Add 'key_may_exist' functions family (#518)

src/db.rs

@@ -844,6 +844,62 @@ impl<T: ThreadMode> DBWithThreadMode<T> {
         Ok(convert_values(values, values_sizes))
     }
 
+    /// Returns `false` if the given key definitely doesn't exist in the database, otherwise returns
+    /// `false`. This function uses default `ReadOptions`.
+    pub fn key_may_exist<K: AsRef<[u8]>>(&self, key: K) -> bool {
+        self.key_may_exist_opt(key, &ReadOptions::default())
+    }
+
+    /// Returns `false` if the given key definitely doesn't exist in the database, otherwise returns
+    /// `false`.
+    pub fn key_may_exist_opt<K: AsRef<[u8]>>(&self, key: K, readopts: &ReadOptions) -> bool {
+        let key = key.as_ref();
+        unsafe {
+            0 != ffi::rocksdb_key_may_exist(
+                self.inner,
+                readopts.inner,
+                key.as_ptr() as *const c_char,
+                key.len() as size_t,
+                ptr::null_mut(), /*value*/
+                ptr::null_mut(), /*val_len*/
+                ptr::null(),     /*timestamp*/
+                0,               /*timestamp_len*/
+                ptr::null_mut(), /*value_found*/
+            )
+        }
+    }
+
+    /// Returns `false` if the given key definitely doesn't exist in the specified column family,
+    /// otherwise returns `false`. This function uses default `ReadOptions`.
+    pub fn key_may_exist_cf<K: AsRef<[u8]>>(&self, cf: impl AsColumnFamilyRef, key: K) -> bool {
+        self.key_may_exist_cf_opt(cf, key, &ReadOptions::default())
+    }
+
+    /// Returns `false` if the given key definitely doesn't exist in the specified column family,
+    /// otherwise returns `false`.
+    pub fn key_may_exist_cf_opt<K: AsRef<[u8]>>(
+        &self,
+        cf: impl AsColumnFamilyRef,
+        key: K,
+        readopts: &ReadOptions,
+    ) -> bool {
+        let key = key.as_ref();
+        0 != unsafe {
+            ffi::rocksdb_key_may_exist_cf(
+                self.inner,
+                readopts.inner,
+                cf.inner(),
+                key.as_ptr() as *const c_char,
+                key.len() as size_t,
+                ptr::null_mut(), /*value*/
+                ptr::null_mut(), /*val_len*/
+                ptr::null(),     /*timestamp*/
+                0,               /*timestamp_len*/
+                ptr::null_mut(), /*value_found*/
+            )
+        }
+    }
+
     fn create_inner_cf_handle(
         &self,
         name: &str,

tests/test_db.rs

@@ -966,6 +966,39 @@ fn multi_get_cf() {
     }
 }
 
+#[test]
+fn key_may_exist() {
+    let path = DBPath::new("_rust_rocksdb_multi_get");
+
+    {
+        let db = DB::open_default(&path).unwrap();
+        assert_eq!(false, db.key_may_exist("nonexistent"));
+        assert_eq!(
+            false,
+            db.key_may_exist_opt("nonexistent", &ReadOptions::default())
+        );
+    }
+}
+
+#[test]
+fn key_may_exist_cf() {
+    let path = DBPath::new("_rust_rocksdb_multi_get_cf");
+
+    {
+        let mut opts = Options::default();
+        opts.create_if_missing(true);
+        opts.create_missing_column_families(true);
+        let db = DB::open_cf(&opts, &path, &["cf"]).unwrap();
+        let cf = db.cf_handle("cf").unwrap();
+
+        assert_eq!(false, db.key_may_exist_cf(cf, "nonexistent"));
+        assert_eq!(
+            false,
+            db.key_may_exist_cf_opt(cf, "nonexistent", &ReadOptions::default())
+        );
+    }
+}
+
 #[test]
 fn test_snapshot_outlive_db() {
     let t = trybuild::TestCases::new();