refactor(hm-vm): ImageRegistry capacity is a bare u64 where 0 silently makes the cache evict every entry

Author: markovejnovic

crates/hm-exec/src/local/backend.rs

@@ -4,6 +4,7 @@
 //! drives the [`hm_vm`] subsystem. The VM backend (Docker, etc.) is injected;
 //! snapshot caching is owned by `hm-vm`'s [`hm_vm::ImageRegistry`].
 
+use std::num::NonZeroU64;
 use std::sync::Arc;
 
 use tokio::sync::mpsc;
@@ -16,7 +17,7 @@ use crate::{BackendError, BackendHandle, Capabilities, ExecutionBackend, Result,
 
 /// Number of cached snapshots the image registry retains before evicting
 /// least-recently-used entries.
-const REGISTRY_CAPACITY: u64 = 64;
+const REGISTRY_CAPACITY: NonZeroU64 = NonZeroU64::new(64).expect("64 is non-zero");
 
 /// Runs the build locally via the in-process DAG scheduler, executing each
 /// step inside a VM provided by the injected [`hm_vm::VmBackend`].

crates/hm-vm/src/registry.rs

@@ -5,6 +5,7 @@
 //! exceeded, returning the evicted snapshot IDs so the caller can clean up
 //! backend resources.
 
+use std::num::NonZeroU64;
 use std::path::Path;
 use std::time::{SystemTime, UNIX_EPOCH};
 
@@ -28,7 +29,7 @@ use crate::types::SnapshotId;
 pub struct ImageRegistry {
     #[debug(skip)]
     conn: Mutex<Connection>,
-    capacity: u64,
+    capacity: NonZeroU64,
 }
 
 /// Returns the current Unix epoch in seconds.
@@ -52,7 +53,7 @@ impl ImageRegistry {
     ///
     /// Returns an error if the database cannot be opened or the schema cannot
     /// be applied.
-    pub fn open(path: &Path, capacity: u64) -> Result<Self> {
+    pub fn open(path: &Path, capacity: NonZeroU64) -> Result<Self> {
         if let Some(parent) = path.parent() {
             std::fs::create_dir_all(parent)?;
         }
@@ -176,11 +177,12 @@ impl ImageRegistry {
     /// its capacity. Returns the snapshot IDs of evicted entries.
     fn evict_overflow(&self) -> Vec<SnapshotId> {
         let count = self.len();
-        if count <= self.capacity {
+        let capacity = self.capacity.get();
+        if count <= capacity {
             return Vec::new();
         }
 
-        let overflow = count - self.capacity;
+        let overflow = count - capacity;
 
         let Ok(conn) = self.conn.lock() else {
             return Vec::new();
@@ -221,6 +223,7 @@ mod tests {
     fn open_temp(capacity: u64) -> (ImageRegistry, tempfile::TempDir) {
         let dir = tempfile::tempdir().expect("failed to create temp dir");
         let db_path = dir.path().join("registry.db");
+        let capacity = NonZeroU64::new(capacity).expect("capacity must be non-zero");
         let registry = ImageRegistry::open(&db_path, capacity).expect("failed to open registry");
         (registry, dir)
     }
@@ -294,14 +297,16 @@ mod tests {
         let dir = tempfile::tempdir().expect("failed to create temp dir");
         let db_path = dir.path().join("registry.db");
 
+        let capacity = NonZeroU64::new(10).expect("capacity must be non-zero");
+
         {
-            let reg = ImageRegistry::open(&db_path, 10).expect("open");
+            let reg = ImageRegistry::open(&db_path, capacity).expect("open");
             reg.put("persistent", &SnapshotId("snap-persist".into()));
             assert_eq!(reg.len(), 1);
             // reg is dropped here, closing the connection.
         }
 
-        let reg2 = ImageRegistry::open(&db_path, 10).expect("reopen");
+        let reg2 = ImageRegistry::open(&db_path, capacity).expect("reopen");
         assert_eq!(reg2.len(), 1);
         let got = reg2.get("persistent");
         assert_eq!(got, Some(SnapshotId("snap-persist".into())));

crates/hm-vm/src/vm.rs

@@ -258,6 +258,8 @@ mod tests {
     fn open_temp_registry(capacity: u64) -> (ImageRegistry, tempfile::TempDir) {
         let dir = tempfile::tempdir().expect("failed to create temp dir");
         let db = dir.path().join("registry.db");
+        let capacity =
+            std::num::NonZeroU64::new(capacity).expect("capacity must be non-zero");
         let reg = ImageRegistry::open(&db, capacity).expect("failed to open registry");
         (reg, dir)
     }