Fix y-sweet path issue (#293)

When a y-sweet service is started in a session backend environment (i.e.
`serve-doc`), it expects to receive the document ID as
`SESSION_BACKEND_KEY`.

When an S3 bucket is attached for storage, an environment variable
`STORAGE_PREFIX` is also generated by combining some configured prefix
with the doc ID.

A problem arises because y-sweet already expects to concatenate the doc
ID when doing a lookup. The result is that we end up with the document
ID repeated in the path.

We would still like to respect `STORAGE_PREFIX` if it is provided, but
we want to avoid duplicating the doc ID. This PR:
- Ensures that the last /-delimited component of the `STORAGE_PREFIX` is
the doc ID, unless `STORAGE_PREFIX` is unset or empty
- Chops the doc ID off and constructs a new `STORAGE_PREFIX`.

Also, we no longer require `STORAGE_PREFIX` (since it is allowed to be
empty), but we do not allow `STORAGE_PREFIX` to be set unless
`STORAGE_BUCKET` also is.

I'd like to chop this code up more so that we can unit test it better,
but for now I've tested locally and it solves the issue I was having.

crates/y-sweet/src/main.rs

@@ -256,27 +256,38 @@ async fn main() -> Result<()> {
         } => {
             let doc_id = env::var("SESSION_BACKEND_KEY").expect("SESSION_BACKEND_KEY must be set");
 
-            let bucket = env::var("STORAGE_BUCKET").ok();
-            let prefix = env::var("STORAGE_PREFIX").ok();
-
-            let store = match (bucket, prefix) {
-                (Some(bucket), Some(prefix)) => {
-                    let prefix = (!prefix.is_empty()).then(|| prefix);
-                    let s3_config = parse_s3_config_from_env_and_args(bucket, prefix)
-                        .context("Failed to parse S3 configuration")?;
-                    let store = S3Store::new(s3_config);
-                    let store: Box<dyn Store> = Box::new(store);
-                    store
-                        .init()
-                        .await
-                        .context("Failed to initialize S3 store")?;
-                    Some(store)
-                }
-                (None, None) => {
-                    tracing::warn!("No store set. Documents will be stored in memory only.");
+            let store = if let Ok(bucket) = env::var("STORAGE_BUCKET") {
+                let prefix = if let Ok(prefix) = env::var("STORAGE_PREFIX") {
+                    // If the prefix is set, it should contain the document ID as its last '/'-separated part.
+                    // We want to pop that, because we will add it back when accessing the doc.
+                    let mut parts: Vec<&str> = prefix.split('/').collect();
+                    if let Some(last) = parts.pop() {
+                        if last != doc_id {
+                            anyhow::bail!("STORAGE_PREFIX must end with the document ID. Found: {} Expected: {}", last, doc_id);
+                        }
+
+                        let prefix = parts.join("/");
+
+                        Some(prefix)
+                    } else {
+                        // As far as y-sweet is concerned, `STORAGE_BUCKET` = "" is equivalent to `STORAGE_BUCKET` not being set.
+                        None
+                    }
+                } else {
                     None
+                };
+
+                let s3_config = parse_s3_config_from_env_and_args(bucket, prefix)?;
+                let store = S3Store::new(s3_config);
+                let store: Box<dyn Store> = Box::new(store);
+                store.init().await?;
+                Some(store)
+            } else {
+                if env::var("STORAGE_PREFIX").is_ok() {
+                    anyhow::bail!("If STORAGE_PREFIX is set, STORAGE_BUCKET must also be set.");
                 }
-                _ => panic!("Invalid store configuration. Expected both STORAGE_BUCKET and STORAGE_PREFIX environment variables to be set."),
+
+                None
             };
 
             let cancellation_token = CancellationToken::new();