In Ruby, why can a long-running service using Tempfile.new fill the temp directory, and how do Tempfile.create and Dir.mktmpdir make cleanup predictable?

## Two ways to make a temporary file `require "tempfile"` gives you two creation styles, and the library's own documentation ranks them: - **`Tempfile.create`** — *recommended*. Returns an ordinary `File`. Deletion timing is predictable. - **`Tempfile.new` / `Tempfile.open`** — kept mostly for backward compatibility. Returns a `Tempfile` (a delegator around `File`) whose file is removed by the garbage collector's finalizer. Both create a uniquely named file in the system temporary directory (`Dir.tmpdir`), open for reading and writing with permissions `0600`, and pick names safely across threads and processes. ## Why Tempfile.new leaks in production A `Tempfile` object registers a finalizer that deletes its file. That means: 1. The file is deleted when the GC collects the object or when Ruby exits normally — not when you are done with it. 2. In a busy service with long GC intervals, thousands of finished temp files can sit on disk at once. 3. If the process dies from `SIGKILL`, an out-of-memory kill or a segfault, finalizers never run and the files stay forever. 4. The legacy fix is manual: `ensure file.close; file.unlink` (or `file.close!`, which does both). A worker that converts uploaded images with `Tempfile.new` and relies on GC is exactly how a temp directory fills up over weeks. ## Tempfile.create: deletion you can predict | Form | When the file is removed | |---|---| | `Tempfile.create { \|f\| ... }` | when the block exits, even on an exception | | `Tempfile.create` (no block) | never automatically — close it and `File.unlink(f.path)` | | `Tempfile.create(anonymous: true)` | before it returns; no name ever visible | | `Tempfile.create(anonymous: true) { ... }` | before the block runs | With a block, `Tempfile.create` returns the **block's value**, closes the file and removes it. The object is a plain `File`, so there is no delegation overhead. You can pass a `[prefix, suffix]` basename such as `["upload", ".jpg"]` when a tool needs the right extension. `anonymous: true` (added in Ruby 3.4) creates the file and removes its directory entry at once — `O_TMPFILE` on Linux, `FILE_SHARE_DELETE` on Windows. The open handle still works; `f.path` returns only the temp directory. Because nothing has a name, the operating system reclaims the space when the handle closes or the process ends, **even after an abnormal exit**. Use it for large scratch buffers that no other process needs to open. ## Dir.mktmpdir for several files `require "tmpdir"` adds `Dir.mktmpdir`: - `Dir.mktmpdir { |dir| ... }` creates a new directory with permissions `0700`, yields its path, and after the block removes the directory **and everything in it** with `FileUtils.remove_entry`, returning the block's value. - `Dir.mktmpdir("resize")` sets a name prefix; a second argument chooses the parent directory. - Without a block it returns the path and removes nothing; the caller must clean up in an `ensure`. It is the right tool when an external command writes several outputs, or when a test needs an isolated working tree. ## Temporary files in tests Test suites are where temp files multiply fastest, because every example may create some: 1. Create a per-example directory with `Dir.mktmpdir` in the setup hook and remove it in teardown, or wrap the example body in the block form so removal is automatic. 2. Point the code under test at that directory through a parameter or configuration value instead of letting it write into the project tree. 3. Prefer `Tempfile.create` with a block for single files, so a failing assertion inside the block still removes the file. `Dir.mktmpdir` directories are created with mode `0700`, so other users on the machine cannot read them, and each call gets a unique name, so parallel test processes never share a directory by accident. ## Choosing - One scratch file you read back yourself: `Tempfile.create(anonymous: true) { ... }`. - A named file another program must open: `Tempfile.create(["in", ".png"]) { |f| ... }`. - Several files or a working directory: `Dir.mktmpdir { |dir| ... }`. - `Tempfile.new` only when an API insists on a `Tempfile`, and then with `close!` in an `ensure`.