1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111

use super::*;

/// A pending log reservation which can be aborted or completed.
/// NB the holder should quickly call `complete` or `abort` as
/// taking too long to decide will cause the underlying IO
/// buffer to become blocked.
pub struct Reservation<'a> {
    pub(super) iobufs: &'a IoBufs,
    pub(super) idx: usize,
    pub(super) data: Vec<u8>,
    pub(super) destination: &'a mut [u8],
    pub(super) flushed: bool,
    pub(super) lsn: Lsn,
    pub(super) lid: LogID,
    pub(super) is_external: bool,
}

impl<'a> Drop for Reservation<'a> {
    fn drop(&mut self) {
        // We auto-abort if the user never uses a reservation.
        let should_flush = !self.data.is_empty() && !self.flushed;
        if should_flush {
            self.flush(false).unwrap();
        }
    }
}

impl<'a> Reservation<'a> {
    /// Cancel the reservation, placing a failed flush on disk, returning
    /// the (cancelled) log sequence number and file offset.
    pub fn abort(mut self) -> CacheResult<(Lsn, DiskPtr), ()> {
        if self.is_external {
            let blob_ptr = self.external_ptr().unwrap();

            assert_eq!(self.lsn, blob_ptr);

            trace!(
                "removing blob for aborted reservation at lsn {}",
                blob_ptr
            );
            remove_blob(blob_ptr, &self.iobufs.config)?;
        }

        self.flush(false)
    }

    /// Complete the reservation, placing the buffer on disk. returns
    /// the log sequence number of the write, and the file offset.
    pub fn complete(mut self) -> CacheResult<(Lsn, DiskPtr), ()> {
        self.flush(true)
    }

    /// Get the log file offset for reading this buffer in the future.
    pub fn lid(&self) -> LogID {
        self.lid
    }

    /// Get the log sequence number for this update.
    pub fn lsn(&self) -> Lsn {
        self.lsn
    }

    /// Get the underlying storage location for the written value.
    /// Note that an external write still has a pointer in the
    /// log at the provided lid location.
    pub fn ptr(&self) -> DiskPtr {
        if let Some(external_ptr) = self.external_ptr() {
            DiskPtr::new_external(self.lid, external_ptr)
        } else {
            DiskPtr::new_inline(self.lid)
        }
    }

    fn external_ptr(&self) -> Option<ExternalPointer> {
        if self.is_external {
            let mut blob_ptr_bytes =
                [0u8; std::mem::size_of::<Lsn>()];
            blob_ptr_bytes
                .copy_from_slice(&self.data[MSG_HEADER_LEN..]);
            let blob_ptr: ExternalPointer =
                unsafe { std::mem::transmute(blob_ptr_bytes) };

            Some(blob_ptr)
        } else {
            None
        }
    }

    fn flush(
        &mut self,
        valid: bool,
    ) -> CacheResult<(Lsn, DiskPtr), ()> {
        if self.flushed {
            panic!("flushing already-flushed reservation!");
        }

        self.flushed = true;

        if !valid {
            self.data[0] = FAILED_FLUSH;
            // don't actually zero the message, still check its hash
            // on recovery to find corruption.
        }

        self.destination.copy_from_slice(&*self.data);

        self.iobufs.exit_reservation(self.idx)?;

        Ok((self.lsn(), self.ptr()))
    }
}