impl: backends independent from filesystems

- New `KvsBackend` API - independent from filesystem.
- Make `JsonBackend` explicitly default, allow others.
- Add `JsonBackendBuilder`.
  - This is required to allow for defaults override without setters.
  - E.g., `snapshot_max_count` can be set, but shouldn't change after
    init.
- Update tests.

Author: arkjedrz

src/rust/rust_kvs/examples/basic.rs

@@ -8,9 +8,10 @@ use std::collections::HashMap;
 use tempfile::tempdir;
 
 fn main() -> Result<(), ErrorCode> {
-    // Temporary directory.
+    // Temporary directory and common backend.
     let dir = tempdir()?;
-    let dir_string = dir.path().to_string_lossy().to_string();
+    let dir_path = dir.path().to_path_buf();
+    let backend = Box::new(JsonBackendBuilder::new().working_dir(dir_path).build());
 
     // Instance ID for KVS object instances.
     let instance_id = InstanceId(0);
@@ -20,7 +21,7 @@ fn main() -> Result<(), ErrorCode> {
         // `kvs_load` is explicitly set to `KvsLoad::Optional`, but this is the default value.
         // KVS files are not required.
         let builder = KvsBuilder::new(instance_id)
-            .dir(dir_string.clone())
+            .backend(backend.clone())
             .kvs_load(KvsLoad::Optional);
         let kvs = builder.build()?;
 
@@ -65,7 +66,7 @@ fn main() -> Result<(), ErrorCode> {
         // Build KVS instance for given instance ID and temporary directory.
         // `kvs_load` is set to `KvsLoad::Required` - KVS files must already exist from previous KVS instance.
         let builder = KvsBuilder::new(instance_id)
-            .dir(dir_string)
+            .backend(backend)
             .kvs_load(KvsLoad::Required);
         let kvs = builder.build()?;
 

src/rust/rust_kvs/examples/defaults.rs

@@ -31,9 +31,10 @@ fn create_defaults_file(dir_path: PathBuf, instance_id: InstanceId) -> Result<()
 }
 
 fn main() -> Result<(), ErrorCode> {
-    // Temporary directory.
+    // Temporary directory and common backend.
     let dir = tempdir()?;
-    let dir_string = dir.path().to_string_lossy().to_string();
+    let dir_path = dir.path().to_path_buf();
+    let backend = Box::new(JsonBackendBuilder::new().working_dir(dir_path).build());
 
     // Instance ID for KVS object instances.
     let instance_id = InstanceId(0);
@@ -44,7 +45,7 @@ fn main() -> Result<(), ErrorCode> {
     // Build KVS instance for given instance ID and temporary directory.
     // `defaults` is set to `KvsDefaults::Required` - defaults are required.
     let builder = KvsBuilder::new(instance_id)
-        .dir(dir_string)
+        .backend(backend.clone())
         .defaults(KvsDefaults::Required);
     let kvs = builder.build()?;
 

src/rust/rust_kvs/examples/snapshots.rs

@@ -6,9 +6,10 @@ use rust_kvs::prelude::*;
 use tempfile::tempdir;
 
 fn main() -> Result<(), ErrorCode> {
-    // Temporary directory.
+    // Temporary directory and common backend.
     let dir = tempdir()?;
-    let dir_string = dir.path().to_string_lossy().to_string();
+    let dir_path = dir.path().to_path_buf();
+    let backend = Box::new(JsonBackendBuilder::new().working_dir(dir_path).build());
 
     // Instance ID for KVS object instances.
     let instance_id = InstanceId(0);
@@ -17,7 +18,7 @@ fn main() -> Result<(), ErrorCode> {
         println!("-> `snapshot_count` and `snapshot_max_count` usage");
 
         // Build KVS instance for given instance ID and temporary directory.
-        let builder = KvsBuilder::new(instance_id).dir(dir_string.clone());
+        let builder = KvsBuilder::new(instance_id).backend(backend.clone());
         let kvs = builder.build()?;
 
         let max_count = kvs.snapshot_max_count() as u32;
@@ -38,7 +39,7 @@ fn main() -> Result<(), ErrorCode> {
         println!("-> `snapshot_restore` usage");
 
         // Build KVS instance for given instance ID and temporary directory.
-        let builder = KvsBuilder::new(instance_id).dir(dir_string.clone());
+        let builder = KvsBuilder::new(instance_id).backend(backend.clone());
         let kvs = builder.build()?;
 
         let max_count = kvs.snapshot_max_count() as u32;