WIP cloudstorage

taskchampion/taskchampion/src/server/cloud/cloudstorage.rs

@@ -0,0 +1,30 @@
+use crate::errors::Result;
+
+/// An abstraction of a cloud-storage service.
+///
+/// The underlying cloud storage is assumed to be a map from object names to object values,
+/// similar to a HashMap, with the addition of a compare-and-swap operation. Object names
+/// are always simple strings from the character set `[a-zA-Z0-9-]`, no more than 100 characters
+/// in length.
+pub(in crate::server::cloud) trait CloudStorage {
+    /// Put an object into cloud storage. If the object exists, it is overwritten.
+    fn put(&mut self, name: &[u8], value: &[u8]) -> Result<()>;
+
+    /// Get an object from cloud storage, or None if the object does not exist.
+    fn get(&mut self, name: &[u8]) -> Result<Option<Vec<u8>>>;
+
+    /// Delete an object. Does nothing if the object does not exist.
+    fn del(&mut self, name: &[u8]) -> Result<()>;
+
+    /// Enumerate objects with the given prefix.
+    fn list<'a>(&'a mut self, prefix: &'a [u8]) -> Result<Box<dyn Iterator<Item = &'a [u8]> + 'a>>;
+
+    /// Compare the existing object with `existing_value`, and replace with `new_value` only if the
+    /// values match. Returns true if the replacement occurred.
+    fn compare_and_swap(
+        &mut self,
+        name: &[u8],
+        existing_value: Option<Vec<u8>>,
+        new_value: Vec<u8>,
+    ) -> Result<bool>;
+}

taskchampion/taskchampion/src/server/cloud/mod.rs

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+/*!
+* Support for cloud-storage-backed sync.
+*
+* All of these operate using a similar approach, with specific patterns of object names. The
+* process of adding a new version requires a compare-and-swap operation that sets a new version
+* as the "latest" only if the existing "latest" has the expected value. This ensures a continuous
+* chain of versions, even if multiple replicas attempt to sync at the same time.
+*/
+
+mod cloudstorage;
+mod server;