hive-priv: test seams are traits, not closure bags

hive-priv/src/shared.rs

@@ -7,6 +7,7 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{Context as _, Result, bail};
 use hive_priv_sock::AGENT_PREFIX;
+use tokio::process::Command;
 
 /// Return the system container name for a logical agent name.
 /// All agents (including the manager) use the `h-` prefix.
@@ -210,13 +211,60 @@ impl Drop for StagedFile<'_> {
     }
 }
 
-/// Scratch-directory helpers shared by every module's tests. One counter for
+/// How an op starts a host program. [`RealSpawn`] in production; tests
+/// substitute stand-ins, so no root, container or btrfs filesystem is needed.
+pub(crate) trait Spawn {
+    /// The `program` command, ready for arguments.
+    fn command(&self, program: &str) -> Command;
+}
+
+/// Starts the program `program` names on `PATH`.
+pub(crate) struct RealSpawn;
+
+impl Spawn for RealSpawn {
+    fn command(&self, program: &str) -> Command {
+        Command::new(program)
+    }
+}
+
+/// Scratch directories and stand-in tools shared by every module's tests. One counter for
 /// the whole crate, so no two tests get the same directory.
 #[cfg(test)]
 pub(crate) mod test_support {
     use std::path::PathBuf;
     use std::sync::atomic::{AtomicU32, Ordering};
 
+    use tokio::process::Command;
+
+    use super::Spawn;
+
+    /// Runs every program as `sh -c <script> <program> <args…>`.
+    pub(crate) struct ScriptSpawn(pub(crate) String);
+
+    impl Spawn for ScriptSpawn {
+        fn command(&self, program: &str) -> Command {
+            let mut cmd = Command::new("sh");
+            cmd.arg("-c").arg(&self.0).arg(program);
+            cmd
+        }
+    }
+
+    /// Starts a program that does not exist, whatever is asked for.
+    pub(crate) struct MissingTool;
+
+    impl Spawn for MissingTool {
+        fn command(&self, _: &str) -> Command {
+            Command::new("/nonexistent/hive-priv-test-tool")
+        }
+    }
+
+    /// A borrowed stand-in, so one stub can back several fixtures.
+    impl<T: Spawn> Spawn for &T {
+        fn command(&self, program: &str) -> Command {
+            (**self).command(program)
+        }
+    }
+
     /// Unique scratch dir per test, no external tempfile dep.
     pub(crate) fn scratch() -> PathBuf {
         static CTR: AtomicU32 = AtomicU32::new(0);