refactor Offchain Worker examples

[bernardoaraujor]

As discussed in:

• Ideas for a new set of example/template pallets polkadot-sdk#186
• https://forum.polkadot.network/t/offchain-workers-design-assumptions-vulnerabilities/2548

this PR is refactoring FRAME's Offchain Worker examples in order to mitigate common misunderstandings that we've been seeing in the community (which can result in security issues)

the original OCW example was actually split into two separate examples:

Ping Pong

inspired by https://gnunicorn.github.io/substrate-offchain-cb/

this example attempts to illustrate a scenario where some user input triggers some action by the OCWs... in this case, these are simple ping-pong events, where the pong can happen as many kinds of transactions, namely:

• signed (coming from authorized/permissioned accounts)
• unsigned
• unsigned with signed payload

the comments and docs put a strong emphasis into the security issues that come from using unsigned transactions, and the user is encouraged to use signed transactions from authorized/permissioned accounts instead.

Price Oracle

follows the same spirit from the original example (a BTC/USD price oracle), but new prices can only be submitted by authorized/permissioned accounts, which removes the possibilty of malicious actors submitting false prices via unsigned transactions.

The main purpose of this example is to:

• illustrate how to use authorized/permissioned accounts for OCWs
• illustrate how to use HTTP calls to external resources (cryptocompare's API in this case)
• illustrate how to coordinate potentially concurrent OCW threads via local offchain storage

[kianenigma]

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[ggwpez]

Yea looks good, just some nits.

[ggwpez]

Do you know by any chance what license this was originally published under?

[brunopgalvao]

The original offchain-worker example pallet was published under Apache-2.0:

substrate/frame/examples/offchain-worker/src/lib.rs

Line 4 in f81f8eb

// SPDX-License-Identifier: Apache-2.0

[ggwpez]

Suggested change

const RECENTLY_SENT: () = ();

I dont think this adds much.

[brunopgalvao]

I do understand the author's original intent to have a descriptive error. So in the places where it is used, replace it with an empty tuple?

substrate/frame/examples/offchain-worker-price-oracle/src/lib.rs

Lines 257 to 258 in a08af90

	Ok(Some(block)) if block_number < block + T::GracePeriod::get() =>
	Err(RECENTLY_SENT),

[brunopgalvao]

Also want to note that this came from the original implementation:

• substrate/frame/examples/offchain-worker/src/lib.rs

  Line 399 in a6b0d43

  Err(RECENTLY_SENT),

Please let me know how you'd like me to move forward.

[kianenigma]

Please return () as error without any alias, it is not a good rust pattern.

[bernardoaraujor]

99ff657

[kianenigma]

The whole line wrapping here seems 80 rather than 100 🙄

[bernardoaraujor]

@kianenigma it seems that your comment is pointing to the code before this commit, which addresses line wrapping

here's the same snippet after the commit:

substrate/frame/examples/offchain-worker-price-oracle/src/lib.rs

Lines 230 to 236 in 5ec0432

	let val = StorageValueRef::persistent(b"example_ocw::last_send");
	// The Local Storage is persisted and shared between runs of the offchain workers, and
	// offchain workers may run concurrently. We can use the `mutate` function, to write a
	// storage entry in an atomic fashion. Under the hood it uses `compare_and_set`
	// low-level method of local storage API, which means that only one worker will be able
	// to "acquire a lock" and send a transaction if multiple workers happen to be executed
	// concurrently.

[paritytech-cicd-pr]

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