*objDir + "_cgo_export.h")
+		fexp, err := os.Create(*exportHeader)
+		if err != nil {
+			fatalf("%s", err)
+		}
+		fgcch, err := os.Open(filepath.Join(outputDir(), "_cgo_export.h"))
 		if err != nil {
 			fatalf("%s", err)
 		}
@@ -697,8 +700,8 @@ func (p *Package) writeOutput(f *File, srcfile string) {
 	base := srcfile
 	base = strings.TrimSuffix(base, ".go")
 	base = filepath.Base(base)
-	fgo1 := creat(*objDir + base + ".cgo1.go")
-	fgcc := creat(*objDir + base + ".cgo2.c")
+	fgo1 := creat(base + ".cgo1.go")
+	fgcc := creat(base + ".cgo2.c")
 
 	p.GoFiles = append(p.GoFiles, base+".cgo1.go")
 	p.GccFiles = append(p.GccFiles, base+".cgo2.c")
@@ -1395,7 +1398,7 @@ func gccgoToSymbol(ppath string) string {
 				fatalf("unable to locate gccgo: %v", err)
 			}
 		}
-		gccgoMangler, err = pkgpath.ToSymbolFunc(cmd, *objDir)
+		gccgoMangler, err = pkgpath.ToSymbolFunc(cmd, outputDir())
 		if err != nil {
 			fatalf("%v", err)
 		}

src/cmd/cgo/util.go

@@ -10,6 +10,7 @@ import (
 	"go/token"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"slices"
 )
 
@@ -97,10 +98,17 @@ func error_(pos token.Pos, msg string, args ...any) {
 	fmt.Fprintf(os.Stderr, "\n")
 }
 
+// create creates a file in the output directory.
 func creat(name string) *os.File {
-	f, err := os.Create(name)
+	f, err := os.Create(filepath.Join(outputDir(), name))
 	if err != nil {
 		fatalf("%s", err)
 	}
 	return f
 }
+
+// outputDir returns the output directory, making sure that it exists.
+func outputDir() string {
+	os.MkdirAll(*objDir, 0o700)
+	return *objDir
+}