Exercise: Reading and writing database contents

Now that you have an open database, it's time to interact with it by storing and retrieving data.

Preparation

You'll need a few more items from the sys crate:

  • leveldb_readoptions_t: opaque type to specify read operation options
  • leveldb_writeoptions_t: opaque type to specify write operation options
  • leveldb_readoptions_create: creates a default readoptions_t
  • leveldb_readoptions_destroy: deallocates readoptions_t
  • leveldb_writeoptions_create: creates a default writeoptions_t
  • leveldb_writeoptions_destroy: deallocates writeoptions_t
  • leveldb_put: writes a binary value for a given binary key
  • leveldb_get: reads a binary value for a given binary key. Returns a null pointer for "not found", an owned object otherwise.
  • leveldb_free: deallocates a value object returned by leveldb_get

Your tasks

✅ Implement two functions on your Database type:

  • pub fn put(&self, key: &[u8], data: &[u8]) -> Result<(), Error>
  • pub fn get(&self, key: &[u8]) -> Result<Option<Box<[u8]>>, Error>

Be mindful of the API's ownership contract.

✅ Test your implementation.

Help and hints

  • You only need to create (and destroy!) the read/write options objects, not configure them further in any way.
  • Stuck with the wrong primitive type? Try casting it.
  • b"a string" is literal syntax for creating an &[u8] slice.
  • Here's how to put a slice on the heap (AKA box it): 
    let slice = std::slice::from_raw_parts(data as *mut u8, len);
let result = Box::from(slice);
    ❗ note that by creating a Box you've copied the data and still own (and therefore have to free) the data behind the raw pointer.
  • don't free null pointers.