Fix races on temporary file copies (#616)

* Fix races on temporary file copies

* Appease Windows

* Fix ordering

core/core.go

@@ -404,10 +404,17 @@ func downloadBazelIfNecessary(version string, bazeliskHome string, bazelForkOrUR
 }
 
 func atomicWriteFile(path string, contents []byte, perm os.FileMode) error {
-	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+	parent := filepath.Dir(path)
+	if err := os.MkdirAll(parent, 0755); err != nil {
 		return fmt.Errorf("failed to MkdirAll parent of %s: %w", path, err)
 	}
-	tmpPath := path + ".tmp"
+	tmpFile, err := os.CreateTemp(parent, filepath.Base(path)+".tmp")
+	if err != nil {
+		return fmt.Errorf("failed to create temporary file in %s: %w", parent, err)
+	}
+	tmpFile.Close()
+	defer os.Remove(tmpFile.Name())
+	tmpPath := tmpFile.Name()
 	if err := os.WriteFile(tmpPath, contents, perm); err != nil {
 		return fmt.Errorf("failed to write file %s: %w", tmpPath, err)
 	}
@@ -458,12 +465,20 @@ func downloadBazelToCAS(version string, bazeliskHome string, repos *Repositories
 	f.Close()
 	actualSha256 := strings.ToLower(fmt.Sprintf("%x", h.Sum(nil)))
 
-	pathToBazelInCAS := filepath.Join(casDir, actualSha256, "bin", "bazel"+platforms.DetermineExecutableFilenameSuffix())
-	if err := os.MkdirAll(filepath.Dir(pathToBazelInCAS), 0755); err != nil {
+	bazelInCASBasename := "bazel" + platforms.DetermineExecutableFilenameSuffix()
+	pathToBazelInCAS := filepath.Join(casDir, actualSha256, "bin", bazelInCASBasename)
+	dirForBazelInCAS := filepath.Dir(pathToBazelInCAS)
+	if err := os.MkdirAll(dirForBazelInCAS, 0755); err != nil {
 		return "", "", fmt.Errorf("failed to MkdirAll parent of %s: %w", pathToBazelInCAS, err)
 	}
 
-	tmpPathInCorrectDirectory := pathToBazelInCAS + ".tmp"
+	tmpPathFile, err := os.CreateTemp(dirForBazelInCAS, bazelInCASBasename+".tmp")
+	if err != nil {
+		return "", "", fmt.Errorf("failed to create temporary file in %s: %w", dirForBazelInCAS, err)
+	}
+	tmpPathFile.Close()
+	defer os.Remove(tmpPathFile.Name())
+	tmpPathInCorrectDirectory := tmpPathFile.Name()
 	if err := os.Rename(tmpDestPath, tmpPathInCorrectDirectory); err != nil {
 		return "", "", fmt.Errorf("failed to move %s to %s: %w", tmpDestPath, tmpPathInCorrectDirectory, err)
 	}