feat(tvix/store): eliminate generics in BlobStore

To construct various stores at runtime, we need to eliminate associated
types from the BlobService trait, and return Box<dyn …> instead of
specific types.

This also means we can't consume self in the close() method, so
everything we write to is put in an Option<>, and during the first close
we take from there.

Change-Id: Ia523b6ab2f2a5276f51cb5d17e81a5925bce69b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8647
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/blobservice/mod.rs

@@ -14,28 +14,25 @@ pub use self::sled::SledBlobService;
 /// It provides functions to check whether a given blob exists,
 /// a way to get a [io::Read] to a blob, and a method to initiate writing a new
 /// Blob, which returns a [BlobWriter], that can be used
-pub trait BlobService {
-    type BlobReader: io::Read + Send + std::marker::Unpin;
-    type BlobWriter: BlobWriter + Send;
-
+pub trait BlobService: Send + Sync {
     /// Check if the service has the blob, by its content hash.
     fn has(&self, digest: &B3Digest) -> Result<bool, Error>;
 
     /// Request a blob from the store, by its content hash. Returns a Option<BlobReader>.
-    fn open_read(&self, digest: &B3Digest) -> Result<Option<Self::BlobReader>, Error>;
+    fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn io::Read + Send>>, Error>;
 
     /// Insert a new blob into the store. Returns a [BlobWriter], which
     /// implements [io::Write] and a [BlobWriter::close].
     /// TODO: is there any reason we want this to be a Result<>, and not just T?
-    fn open_write(&self) -> Result<Self::BlobWriter, Error>;
+    fn open_write(&self) -> Result<Box<dyn BlobWriter>, Error>;
 }
 
 /// A [io::Write] that you need to close() afterwards, and get back the digest
 /// of the written blob.
-pub trait BlobWriter: io::Write {
+pub trait BlobWriter: io::Write + Send + Sync + 'static {
     /// Signal there's no more data to be written, and return the digest of the
     /// contents written.
     ///
-    /// This consumes self, so it's not possible to close twice.
-    fn close(self) -> Result<B3Digest, Error>;
+    /// Closing a already-closed BlobWriter is a no-op.
+    fn close(&mut self) -> Result<B3Digest, Error>;
 }