chore(ecstore): drop the disk dead_code blanket (#6139)

* chore(ecstore): drop the disk dead_code blanket

Removing the blanket exposes 36 items in the lowest storage layer: 7 deleted, 29 kept with reasoned item-level allows. That is the smallest deletion share of this burn-down, and the reason is a verification limit rather than a judgement call.

disk/local.rs carries 141 `#[cfg(target_os = "linux")]` sites — the densest platform gating in the tree, because O_DIRECT and io_uring only exist there. The direct-I/O cluster (six ENV_RUSTFS_OBJECT_DIRECT_IO_* constants plus is_direct_io_read_enabled, is_direct_io_write_enabled, get_direct_io_read_threshold, direct_write_staging_capacity, direct_write_tail_split and DIRECT_WRITE_STAGING_BYTES) reads as dead on macOS purely because its production callers at local.rs:1766, 3114 and 4605 sit inside Linux-gated blocks. direct_write_staging_capacity even documents itself as "Platform-independent (no O_DIRECT), so it is unit-tested on any host".

Deleting those would leave every local check green — 4096 tests pass, clippy is clean, make pre-commit exits 0 — and break the Linux build in CI, because all four local lanes compile for aarch64-apple-darwin. Cross-checking locally is not available either: cargo check --target x86_64-unknown-linux-gnu fails in the aws-lc-sys build script for want of a Linux C cross-compiler. Their allows name the platform reason so the next reader on a non-Linux host does not repeat the investigation.

Deleted, all in files with no target_os gating at all (os.rs, disk_store.rs):

- HealthDiskCtxKey and HealthDiskCtxValue with its private log_success. Note that DiskHealthTracker::log_success is a different method of the same name and is live from cluster/rpc/peer_s3_client.rs and remote_disk.rs — the two have to be told apart by type, not by name.
- LocalDiskWrapper::new_with_health and check_id.
- os.rs file_exists and lock_destination_directory_for_path_access.

Kept with allows: DiskHealthTracker's set_faulty, mark_offline, waiting_count and last_success have test callers in remote_disk.rs, so they only look dead in the lib target. to_disk_error, remove_all and sync_dir_files are asserted by their own files' tests. The reclaim, mmap and path-cache field groups are written but never read back.

Placement follows the same rule as the earlier roots: per-method allows inside impl DiskHealthTracker and impl LocalDisk, since both are mostly live and a block-level allow would be a smaller version of the blanket this issue removes. Struct-level allows are used only where the warning covers that struct's own fields. The three cached_read_env! functions take their allow inside the macro invocation, before the fn line, because the macro forwards $(#[$meta:meta])* onto the generated item.

Verification, four lanes warning-free: default, --tests, --features rio-v2 --tests, --features test-util --tests. cargo nextest run -p rustfs-ecstore 4096 passed; clippy --lib --tests -D warnings clean; make pre-commit exit 0. The Linux lane is not covered locally and is left to CI.

Ref rustfs/backlog#1823 (step 2).

* chore(ecstore): correct two dead_code reasons in the disk root

check_valid_path and reject_symlink_components have no caller at all -
not even a test - so 'asserted by this file's tests' misreads them as
covered. Both are method wrappers over live free functions; say that
instead.

Ref rustfs/backlog#1823.

crates/ecstore/src/disk/disk_store.rs

@@ -637,14 +637,23 @@ impl Default for DiskOperationMetrics {
 }
 
 impl DiskOperationMetrics {
+    #[allow(
+        dead_code,
+        reason = "internal metrics recorder reached only from record() below (backlog#1823)"
+    )]
     fn record_call(&mut self) {
         self.lifetime_calls.fetch_add(1, Ordering::Relaxed);
     }
 
+    #[allow(
+        dead_code,
+        reason = "internal metrics recorder reached only from record() below (backlog#1823)"
+    )]
     fn record_latency(&mut self, now_sec: u64, elapsed: Duration) {
         self.record_latency_atomic(now_sec, elapsed);
     }
 
+    #[allow(dead_code, reason = "metrics roll-up with no caller in this port (backlog#1823)")]
     fn record(&mut self, now_sec: u64, elapsed: Duration) {
         self.record_call();
         self.record_latency(now_sec, elapsed);
@@ -770,6 +779,7 @@ impl DiskHealthTracker {
     }
 
     /// Set disk as faulty
+    #[allow(dead_code, reason = "asserted by this file's tests (backlog#1823)")]
     pub fn set_faulty(&self) {
         self.status.store(DISK_HEALTH_FAULTY, Ordering::Release);
     }
@@ -850,6 +860,7 @@ impl DiskHealthTracker {
         became_offline
     }
 
+    #[allow(dead_code, reason = "asserted by this file's tests (backlog#1823)")]
     pub fn mark_offline(&self, endpoint: &Endpoint, reason: &'static str) -> bool {
         let current = self.runtime_state();
         if current == RuntimeDriveHealthState::Offline {
@@ -980,11 +991,13 @@ impl DiskHealthTracker {
     }
 
     /// Get waiting operations count
+    #[allow(dead_code, reason = "asserted by this file's tests (backlog#1823)")]
     pub fn waiting_count(&self) -> u32 {
         self.waiting.load(Ordering::Relaxed)
     }
 
     /// Get last success timestamp
+    #[allow(dead_code, reason = "asserted by this file's tests (backlog#1823)")]
     pub fn last_success(&self) -> i64 {
         self.last_success.load(Ordering::Acquire)
     }
@@ -1026,21 +1039,6 @@ impl Default for DiskHealthTracker {
     }
 }
 
-/// Health check context key for tracking disk operations
-#[derive(Debug, Clone)]
-struct HealthDiskCtxKey;
-
-#[derive(Debug)]
-struct HealthDiskCtxValue {
-    last_success: Arc<AtomicI64>,
-}
-
-impl HealthDiskCtxValue {
-    fn log_success(&self) {
-        self.last_success.store(current_unix_nanos(), Ordering::Relaxed);
-    }
-}
-
 /// LocalDiskWrapper wraps a DiskStore with health tracking capabilities.
 /// This is similar to Go's xlStorageDiskIDCheck.
 #[derive(Debug, Clone)]
@@ -1072,10 +1070,6 @@ impl LocalDiskWrapper {
         )
     }
 
-    pub(crate) fn new_with_health(disk: Arc<LocalDisk>, health_check: bool, health: Arc<DiskHealthTracker>) -> Self {
-        Self::new_with_health_and_metrics(disk, health_check, health, Arc::new(DiskHealthMetricEpoch::default()))
-    }
-
     pub(crate) fn new_with_reconnect_state(
         disk: Arc<LocalDisk>,
         health_check: bool,
@@ -1438,20 +1432,6 @@ impl LocalDiskWrapper {
         }
     }
 
-    async fn check_id(&self, want_id: Option<Uuid>) -> Result<()> {
-        if want_id.is_none() {
-            return Ok(());
-        }
-
-        let stored_disk_id = self.disk.get_disk_id().await?;
-
-        if stored_disk_id != want_id {
-            return Err(Error::other(format!("Disk ID mismatch wanted {want_id:?}, got {stored_disk_id:?}")));
-        }
-
-        Ok(())
-    }
-
     /// Check if disk ID is stale
     async fn check_disk_stale(&self) -> Result<()> {
         let Some(current_disk_id) = *self.disk_id.read().await else {