cmd/go: in 'go build -o', allow the destination file to exist if it is empty

This allows the target of 'go build' to be a filename constructed
using ioutil.TempFile or similar, without racily deleting the file
before rebuilding it.

Updates #32407
Updates #28387

Change-Id: I4c5072830a02b93f0c4186b50bffa9de00257afe
Reviewed-on: https://go-review.googlesource.com/c/go/+/206477
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -1610,12 +1610,12 @@ func (b *Builder) copyFile(dst, src string, perm os.FileMode, force bool) error
 
 	// Be careful about removing/overwriting dst.
 	// Do not remove/overwrite if dst exists and is a directory
-	// or a non-object file.
+	// or a non-empty non-object file.
 	if fi, err := os.Stat(dst); err == nil {
 		if fi.IsDir() {
 			return fmt.Errorf("build output %q already exists and is a directory", dst)
 		}
-		if !force && fi.Mode().IsRegular() && !isObject(dst) {
+		if !force && fi.Mode().IsRegular() && fi.Size() != 0 && !isObject(dst) {
 			return fmt.Errorf("build output %q already exists and is not an object file", dst)
 		}
 	}

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

+[short] skip
+
+# Ensure that the target of 'go build -o' can be an existing, empty file so that
+# its name can be reserved using ioutil.TempFile or the 'mktemp` command.
+
+go build -o empty-file$GOEXE main.go
+
+-- main.go --
+package main
+func main() {}
+-- empty-file$GOEXE --