try_get_with_by_ref always triggers init future

[dannygale]

I'm having some trouble with try_get_with_by_ref and similar functions, and i'm not sure if it's something i'm doing wrong, or if I've found a bug. The issue is that try_get_with_by_ref always triggers the init future, even when contains_key returns true in the init future itself. To debug this issue, I'm running on a single-threaded tokio runtime

self.mdcache is a moka::future::Cache<PathBuf, MyStruct>

This code:

        let key: PathBuf = self.path.clone();
        let metadata = self
            .mdcache
            .try_get_with_by_ref(
                &key,
                async {
                    tracing::debug!(
                        "mdcache miss ({})",
                        if self.mdcache.contains_key(&key) {
                            "NOT expected"
                        } else {
                            "expected"
                        },
                    );
                    self.file.read_metadata()
                }
                .await,
            )
            .await
            .unwrap();

Is producing this kind of output consistently -- the only "expected" miss is the first access. For all of the following accesses, contains_key is true, but the init function is getting called and awaited anyway, defeating the purpose of the cache. These are consecutive accesses for the same key, produced by the above snippet:

2022-12-06T17:52:33.327125Z DEBUG mdcache miss (expected)
2022-12-06T17:52:33.330107Z DEBUG mdcache miss (NOT expected)
2022-12-06T17:52:33.332269Z DEBUG mdcache miss (NOT expected) 
...

The following code works as expected, but loses the guarantee of a single in-flight init task for a given key:

        match self.mdcache.get(&self.path) {
            Some(md) => Ok(md),
            None => {
                tracing::debug!("mdcache miss");
                let md = self.file.read_metadata().await?;
                self.mdcache.insert(self.path.clone(), md.clone()).await;

                Ok(md)
            }
        }

Am I misunderstanding how try_get_with and friends are supposed to work, am I misusing it, or is this an issue with the cache implementation?

[dannygale]

Pretty sure this is a cache implementation issue. Created #208

[tatsuya6502]

Thank you for filing #208. I reviewed but found a problem in the reproducing code. I explained how to fix it. I closed the issue as I believe try_get_with_by_ref is working correctly. Please reopen it if needed.

[tatsuya6502]

Thank you for reporting. I reviewed your original code, and I believe you have an extra .await, which causes the async block executed unconditionally before being passed to try_get_with_by_ref(). Please remove it.

        let metadata = self
            .mdcache
            .try_get_with_by_ref(
                &key,
                async {
                    tracing::debug!(
                        "mdcache miss ({})",
                        if self.mdcache.contains_key(&key) {
                            "NOT expected"
                        } else {
                            "expected"
                        },
                    );
                    self.file.read_metadata()
                }
                // Please remove this await. This causes the async block above executed
                // unconditionally _before_ being passed to try_get_with_by_ref().
                .await, 
            )
            .await
            .unwrap();

[tatsuya6502]

I am marking this Q&A as answered. Please let me know if you need any further assistance.