Introduce an example to check the PREV_SNAPSHOT feature

examples/prev-snapshot.rs

@@ -0,0 +1,85 @@
+use std::error::Error;
+
+use heed::types::*;
+use heed::{Database, EnvFlags, EnvOpenOptions};
+
+// In this test we are checking that we can move to a previous environement snapshot.
+fn main() -> Result<(), Box<dyn Error>> {
+    let env_path = tempfile::tempdir()?;
+
+    let env = unsafe {
+        EnvOpenOptions::new()
+            .map_size(10 * 1024 * 1024) // 10MB
+            .max_dbs(3)
+            .open(&env_path)?
+    };
+
+    let mut wtxn = env.write_txn()?;
+    let db: Database<Str, Str> = env.create_database(&mut wtxn, None)?;
+
+    // We fill the db database with entries.
+    db.put(&mut wtxn, "I am here", "to test things")?;
+    db.put(&mut wtxn, "I am here too", "for the same purpose")?;
+
+    wtxn.commit()?;
+
+    env.prepare_for_closing().wait();
+
+    // We can get the env state from before the last commit
+    // and therefore see an empty env.
+    let env = unsafe {
+        EnvOpenOptions::new()
+            .map_size(10 * 1024 * 1024) // 10MB
+            .max_dbs(3)
+            .flags(EnvFlags::PREV_SNAPSHOT)
+            .open(&env_path)?
+    };
+
+    let mut wtxn = env.write_txn()?;
+    let db: Database<Str, Str> = env.create_database(&mut wtxn, None)?;
+
+    assert!(db.is_empty(&wtxn)?);
+
+    wtxn.abort();
+    env.prepare_for_closing().wait();
+
+    // However, if we don't commit we can still get
+    // back the latest version of the env.
+    let env = unsafe {
+        EnvOpenOptions::new()
+            .map_size(10 * 1024 * 1024) // 10MB
+            .max_dbs(3)
+            .open(&env_path)?
+    };
+
+    let mut wtxn = env.write_txn()?;
+    let db: Database<Str, Str> = env.create_database(&mut wtxn, None)?;
+
+    assert_eq!(db.get(&wtxn, "I am here")?, Some("to test things"));
+    assert_eq!(db.get(&wtxn, "I am here too")?, Some("for the same purpose"));
+
+    // And write new stuff in the env.
+    db.put(&mut wtxn, "I will fade away", "I am so sad")?;
+
+    wtxn.commit()?;
+    env.prepare_for_closing().wait();
+
+    // Once again we can get back the previous version
+    // of the env and see some entries disappear.
+    let env = unsafe {
+        EnvOpenOptions::new()
+            .map_size(10 * 1024 * 1024) // 10MB
+            .max_dbs(3)
+            .flags(EnvFlags::PREV_SNAPSHOT)
+            .open(&env_path)?
+    };
+
+    let rtxn = env.read_txn()?;
+    let db: Database<Str, Str> = env.open_database(&rtxn, None)?.unwrap();
+
+    assert_eq!(db.get(&rtxn, "I am here")?, Some("to test things"));
+    assert_eq!(db.get(&rtxn, "I am here too")?, Some("for the same purpose"));
+    assert_eq!(db.get(&rtxn, "I will fade away")?, None);
+
+    Ok(())
+}

heed/Cargo.toml

@@ -131,6 +131,10 @@ path = "../examples/multi-env.rs"
 name = "nested"
 path = "../examples/nested.rs"
 
+[[example]]
+name = "prev-snapshot"
+path = "../examples/prev-snapshot.rs"
+
 [[example]]
 name = "rmp-serde"
 path = "../examples/rmp-serde.rs"

heed3/Cargo.toml

@@ -106,6 +106,10 @@ longer-keys = ["lmdb-master3-sys/longer-keys"]
 
 # Examples are located outside the standard heed/examples directory to prevent
 # conflicts between heed3 and heed examples when working on both crates.
+[[example]]
+name = "prev-snapshot"
+path = "../examples/prev-snapshot.rs"
+
 [[example]]
 name = "heed3-encrypted"
 path = "../examples/heed3-encrypted.rs"