refactor bucket storage file open (#33314)

bucket_map/src/bucket_storage.rs

@@ -6,7 +6,7 @@ use {
     std::{
         fs::{remove_file, OpenOptions},
         io::{Seek, SeekFrom, Write},
-        path::PathBuf,
+        path::{Path, PathBuf},
         sync::{
             atomic::{AtomicU64, Ordering},
             Arc,
@@ -342,57 +342,79 @@ impl<O: BucketOccupied> BucketStorage<O> {
         unsafe { std::slice::from_raw_parts_mut(ptr, len as usize) }
     }
 
-    /// allocate a new memory mapped file of size `bytes` on one of `drives`
-    fn new_map(drives: &[PathBuf], bytes: u64, stats: &BucketStats) -> (MmapMut, PathBuf, u128) {
+    /// open a disk bucket file and mmap it
+    /// optionally creates it.
+    fn map_open_file(
+        path: impl AsRef<Path> + std::fmt::Debug + Clone,
+        create: bool,
+        create_bytes: u64,
+        stats: &BucketStats,
+    ) -> Option<MmapMut> {
         let mut measure_new_file = Measure::start("measure_new_file");
-        let r = thread_rng().gen_range(0..drives.len());
-        let drive = &drives[r];
-        let file_random = thread_rng().gen_range(0..u128::MAX);
-        let pos = format!("{}", file_random,);
-        let file = drive.join(pos);
-        let mut data = OpenOptions::new()
+        let data = OpenOptions::new()
             .read(true)
             .write(true)
-            .create(true)
-            .open(file.clone())
-            .map_err(|e| {
-                panic!(
-                    "Unable to create data file {} in current dir({:?}): {:?}",
-                    file.display(),
-                    std::env::current_dir(),
-                    e
-                );
-            })
-            .unwrap();
+            .create(create)
+            .open(path.clone());
+        if let Err(e) = data {
+            if !create {
+                // we can't load this file, so bail without error
+                return None;
+            }
+            panic!(
+                "Unable to create data file {:?} in current dir({:?}): {:?}",
+                path,
+                std::env::current_dir(),
+                e
+            );
+        }
+        let mut data = data.unwrap();
 
-        // Theoretical performance optimization: write a zero to the end of
-        // the file so that we won't have to resize it later, which may be
-        // expensive.
-        //debug!("GROWING file {}", capacity * cell_size as u64);
-        data.seek(SeekFrom::Start(bytes - 1)).unwrap();
-        data.write_all(&[0]).unwrap();
-        data.rewind().unwrap();
-        measure_new_file.stop();
-        let mut measure_flush = Measure::start("measure_flush");
-        data.flush().unwrap(); // can we skip this?
-        measure_flush.stop();
+        if create {
+            // Theoretical performance optimization: write a zero to the end of
+            // the file so that we won't have to resize it later, which may be
+            // expensive.
+            //debug!("GROWING file {}", capacity * cell_size as u64);
+            data.seek(SeekFrom::Start(create_bytes - 1)).unwrap();
+            data.write_all(&[0]).unwrap();
+            data.rewind().unwrap();
+            measure_new_file.stop();
+            let measure_flush = Measure::start("measure_flush");
+            data.flush().unwrap(); // can we skip this?
+            stats
+                .flush_file_us
+                .fetch_add(measure_flush.end_as_us(), Ordering::Relaxed);
+        }
         let mut measure_mmap = Measure::start("measure_mmap");
-        let res = (
-            unsafe { MmapMut::map_mut(&data).unwrap() },
-            file,
-            file_random,
-        );
+        let res = unsafe { MmapMut::map_mut(&data) };
+        if let Err(e) = res {
+            panic!(
+                "Unable to mmap file {:?} in current dir({:?}): {:?}",
+                path,
+                std::env::current_dir(),
+                e
+            );
+        }
         measure_mmap.stop();
         stats
             .new_file_us
             .fetch_add(measure_new_file.as_us(), Ordering::Relaxed);
-        stats
-            .flush_file_us
-            .fetch_add(measure_flush.as_us(), Ordering::Relaxed);
         stats
             .mmap_us
             .fetch_add(measure_mmap.as_us(), Ordering::Relaxed);
-        res
+        res.ok()
+    }
+
+    /// allocate a new memory mapped file of size `bytes` on one of `drives`
+    fn new_map(drives: &[PathBuf], bytes: u64, stats: &BucketStats) -> (MmapMut, PathBuf, u128) {
+        let r = thread_rng().gen_range(0..drives.len());
+        let drive = &drives[r];
+        let file_random = thread_rng().gen_range(0..u128::MAX);
+        let pos = format!("{}", file_random,);
+        let file = drive.join(pos);
+        let res = Self::map_open_file(file.clone(), true, bytes, stats).unwrap();
+
+        (res, file, file_random)
     }
 
     /// copy contents from 'old_bucket' to 'self'