Use 'tokio_executor::blocking' when acquiring connections from synchronous database pools to avoid tying up the executor.

contrib/codegen/src/database.rs

@@ -73,6 +73,7 @@ pub fn database_attr(attr: TokenStream, input: TokenStream) -> Result<TokenStrea
     let databases = quote_spanned!(span => ::rocket_contrib::databases);
     let Poolable = quote_spanned!(span => #databases::Poolable);
     let r2d2 = quote_spanned!(span => #databases::r2d2);
+    let tokio_executor = quote_spanned!(span => #databases::tokio_executor);
     let request = quote!(::rocket::request);
 
     let generated_types = quote_spanned! { span =>
@@ -140,17 +141,21 @@ pub fn database_attr(attr: TokenStream, input: TokenStream) -> Result<TokenStrea
             }
         }
 
-        impl<'a, 'r> #request::FromRequest<'a, 'r> for #guard_type {
+        impl<'a, 'r> #request::FromRequestAsync<'a, 'r> for #guard_type {
             type Error = ();
 
-            fn from_request(request: &'a #request::Request<'r>) -> #request::Outcome<Self, ()> {
+            fn from_request<'fut>(request: &'a #request::Request<'r>) -> #request::FromRequestFuture<'fut, Self, Self::Error> where 'a: 'fut {
                 use ::rocket::{Outcome, http::Status};
-                let pool = ::rocket::try_outcome!(request.guard::<::rocket::State<#pool_type>>());
+                Box::pin(async move {
+                    let pool = ::rocket::try_outcome!(request.guard::<::rocket::State<'_, #pool_type>>()).0.clone();
 
-                match pool.0.get() {
+                    #tokio_executor::blocking::run(move || {
+                        match pool.get() {
                             Ok(conn) => Outcome::Success(#guard_type(conn)),
                             Err(_) => Outcome::Failure((Status::ServiceUnavailable, ())),
                         }
+                    }).await
+                })
             }
         }
     }.into())

contrib/lib/Cargo.toml

@@ -14,7 +14,7 @@ edition = "2018"
 [features]
 # Internal use only.
 templates = ["serde", "serde_json", "glob", "notify"]
-databases = ["r2d2", "rocket_contrib_codegen/database_attribute"]
+databases = ["r2d2", "tokio-executor", "rocket_contrib_codegen/database_attribute"]
 
 # User-facing features.
 default = ["json", "serve"]
@@ -66,6 +66,7 @@ uuid = { version = ">=0.7.0, <0.9.0", optional = true }
 diesel = { version = "1.0", default-features = false, optional = true }
 postgres = { version = "0.17", optional = true }
 r2d2 = { version = "0.8", optional = true }
+tokio-executor = { version = "0.2.0-alpha.6", optional = true }
 r2d2_postgres = { version = "0.16", optional = true }
 mysql = { version = "17.0", optional = true }
 r2d2_mysql = { version = "17.0", optional = true }

contrib/lib/src/databases.rs

@@ -397,6 +397,9 @@
 
 pub extern crate r2d2;
 
+#[doc(hidden)]
+pub extern crate tokio_executor;
+
 #[cfg(any(feature = "diesel_sqlite_pool",
           feature = "diesel_postgres_pool",
           feature = "diesel_mysql_pool"))]