store: use block_on from pollster when reading conflicts

This avoids rust-lang/futures-rs#2090. I
don't think we need to worry about reading legacy conflicts
asynchronously - async is really only useful for Google's backend
right now, and we don't use the legacy format at Google. In
particular, I don't want `MergedTree::value()` to have to be async.

It's unfortunate that we need another dependency just for this, but
it's a tiny crate at least.

Cargo.toml

@@ -53,6 +53,7 @@ num_cpus = "1.16.0"
 once_cell = "1.18.0"
 pest = "2.7.5"
 pest_derive = "2.7.5"
+pollster = "0.3.0"
 pretty_assertions = "1.4.0"
 prost = "0.11.9"
 prost-build = "0.11.9"

lib/Cargo.toml

@@ -19,7 +19,7 @@ harness = false
 version_check = { workspace = true }
 
 [dependencies]
-async-trait = { workspace = true}
+async-trait = { workspace = true }
 backoff = { workspace = true }
 blake2 = { workspace = true }
 byteorder = { workspace = true }
@@ -37,6 +37,7 @@ maplit = { workspace = true }
 once_cell = { workspace = true }
 pest = { workspace = true }
 pest_derive = { workspace = true }
+pollster = { workspace = true }
 prost = { workspace = true }
 rand = { workspace = true }
 rand_chacha = { workspace = true }

lib/src/store.rs

@@ -21,6 +21,7 @@ use std::io::Read;
 use std::sync::{Arc, RwLock};
 
 use futures::executor::block_on;
+use pollster::FutureExt as _;
 
 use crate::backend;
 use crate::backend::{
@@ -229,7 +230,7 @@ impl Store {
         path: &RepoPath,
         id: &ConflictId,
     ) -> BackendResult<MergedTreeValue> {
-        let backend_conflict = block_on(self.backend.read_conflict(path, id))?;
+        let backend_conflict = self.backend.read_conflict(path, id).block_on()?;
         Ok(Merge::from_backend_conflict(backend_conflict))
     }