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Texting Robots: A Rust native `robots.txt` parser with thorough unit testing

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# Texting Robots

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Crate `texting_robots` is a library for parsing `robots.txt` files.
A key design goal of this crate is to have a thorough test suite tested
against real world data across millions of sites. While `robots.txt` is a
simple specification itself the scale and complexity of the web teases out
every possible edge case.

To read more about the `robots.txt` specification a good starting point is
[How Google interprets the robots.txt specification][google-spec].

This library cannot guard you against all possible edge cases but should
give you a strong starting point from which to ensure you and your code
constitute a positive addition to the internet at large.

[google-spec]: https://developers.google.com/search/docs/advanced/robots/robots_txt

## Installation

You can install the library by adding this entry:

```plain
[dependencies]
texting_robots = "0.2"
```

to your `Cargo.toml` dependency list.

## Overview of usage

This crate provides a simple high level usage through the `Robot` struct.

The `Robot` struct is responsible for consuming the `robots.txt` file,
processing the contents, and deciding whether a given URL is allow for
your bot or not. Additional information such as your bot's crawl delay
and any sitemaps that may exist are also available.

Given the many options and potential preferences Texting Robots does not
perform caching or a HTTP GET request of the `robots.txt` files themselves.
This step is up to the user of the library.

```rust
use texting_robots::{Robot, get_robots_url};

// If you want to fetch a URL we'll find the URL for `robots.txt`
let url = "https://www.rust-lang.org/learn";
let robots_url = get_robots_url(url);
// Then we fetch `robots.txt` from robots_url to parse as below

// A `robots.txt` file in String or byte format.
let txt = r"User-Agent: FerrisCrawler
Allow: /ocean
Disallow: /rust
Disallow: /forest*.py
Crawl-Delay: 10
User-Agent: *
Disallow: /
Sitemap: https://www.example.com/site.xml";

// Build the Robot for our friendly User-Agent
let r = Robot::new("FerrisCrawler", txt.as_bytes()).unwrap();

// Ferris has a crawl delay of one second per limb
// (Crabs have 10 legs so Ferris must wait 10 seconds!)
assert_eq!(r.delay, Some(10.0));

// Any listed sitemaps are available for any user agent who finds them
assert_eq!(r.sitemaps, vec!["https://www.example.com/site.xml"]);

// We can also check which pages Ferris is allowed to crawl
// Notice we can supply the full URL or a relative path?
assert_eq!(r.allowed("https://www.rust-lang.org/ocean"), true);
assert_eq!(r.allowed("/ocean"), true);
assert_eq!(r.allowed("/ocean/reef.html"), true);
// Sadly Ferris is allowed in the ocean but not in the rust
assert_eq!(r.allowed("/rust"), false);
// Ferris is also friendly but not very good with pythons
assert_eq!(r.allowed("/forest/tree/snake.py"), false);
```

## Crawling considerations

### Obtaining `robots.txt`

To obtain `robots.txt` requires performing an initial HTTP GET request to the
domain in question. When handling the HTTP status codes and how they impact `robots.txt`
the [suggestions made by Google are recommended][google-spec].

- 2xx (success): Attempt to process the resulting payload
- 3xx (redirection): Follow a reasonable number of redirects
- 4xx (client error): Assume there are no crawl restrictions except for:
  - 429 "Too Many Requests": Retry after a reasonable amount of time
  (potentially set by the "[Retry-After][mozilla-ra]" header)
- 5xx (server errors): Assume you should not crawl until fixed and/or interpret with care

Even when directed to "assume no crawl restrictions" it is likely reasonable and
polite to use a small fetch delay between requests.

#### Always set a User Agent

For crawling `robots.txt` (and especially for crawling in general) you should
include a user agent in your request. Most crawling libraries offer adding the
user agent in a single line.

```rust
ClientBuilder.new().user_agent("FerrisCrawler/0.1 (https://ferris.rust/about-this-robot)")...
```

Beyond respecting `robots.txt` providing a good user agent provides a line of
communication between you and the web master.

### Beyond the `robots.txt` specification and general suggestions

`texting_robots` provides much of what you need for safe and respectful
crawling but is not a full solution by itself.

As an example, the HTTP error code 429 ([Too Many Requests][mozilla-tmr]) must be
tracked when requesting pages on a given site. When a 429 is seen the crawler
should slow down, even if obeying the Crawl-Delay set in `robots.txt`, and
potentially using the delay set by the server's [Retry-After][mozilla-ra] header.

An even more complex example is that multiple domains may back on to the same
backend web server. This is a common scenario for specific products or services
that host thousands or millions of domains. How you rate limit fairly using the
`Crawl-Delay` is entirely up to the end user (and potentially the service when
using HTTP error code 429 to rate limit traffic).

To protect against adverse input the user of Texting Robots is also suggested to
follow [Google's recommendations][google-spec] and limit input to 500 kibibytes.
This is not yet done at the library level in case a larger input may be desired
but may be revisited depending on community feedback.

[mozilla-tmr]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/429
[mozilla-ra]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After

### Usage of Texting Robots in other languages

While not yet specifically supporting any languages other than Rust the
library was designed to support language integrations in the future. Battle
testing this intepretation of the `robots.txt` specification against the web is
easier done testing with friends!

A C API through Rust FFI should be relatively easy to provide given Texting Robots
only relies on strings, floats, and booleans. The lack of native fetching abilities
should ensure the library is portable across platforms, situations, and languages.

A proof of concept was performed in [WASI][wasi], the "WebAssembly System Interface",
showing that the library compiles happily and only experiences a 50% or 75% speed
penalty when used with the [Wasmer][wasmer] (LLVM backend) and [Wasmtime][wasmtime]
runtimes respectively. No optimizations have been done thus far and there's likely
low hanging fruit to reap.

See `wasi_poc.sh` for details.

[wasi]: https://wasi.dev/
[wasmer]: https://wasmer.io/
[wasmtime]: https://wasmtime.dev/

## Testing

To run the majority of core tests simply execute `cargo test`.

### Unit and Integration Tests

To check Texting Robot's behaviour against the `robots.txt` specification
almost all unit tests from [Google's C++ robots.txt parser][google-cpp] and
[Moz's reppy][moz-reppy] have been translated and included.

Certain aspects of the Google and Moz interpretation disagree with each other.
When this occurred the author deferred to as much common sense as they
were able to muster.

For a number of popular domains the `robots.txt` of the given domain was
saved and tests written against them.

[google-cpp]: https://github.com/google/robotstxt
[moz-reppy]: https://github.com/seomoz/reppy

### Common Crawl Test Harness

To ensure that the `robots.txt` parser will not panic in real world situations
over 34 million `robots.txt` responses were passed through Texting Robots.
While this test doesn't guarantee the `robots.txt` files were handled correctly
it does ensure the parser is unlikely to panic during practice.

Many problematic, invalid, outrageous, and even adversarial `robots.txt`
examples were discovered in this process.

For full details see [the Common Crawl testing harness][cc-test].

[cc-test]: https://github.com/Smerity/texting_robots_cc_test

### Fuzz Testing

In the local `fuzz` directory is a fuzz testing harness. The harness is not
particularly sophisticated but does utilize a low level of structure awareness
through utilizing [dictionary guided fuzzing][dgf]. The harness has already
revealed one low level unwrap panic.

To run:

```bash
cargo fuzz run fuzz_target_1 -- -max_len=512 -dict=keywords.dict
```

Note:

- `cargo fuzz` requires nightly (i.e. run `rustup default nightly` in the `fuzz` directory)
- If you have multiple processors you may wish to add `--jobs N` after `cargo run`

[dgf]: https://llvm.org/docs/LibFuzzer.html#dictionaries

### Code Coverage with Tarpaulin

This project uses [Tarpaulin](https://github.com/xd009642/tarpaulin) to perform
code coverage reporting. Given the relatively small surface area of the parser
and Robot struct the coverage is high. Unit testing is more important for ensuring
behavioural correctness however.

To get line numbers for uncovered code run:

```bash
cargo tarpaulin --ignore-tests -v
```

## License

Licensed under either of

* Apache License, Version 2.0
   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license
   ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.

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