spawn errors: impl HasKind for futures::SpawnError

This needs two kinds.  We have decided to treat a non-shutdown
SpawnError as "unexplained" rather than as an InternalError.

There are many crates whose
  From<futures::task::SpawnError> for Error
erroneously treat it as an internal error.  We will fix them in a moment.

Cargo.lock

@@ -3078,6 +3078,8 @@ version = "0.0.1"
 dependencies = [
  "backtrace",
  "derive_more",
+ "futures",
+ "thiserror",
 ]
 
 [[package]]
@@ -3107,6 +3109,7 @@ dependencies = [
  "serde",
  "thiserror",
  "tor-config",
+ "tor-error",
  "tor-linkspec",
  "tor-llcrypto",
  "tor-netdir",

crates/tor-error/Cargo.toml

@@ -16,5 +16,7 @@ default = ["backtrace"]
 [dependencies]
 backtrace = { version = "0.3", optional = true }
 derive_more = "0.99"
+futures = "0.3"
+thiserror = "1"
 
 [dev-dependencies]

crates/tor-error/src/lib.rs

@@ -77,6 +77,14 @@ pub enum ErrorKind {
     #[display(fmt = "could not write to read-only persistent state")]
     PersistentStateReadOnly,
 
+    /// Tor client's Rust async reactor is shutting down
+    #[display(fmt = "shutting down")]
+    ReactorShuttingDown,
+
+    /// Tor client's Rust async reactor could not spawn a task for unexplained reasons
+    #[display(fmt = "unexplained rust async task spawn failure")]
+    UnexplainedTaskSpawnFailure,
+
     /// Internal error (bug)
     ///
     /// A supposedly impossible problem has arisen.  This indicates a bug in Arti.
@@ -89,3 +97,14 @@ pub trait HasKind {
     /// The kind
     fn kind(&self) -> ErrorKind;
 }
+
+impl HasKind for futures::task::SpawnError {
+    fn kind(&self) -> ErrorKind {
+        use ErrorKind as EK;
+        if self.is_shutdown() {
+            EK::ReactorShuttingDown
+        } else {
+            EK::UnexplainedTaskSpawnFailure
+        }
+    }
+}

crates/tor-guardmgr/Cargo.toml

@@ -19,6 +19,7 @@ testing = []
 
 [dependencies]
 tor-config = { path="../tor-config", version = "0.0.4"}
+tor-error = { path="../tor-error", version = "0.0.1" }
 tor-netdir = { path="../tor-netdir", version = "0.0.4"}
 tor-linkspec = { path="../tor-linkspec", version = "0.0.3"}
 tor-llcrypto = { path="../tor-llcrypto", version = "0.0.3"}