First implementation of K2V (#293)

**Specification:**

View spec at [this URL](https://git.deuxfleurs.fr/Deuxfleurs/garage/src/branch/k2v/doc/drafts/k2v-spec.md)

- [x] Specify the structure of K2V triples
- [x] Specify the DVVS format used for causality detection
- [x] Specify the K2V index (just a counter of number of values per partition key)
- [x] Specify single-item endpoints: ReadItem, InsertItem, DeleteItem
- [x] Specify index endpoint: ReadIndex
- [x] Specify multi-item endpoints: InsertBatch, ReadBatch, DeleteBatch
- [x] Move to JSON objects instead of tuples
- [x] Specify endpoints for polling for updates on single values (PollItem)

**Implementation:**

- [x] Table for K2V items, causal contexts
- [x] Indexing mechanism and table for K2V index
- [x] Make API handlers a bit more generic
- [x] K2V API endpoint
- [x] K2V API router
- [x] ReadItem
- [x] InsertItem
- [x] DeleteItem
- [x] PollItem
- [x] ReadIndex
- [x] InsertBatch
- [x] ReadBatch
- [x] DeleteBatch

**Testing:**

- [x] Just a simple Python script that does some requests to check visually that things are going right (does not contain parsing of results or assertions on returned values)
- [x] Actual tests:
  - [x] Adapt testing framework
  - [x] Simple test with InsertItem + ReadItem
  - [x] Test with several Insert/Read/DeleteItem + ReadIndex
  - [x] Test all combinations of return formats for ReadItem
  - [x] Test with ReadBatch, InsertBatch, DeleteBatch
  - [x] Test with PollItem
  - [x] Test error codes
- [ ] Fix most broken stuff
  - [x] test PollItem broken randomly
  - [x] when invalid causality tokens are given, errors should be 4xx not 5xx

**Improvements:**

- [x] Descending range queries
  - [x] Specify
  - [x] Implement
  - [x] Add test
- [x] Batch updates to index counter
- [x] Put K2V behind `k2v` feature flag

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/293
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/util/Cargo.toml

@@ -41,3 +41,6 @@ http = "0.2"
 hyper = "0.14"
 
 opentelemetry = { version = "0.17", features = [ "rt-tokio", "metrics", "trace" ] }
+
+[features]
+k2v = []

src/util/config.rs

@@ -73,7 +73,11 @@ pub struct Config {
 	pub sled_flush_every_ms: u64,
 
 	/// Configuration for S3 api
-	pub s3_api: ApiConfig,
+	pub s3_api: S3ApiConfig,
+
+	/// Configuration for K2V api
+	#[cfg(feature = "k2v")]
+	pub k2v_api: Option<K2VApiConfig>,
 
 	/// Configuration for serving files as normal web server
 	pub s3_web: WebConfig,
@@ -85,7 +89,7 @@ pub struct Config {
 
 /// Configuration for S3 api
 #[derive(Deserialize, Debug, Clone)]
-pub struct ApiConfig {
+pub struct S3ApiConfig {
 	/// Address and port to bind for api serving
 	pub api_bind_addr: SocketAddr,
 	/// S3 region to use
@@ -95,6 +99,14 @@ pub struct ApiConfig {
 	pub root_domain: Option<String>,
 }
 
+/// Configuration for K2V api
+#[cfg(feature = "k2v")]
+#[derive(Deserialize, Debug, Clone)]
+pub struct K2VApiConfig {
+	/// Address and port to bind for api serving
+	pub api_bind_addr: SocketAddr,
+}
+
 /// Configuration for serving files as normal web server
 #[derive(Deserialize, Debug, Clone)]
 pub struct WebConfig {

src/util/error.rs

@@ -44,6 +44,9 @@ pub enum Error {
 	#[error(display = "Tokio semaphore acquire error: {}", _0)]
 	TokioSemAcquire(#[error(source)] tokio::sync::AcquireError),
 
+	#[error(display = "Tokio broadcast receive error: {}", _0)]
+	TokioBcastRecv(#[error(source)] tokio::sync::broadcast::error::RecvError),
+
 	#[error(display = "Remote error: {}", _0)]
 	RemoteError(String),