Fix memory leak when evicting or expiring an entry (v0.7.x)

[tatsuya6502]

This PR addresses the memory leak issue reported by #64.

The memory leak happens when evicting/expiring an entry or manually invalidating an entry. It should not happen when dropping a cache.

The leaking data is the key of an entry, last modified timestamp (u64) and last accessed timestamp (u64). It does not include the value of an entry.

How to reproduce

• This issue can be reproduced by the code in the description of Memory leak #64.
• Or by running the following commands. Miri will detect similar issue in Deque's unit tests:

$ rustup update nightly
$ rustup +nightly component add miri
$ cargo +nightly miri test deque

running 4 tests
test common::deque::tests::basics ... ok
test common::deque::tests::drop ... ok
test common::deque::tests::iter ... ok
test common::deque::tests::next_node ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 39 filtered out

The following memory was leaked: alloc92503 (Rust heap, size: 1, align: 1) {
    61                                              │ a
}
alloc92586 (Rust heap, size: 48, align: 8) {
    0x00 │ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 │ ................
    0x10 │ ╾0x23b1f7[a92503]<untagged> (8 ptr bytes)╼ 01 00 00 00 00 00 00 00 │ ╾──────╼........
    0x20 │ 01 00 00 00 00 00 00 00 01 __ __ __ __ __ __ __ │ .........░░░░░░░
}
alloc93007 (Rust heap, size: 1, align: 1) { ... }
alloc93087 (Rust heap, size: 48, align: 8) { ... }
alloc93620 (Rust heap, size: 1, align: 1) { ... }
alloc93700 (Rust heap, size: 48, align: 8) { ... }
alloc105252 (Rust heap, size: 1, align: 1) { ... }
alloc105323 (Rust heap, size: 48, align: 8) { ... }
alloc114558 (Rust heap, size: 1, align: 1) { ... }
alloc114629 (Rust heap, size: 48, align: 8) { ... }

error: the evaluated program leaked memory

Cause

The root cause is forgetting to drop an unlinked DeqNode after calling an unsafe method moka::common::Deque::unlink.

Fixes

• Added unsafe unlink_and_drop method to Deque and updated the callers of unlink method to call unlink_and_drop. As the name implies, unlink_and _drop will unlink the DeqNode and then drop it.
• Added Miri test to the CI to detect similar programming errors in future.

This PR is for the master branch (v0.7.x). There is another PR #66 with the same fixes for the maint-06 branch (v0.6.x).

Affected Versions

• v0.1.0 to v0.5.4
• v0.6.0 to v0.6.3. (Fixed in v0.6.4)
• v0.7.0. (Fixed in v0.7.1)

Notes

We are not yanking the affected versions at crates.io although this issue will have impact to almost all Moka users, and may lead out of memory error in some use cases. This is because memory leak is not considered a memory safety issue and does not lead undefined behavior (UB).

Instead, we are doing patch releases v0.6.4 and v0.7.1 with the fixes, so that existing v0.6 and v0.7 users will automatically move to one of these fixed versions when cargo update is executed.

---

Fixes #64.

[tatsuya6502]

Merging.