File Creation Shenanigans

Fri, 30 Oct 2026 00:00:00 +0000

File Creation Shenanigans

Lately I’ve been working on building a concurrent CLI download manager with Go. Initially I wrote the following function to download a file:

// Excerpt from Download: HTTP and progress-reporting code omitted.
if _, err := os.Stat(outputPath); err == nil {
	return fmt.Errorf("output file already exists: %s", outputPath)
} else if !errors.Is(err, os.ErrNotExist) {
	return fmt.Errorf("check output file: %w", err)
}

// ... fetch the HTTP response ...

partPath := outputPath + ".part"
partFile, err := os.Create(partPath)
if err != nil {
	return fmt.Errorf("create file: %w", err)
}

// ... stream into partFile, close it, and check both operations for errors ...

if err := os.Rename(partPath, outputPath); err != nil {
	_ = os.Remove(partPath)
	return fmt.Errorf("rename file: %w", err)
}

Initially, I check whether the destination file exists. If it does, I return an error. However, this check does not reserve the file name. Another process could create the destination while my download is still running. After receiving the HTTP response headers, I construct a temporary path by appending .part to the destination filename. I then use os.Create to open a file for writing, creating it if it does not exist or truncating it if it already exists. The response body is then streamed into the temporary file. After streaming completes successfully, I close the temporary file and check for errors before renaming it to the destination filename.

API on BurrowedInCode

File Creation Shenanigans

File Creation Shenanigans