src/tools/process_locker.rs

   1 //! Inter-process reader-writer lock builder.
   2 //!
   3 //! This implemenation uses fcntl record locks with non-blocking
   4 //! F_SETLK command (never blocks).
   5 //!
   6 //! We maintain a map of shared locks with time stamps, so you can get
   7 //! the timestamp for the oldest open lock with
   8 //! `oldest_shared_lock()`.
   9
  10 use failure::*;
  11
  12 use std::sync::{Arc, Mutex};
  13 use std::os::unix::io::AsRawFd;
  14 use std::collections::HashMap;
  15
  16 // fixme: use F_OFD_ locks when implemented with nix::fcntl
  17
  18 // Note: flock lock conversion is not atomic, so we need to use fcntl
  19
  20 /// Inter-process reader-writer lock
  21 pub struct ProcessLocker {
  22     file: std::fs::File,
  23     exclusive: bool,
  24     writers: usize,
  25     next_guard_id: u64,
  26     shared_guard_list: HashMap<u64, i64>, // guard_id => timestamp
  27 }
  28
  29 /// Lock guard for shared locks
  30 ///
  31 /// Release the lock when it goes out of scope.
  32 pub struct ProcessLockSharedGuard {
  33     guard_id: u64,
  34     locker: Arc<Mutex<ProcessLocker>>,
  35 }
  36
  37 impl  Drop for ProcessLockSharedGuard {
  38     fn drop(&mut self) {
  39         let mut data = self.locker.lock().unwrap();
  40
  41         if data.writers == 0 { panic!("unexpected ProcessLocker state"); }
  42
  43         data.shared_guard_list.remove(&self.guard_id);
  44
  45         if data.writers == 1 && !data.exclusive {
  46
  47             let op = libc::flock {
  48                 l_type: libc::F_UNLCK as i16,
  49                 l_whence: libc::SEEK_SET as i16,
  50                 l_start: 0,
  51                 l_len: 0,
  52                 l_pid: 0,
  53             };
  54
  55             if let Err(err) = nix::fcntl::fcntl(data.file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op)) {
  56                 panic!("unable to drop writer lock - {}", err);
  57             }
  58             data.writers = 0;
  59         }
  60     }
  61 }
  62
  63 /// Lock guard for exclusive locks
  64 ///
  65 /// Release the lock when it goes out of scope.
  66 pub struct ProcessLockExclusiveGuard {
  67     locker: Arc<Mutex<ProcessLocker>>,
  68 }
  69
  70 impl  Drop for ProcessLockExclusiveGuard {
  71     fn drop(&mut self) {
  72         let mut data = self.locker.lock().unwrap();
  73
  74         if !data.exclusive { panic!("unexpected ProcessLocker state"); }
  75
  76         let ltype = if data.writers != 0 { libc::F_RDLCK } else { libc::F_UNLCK };
  77         let op = libc::flock {
  78             l_type: ltype as i16,
  79             l_whence: libc::SEEK_SET as i16,
  80             l_start: 0,
  81             l_len: 0,
  82             l_pid: 0,
  83         };
  84
  85         if let Err(err) = nix::fcntl::fcntl(data.file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLKW(&op)) {
  86             panic!("unable to drop exclusive lock - {}", err);
  87         }
  88
  89         data.exclusive = false;
  90     }
  91 }
  92
  93 impl ProcessLocker {
  94
  95     /// Create a new instance for the specified file.
  96     ///
  97     /// This simply creates the file if it does not exist.
  98     pub fn new<P: AsRef<std::path::Path>>(lockfile: P) -> Result<Arc<Mutex<Self>>, Error> {
  99
 100         let file = std::fs::OpenOptions::new()
 101             .create(true)
 102             .read(true)
 103             .write(true)
 104             .open(lockfile)?;
 105
 106         Ok(Arc::new(Mutex::new(Self {
 107             file,
 108             exclusive: false,
 109             writers: 0,
 110             next_guard_id: 0,
 111             shared_guard_list: HashMap::new(),
 112         })))
 113     }
 114
 115     fn try_lock(file: &std::fs::File, ltype: i32) -> Result<(), Error> {
 116
 117         let op = libc::flock {
 118             l_type: ltype as i16,
 119             l_whence: libc::SEEK_SET as i16,
 120             l_start: 0,
 121             l_len: 0,
 122             l_pid: 0,
 123         };
 124
 125         nix::fcntl::fcntl(file.as_raw_fd(), nix::fcntl::FcntlArg::F_SETLK(&op))?;
 126
 127         Ok(())
 128     }
 129
 130     /// Try to aquire a shared lock
 131     ///
 132     /// On sucess, this makes sure that no other process can get an exclusive lock for the file.
 133     pub fn try_shared_lock(locker: Arc<Mutex<Self>>) -> Result<ProcessLockSharedGuard, Error> {
 134
 135         let mut data = locker.lock().unwrap();
 136
 137         if data.writers == 0 && !data.exclusive {
 138             if let Err(err) = Self::try_lock(&data.file, libc::F_RDLCK) {
 139                 bail!("unable to get shared lock - {}", err);
 140             }
 141         }
 142
 143         data.writers += 1;
 144
 145         let guard = ProcessLockSharedGuard { locker: locker.clone(), guard_id: data.next_guard_id };
 146         data.next_guard_id += 1;
 147
 148         let now = unsafe { libc::time(std::ptr::null_mut()) };
 149
 150         data.shared_guard_list.insert(guard.guard_id, now);
 151
 152         Ok(guard)
 153     }
 154
 155     /// Get oldest shared lock timestamp
 156     pub fn oldest_shared_lock(locker: Arc<Mutex<Self>>) -> Option<i64> {
 157         let mut result = None;
 158
 159         let data = locker.lock().unwrap();
 160
 161         for v in data.shared_guard_list.values() {
 162             result = match result {
 163                 None => Some(*v),
 164                 Some(x) => if x < *v { Some(x) } else { Some(*v) },
 165             };
 166         }
 167
 168         result
 169     }
 170
 171     /// Try to aquire a exclusive lock
 172     ///
 173     /// Make sure the we are the only process which has locks for this file (shared or exclusive).
 174     pub fn try_exclusive_lock(locker: Arc<Mutex<Self>>) -> Result<ProcessLockExclusiveGuard, Error> {
 175
 176         let mut data = locker.lock().unwrap();
 177
 178         if data.exclusive {
 179             bail!("already locked exclusively");
 180         }
 181
 182         if let Err(err) = Self::try_lock(&data.file, libc::F_WRLCK) {
 183             bail!("unable to get exclusive lock - {}", err);
 184         }
 185
 186         data.exclusive = true;
 187
 188         Ok(ProcessLockExclusiveGuard { locker: locker.clone() })
 189     }
 190 }