incr len in with_capacity_aligned_zeroed add a test

Signed-off-by: Yuchen Liang <yuchen@neon.tech>

pageserver/src/page_cache.rs

@@ -646,7 +646,8 @@ impl PageCache {
     fn new(num_pages: usize, align: usize) -> Self {
         assert!(num_pages > 0, "page cache size must be > 0");
 
-        let page_buffer = IoBufferMut::with_capacity_aligned(num_pages * PAGE_SZ, align).leak();
+        let page_buffer =
+            IoBufferMut::with_capacity_aligned_zeroed(num_pages * PAGE_SZ, align).leak();
 
         let size_metrics = &crate::metrics::PAGE_CACHE_SIZE;
         size_metrics.max_bytes.set_page_sz(num_pages);

pageserver/src/virtual_file/dio.rs

@@ -64,6 +64,7 @@ impl IoBufferMut {
         use bytes::BufMut;
         let mut buf = Self::with_capacity_aligned(capacity, align);
         buf.put_bytes(0, capacity);
+        buf.len = capacity;
         buf
     }
 
@@ -223,7 +224,7 @@ impl DerefMut for IoBufferMut {
     }
 }
 
-/// SAFETY: See [`IoBufferMut::advance_mut`]
+/// SAFETY: When advancing the internal cursor, the caller needs to make sure the bytes advcanced past have been initialized.
 unsafe impl bytes::BufMut for IoBufferMut {
     #[inline]
     fn remaining_mut(&self) -> usize {
@@ -235,7 +236,7 @@ unsafe impl bytes::BufMut for IoBufferMut {
     // SAFETY: Caller needs to make sure the bytes being advanced past have been initialized.
     #[inline]
     unsafe fn advance_mut(&mut self, cnt: usize) {
-        let len = self.len();
+        let len: usize = self.len();
         let remaining = self.remaining_mut();
 
         if remaining < cnt {
@@ -317,6 +318,17 @@ mod tests {
         assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
     }
 
+    #[test]
+    fn test_with_capacity_aligned_zeroed() {
+        const ALIGN: usize = 4 * 1024;
+        let v = IoBufferMut::with_capacity_aligned_zeroed(ALIGN, ALIGN);
+        assert_eq!(v.len(), ALIGN);
+        assert_eq!(v.capacity(), ALIGN);
+        assert_eq!(v.align(), ALIGN);
+        assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+        assert_eq!(&v[..], &[0; ALIGN])
+    }
+
     #[test]
     fn test_reserve() {
         use bytes::BufMut;