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/memory.rs

@@ -1,4 +1,4 @@
-use std::io::Cursor;
+use std::io::{self, Cursor};
 use std::{
     collections::HashMap,
     sync::{Arc, RwLock},
@@ -14,63 +14,102 @@ pub struct MemoryBlobService {
 }
 
 impl BlobService for MemoryBlobService {
-    type BlobReader = Cursor<Vec<u8>>;
-    type BlobWriter = MemoryBlobWriter;
-
     #[instrument(skip(self, digest), fields(blob.digest=%digest))]
     fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
         let db = self.db.read().unwrap();
         Ok(db.contains_key(digest))
     }
 
-    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> {
         let db = self.db.read().unwrap();
 
-        Ok(db.get(digest).map(|x| Cursor::new(x.clone())))
+        match db.get(digest).map(|x| Cursor::new(x.clone())) {
+            Some(result) => Ok(Some(Box::new(result))),
+            None => Ok(None),
+        }
     }
 
     #[instrument(skip(self))]
-    fn open_write(&self) -> Result<Self::BlobWriter, Error> {
-        Ok(MemoryBlobWriter::new(self.db.clone()))
+    fn open_write(&self) -> Result<Box<dyn BlobWriter>, Error> {
+        Ok(Box::new(MemoryBlobWriter::new(self.db.clone())))
     }
 }
 
 pub struct MemoryBlobWriter {
     db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>,
 
-    buf: Vec<u8>,
+    /// Contains the Vec and hasher, or None if already closed
+    writers: Option<(Vec<u8>, blake3::Hasher)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
 }
 
 impl MemoryBlobWriter {
     fn new(db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>) -> Self {
         Self {
-            buf: Vec::new(),
             db,
+            writers: Some((Vec::new(), blake3::Hasher::new())),
+            digest: None,
         }
     }
 }
 impl std::io::Write for MemoryBlobWriter {
-    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
-        self.buf.write(buf)
+    fn write(&mut self, b: &[u8]) -> std::io::Result<usize> {
+        match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some((ref mut buf, ref mut hasher)) => {
+                let bytes_written = buf.write(b)?;
+                hasher.write(&buf[..bytes_written])
+            }
+        }
     }
 
     fn flush(&mut self) -> std::io::Result<()> {
-        self.buf.flush()
+        match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some(_) => Ok(()),
+        }
     }
 }
 
 impl BlobWriter for MemoryBlobWriter {
-    fn close(self) -> Result<B3Digest, Error> {
-        // in this memory implementation, we don't actually bother hashing
-        // incrementally while writing, but do it at the end.
-        let mut hasher = blake3::Hasher::new();
-        hasher.update(&self.buf);
-        let digest = B3Digest::from_vec(hasher.finalize().as_bytes().to_vec()).unwrap();
+    fn close(&mut self) -> Result<B3Digest, Error> {
+        if self.writers.is_none() {
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (buf, hasher) = self.writers.take().unwrap();
 
-        // open the database for writing.
-        let mut db = self.db.write()?;
-        db.insert(digest.clone(), self.buf);
+            // We know self.hasher is doing blake3 hashing, so this won't fail.
+            let digest = B3Digest::from_vec(hasher.finalize().as_bytes().to_vec()).unwrap();
 
-        Ok(digest)
+            // Only insert if the blob doesn't already exist.
+            let db = self.db.read()?;
+            if !db.contains_key(&digest) {
+                // drop the read lock, so we can open for writing.
+                drop(db);
+
+                // open the database for writing.
+                let mut db = self.db.write()?;
+
+                // and put buf in there. This will move buf out.
+                db.insert(digest.clone(), buf);
+            }
+
+            self.digest = Some(digest.clone());
+
+            Ok(digest)
+        }
     }
 }