worker: add option to mount cached input directories as read-only

.github/styles/config/vocabularies/TraceMachina/accept.txt

@@ -317,6 +317,7 @@ deserialization
 Dockerised
 Merkle
 subtree
+subtrees
 hardlink
 multiplicatively
 SELinux

nativelink-config/src/cas_server.rs

@@ -1538,6 +1538,42 @@ pub struct LocalWorkerConfig {
     #[serde(default)]
     pub experimental_active_input_leases: bool,
 
+    /// Reuse immutable input directories through read-only bind mounts on Linux.
+    /// The worker prepares each selected subtree once per REAPI `Directory`
+    /// digest in the directory cache, then mounts it into each action instead of
+    /// recreating its filesystem entries.
+    ///
+    /// Paths are relative to the action input root, for example `["out/x/sdk"]`.
+    /// They must be nonempty, canonical, and non-overlapping. Absolute paths,
+    /// empty path components, and `.`/`..` components are rejected.
+    /// Requires `directory_cache`, `use_namespaces: true`, and
+    /// `use_mount_namespace: true`.
+    ///
+    /// Actions declaring overlapping outputs, working inside a selected subtree,
+    /// or using a persistent worker fall back to ordinary materialization.
+    /// Output paths reached through symlink aliases cannot be checked for
+    /// overlap. Use this only with trusted workloads that do not modify the
+    /// selected trees; read-only input mounts are not a security boundary.
+    ///
+    /// Select trees whose contents (and Directory digest) are reused across
+    /// many actions. Trees that change per action add cache construction work
+    /// without reuse. When mounts apply, other inputs use ordinary downloads,
+    /// bypassing both root and subtree directory caching for that action.
+    /// Failed mount preparation retries once without mounts; a failed mount
+    /// syscall still aborts the action before its command runs.
+    ///
+    /// An action can remount its inputs writable in its own user namespace
+    /// and corrupt the shared cache. Later actions can then produce incorrect
+    /// results from those inputs. Enable only for trusted commands.
+    ///
+    /// Cache entries remain pinned until action cleanup and may temporarily
+    /// exceed the cache's eviction budget. Pinned bytes have no separate limit,
+    /// so concurrent distinct trees can exhaust the disk. Leave headroom.
+    ///
+    /// Default: [] (disabled)
+    #[serde(default)]
+    pub experimental_readonly_input_mounts: Vec<String>,
+
     /// Whether to use namespaces to isolate the execution. This is only available
     /// on Linux. It is highly recommended as it avoids a number of issues with
     /// zombie processes and also provides additional hermeticity. If explicitly set

nativelink-config/src/schedulers.rs

@@ -375,7 +375,7 @@ pub struct SimpleSpec {
     /// operation from the store, on every scheduler replica: with a shared
     /// Redis that is a full index read per interval per replica, which is
     /// why it is off unless asked for. Turn it on for a look, not for good.
-    /// Defaults to -1 (disabled). Any positive value enables it.
+    /// Defaults to `-1` (disabled). Any positive value enables it.
     #[serde(
         default = "default_worker_match_logging_interval_s",
         deserialize_with = "convert_duration_with_shellexpand_and_negative"

nativelink-util/src/metrics.rs

@@ -888,6 +888,43 @@ pub fn execution_output_bytes(action_result: &ActionResult) -> u64 {
         + action_result.stderr_digest.size_bytes()
 }
 
+/// Input mount counters, emitted by workers rather than schedulers.
+pub static INPUT_MOUNT_METRICS: LazyLock<InputMountMetrics> = LazyLock::new(|| {
+    let meter = global::meter_with_scope(InstrumentationScope::builder("nativelink").build());
+    InputMountMetrics {
+        cache_hits: meter
+            .u64_counter("worker.input_mount.cache_hits")
+            .with_description("Cached directories reused for input mount preparation")
+            .build(),
+        cache_misses: meter
+            .u64_counter("worker.input_mount.cache_misses")
+            .with_description("Input mount directories requiring cache construction")
+            .build(),
+        fallbacks: meter
+            .u64_counter("worker.input_mount.fallbacks")
+            .with_description(
+                "Actions using ordinary input preparation despite configured input mounts",
+            )
+            .build(),
+        prepare_failures: meter
+            .u64_counter("worker.input_mount.prepare_failures")
+            .with_description("Failed input mount preparations retried without mounts")
+            .build(),
+    }
+});
+
+#[derive(Debug)]
+pub struct InputMountMetrics {
+    /// Cached directories reused for input mount preparation.
+    pub cache_hits: metrics::Counter<u64>,
+    /// Input mount directories requiring cache construction.
+    pub cache_misses: metrics::Counter<u64>,
+    /// Actions using ordinary input preparation despite configured input mounts.
+    pub fallbacks: metrics::Counter<u64>,
+    /// Failed input mount preparations retried without mounts.
+    pub prepare_failures: metrics::Counter<u64>,
+}
+
 /// Global worker fleet metrics instruments.
 pub static WORKER_METRICS: LazyLock<WorkerMetrics> = LazyLock::new(|| {
     let meter = global::meter_with_scope(InstrumentationScope::builder("nativelink").build());

nativelink-worker/BUILD.bazel

@@ -13,6 +13,7 @@ rust_library(
         "src/buck2_file_capture.rs",
         "src/capacity.rs",
         "src/directory_cache.rs",
+        "src/input_mounts.rs",
         "src/lib.rs",
         "src/local_worker.rs",
         "src/persistent_worker/live_worker.rs",