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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503
//! Crate for identifying and expiring long-running database activities.
//! The core primitive provided by the crate is a `PleaseHandle`. These
//! handles represent long-running operations, and can be used as the
//! basis for implementing exclusive locking behaviour when a lock may
//! be held for too long for transaction-level locking to be acceptable.
//!
//! # Setup
//!
//! This crate requires that certain tables exist, and for this reason
//! it exports its own migrations. To manage migrations of third-party
//! crates you can install the `diesel-setup-deps` tool:
//!
//! ```ignore
//! cargo install diesel-setup-deps
//! diesel setup
//! diesel-setup-deps
//! ```
//!
//! The necessary migrations will be automatically added to your diesel
//! migrations directory, and these should be committed to version
//! control along with your other migrations.
//!
//! # Usage
//!
//! To begin with, you will typically have a long-running operation
//! which you want to have exclusive access to some part of your database.
//!
//! A good example might be generating a report: this operation will process
//! large amounts of data, and then save the result to the `output` field
//! in our `reports` table. We want the operation to have exclusive access
//! to the `output` field, so that nobody else tries to generate the report
//! whilst we are running, and we also may want to track whether a report
//! is in progress or not.
//!
//! This operation can be implemented as a single function:
//!
//! ```ignore
//! fn generate_report(
//! connection_pool: Arc<ConnectionPool>,
//! report_id: i32
//! ) -> PleaseResult<ConnectionPoolError> {
//! ```
//!
//! First we obtain a handle to represent our operation:
//!
//! ```ignore
//! let mut handle = PleaseHandle::new_with_cleanup(
//! connection_pool, "generating report"
//! )?;
//! ```
//!
//! Next we store our handle's ID in the reports table:
//!
//! ```ignore
//! handle.transaction(|conn, handle_id| {
//! diesel::update(
//! reports::table
//! .filter(reports::id.eq(report_id))
//! .filter(reports::operation_id.is_null())
//! )
//! .set(reports::operation_id.eq(Some(handle_id)))
//! .execute(conn)
//! })?;
//! ```
//!
//! Importantly, we fail if the operation ID is already set.
//!
//! Now, we can perform whatever work is required to generate the report.
//! If the report may take longer than the operation timout, then you can
//! either increase the timeout, or call `handle.refresh()` every so often
//! to ensure the timeout is never reached.
//!
//! When we have our result, we simply save it back, and close the handle:
//!
//! ```ignore
//! handle.transaction(|conn, handle_id| {
//! diesel::update(
//! reports::table
//! .filter(reports::id.eq(report_id))
//! )
//! .set(reports::output.eq(Some(result)))
//! .execute(conn)
//! })?;
//!
//! handle.close()?;
//! Ok(())
//! })
//! ```
//!
//! The handle will be automatically closed if it is instead allowed to
//! fall out of scope, but errors will be ignored.
//!
//! # Database Schema
//!
//! In the above example, it is expected that the `reports::operation_id` column
//! is a nullable integer column, and a foreign key into the `please_ids`
//! table.
//!
//! Ideally you should set up cascade rules such that when rows are deleted
//! from the `please_ids` table, corresponding reports have their `operation_id`
//! column set to null.
//!
//! # Operation Timeouts
//!
//! If no activity happens on a `PleaseHandle` for longer than the *operation timeout*
//! then the handle may expire. This will happen automatically whenever
//! `perform_cleanup` or `new_with_cleanup` is called from another thread or another
//! database client.
//!
//! Calling the methods `transaction` or `refresh` are considered activity, and will
//! prevent the handle from expiring, assuming it has not already expired. Both methods
//! will fail-fast if the operation has already expired. In this case you should cancel
//! any work you were doing as part of the operation.
//!
//! The operation timeout is controlled by a database function: `please_timeout()`.
//! To change the timeout, use a migration to alter this function and return a
//! different value. It is not currently possible to change the timeout on a per-operation
//! basis.
//!
//! It is recommended to set the operation timeout to as short a time as possible, so
//! that if your application crashes, is terminated unexpectedly, or simply loses
//! connectivity to the database, any locks it might have held are released as
//! soon as possible.
//!
//! The operation timeout is by default set to *two minutes*.
#![allow(proc_macro_derive_resolution_fallback)]
#![deny(missing_docs)]
use std::marker::PhantomData;
use std::error::Error;
use std::fmt::{Formatter, Display, self};
#[macro_use]
extern crate diesel;
extern crate chrono;
use chrono::NaiveDateTime;
use diesel::Connection;
use diesel::pg::Pg;
use diesel::prelude::*;
use diesel::dsl;
mod schema;
#[derive(Queryable, Debug, Clone)]
struct PleaseId {
id: i32,
creation: NaiveDateTime,
expiry: NaiveDateTime,
title: String,
refresh_count: i32,
}
/// Expired ID, only used for logging/debug purposes
#[derive(Debug, Clone)]
pub struct ExpiredId(PleaseId);
/// Error type returned by this library
#[derive(Debug, PartialEq)]
pub enum PleaseError<P> {
/// An error occurred whilst obtaining a connection from the
/// connection provider.
Provider(P),
/// An error occurred whilst querying the database.
Query(diesel::result::Error),
/// Tried to use a handle which had already expired.
Expired,
#[doc(hidden)]
__Nonexhaustive,
}
impl<P: Error> Display for PleaseError<P> {
fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
match self {
&PleaseError::Provider(ref e) => Display::fmt(e, f),
&PleaseError::Query(ref e) => Display::fmt(e, f),
&PleaseError::Expired => Display::fmt(self.description(), f),
&PleaseError::__Nonexhaustive => unreachable!(),
}
}
}
impl<P: Error> Error for PleaseError<P> {
fn description(&self) -> &str {
match self {
&PleaseError::Provider(ref e) => e.description(),
&PleaseError::Query(ref e) => e.description(),
&PleaseError::Expired => "The `please` handle has expired and can no longer be used",
&PleaseError::__Nonexhaustive => unreachable!(),
}
}
fn cause(&self) -> Option<&Error> {
match self {
&PleaseError::Provider(ref e) => Some(e),
&PleaseError::Query(ref e) => Some(e),
&PleaseError::Expired => None,
&PleaseError::__Nonexhaustive => unreachable!(),
}
}
}
impl<P> From<diesel::result::Error> for PleaseError<P> {
fn from(other: diesel::result::Error) -> Self {
PleaseError::Query(other)
}
}
struct ErrorWrapper<E, P>(E, PhantomData<fn(P) -> P>);
impl<P, E: From<PleaseError<P>>> From<diesel::result::Error> for ErrorWrapper<E, P> {
fn from(other: diesel::result::Error) -> Self {
ErrorWrapper(PleaseError::Query(other).into(), PhantomData)
}
}
impl<P, E: From<PleaseError<P>>> From<PleaseError<P>> for ErrorWrapper<E, P> {
fn from(other: PleaseError<P>) -> Self {
ErrorWrapper(other.into(), PhantomData)
}
}
/// Convenient alias for `Result` types returned from this library.
pub type PleaseResult<T, P> = Result<T, PleaseError<P>>;
/// Trait for types providing database connections. This is typically
/// implemented by an `Arc<ConnectionPool>` or functionally equivalent
/// type.
pub trait ConnectionProvider {
/// Error type which may be returned when the provider is unable
/// to obtain a database connection.
type Error;
/// Type of the connection returned. Currently the connection must
/// use the postgres backend.
type Connection: Connection<Backend=Pg>;
/// Obtain a connection from this provider.
fn get(&self) -> Result<Self::Connection, Self::Error>;
}
/// This handle identifies a long-running operation using a unique integer.
#[derive(Debug)]
pub struct PleaseHandle<P: ConnectionProvider> {
provider: P,
id: i32
}
impl<P: ConnectionProvider> PleaseHandle<P> {
fn transaction_internal<R, E: From<PleaseError<P::Error>>, F: FnOnce(&P::Connection) -> Result<R, E>>(provider: &P, f: F) -> Result<R, E> {
let conn = provider.get()
.map_err(PleaseError::Provider)?;
conn.transaction(|| f(&conn).map_err(|e| ErrorWrapper(e, PhantomData)))
.map_err(|ErrorWrapper(e, _)| e)
}
/// Construct a new handle using the specified connection provider.
///
/// The connection provider is typically a connection pool of some kind,
/// or it may be implemented by establishing a new connection each time.
///
/// The title is used as a human-readable name to identify this handle.
/// This is useful if you are inspecting the database, or for logging
/// expired handles.
pub fn new(provider: P, title: &str) -> PleaseResult<Self, P::Error> {
use self::schema::*;
// Allocate a new ID
let id: i32 = Self::transaction_internal(&provider, |conn| -> PleaseResult<i32, P::Error> {
Ok(diesel::insert_into(please_ids::table)
.values(&please_ids::title.eq(title))
.returning(please_ids::id)
.get_result(conn)?)
})?;
Ok(PleaseHandle { provider, id })
}
/// Convenience constructor.
///
/// Equivalent to calling `perform_cleanup` followed by `new`.
/// If you wish to handle expired handles (eg. record them to a log) then
/// call the methods individually.
pub fn new_with_cleanup(provider: P, title: &str) -> PleaseResult<Self, P::Error> {
let _ = Self::perform_cleanup(&provider);
Self::new(provider, title)
}
/// Constructor to use from within an existing transaction.
///
/// Allows conditionally creating a handle without losing the atomicity
/// of a single transaction.
pub fn new_with_connection<C>(provider: P, title: &str, conn: &C) -> QueryResult<Self>
where
C: Connection<Backend=Pg> + ?Sized
{
use self::schema::*;
let id = diesel::insert_into(please_ids::table)
.values(&please_ids::title.eq(title))
.returning(please_ids::id)
.get_result(conn)?;
Ok(PleaseHandle {
provider,
id,
})
}
/// Explicitly clean up old handles. It is recommended to call this before
/// creating a new handle.
///
/// This function returns the expired handles (if any) so that you can log them
/// or use them for debugging.
pub fn perform_cleanup(provider: &P) -> PleaseResult<Vec<ExpiredId>, P::Error> {
use self::schema::*;
Self::transaction_internal(provider, |conn| {
diesel::delete(please_ids::table.filter(please_ids::expiry.lt(dsl::now)))
.get_results(conn)
.map_err(PleaseError::Query)
.map(|v| {
v.into_iter().map(ExpiredId).collect()
})
})
}
/// Run a transaction as part of the operation this handle represents.
///
/// After beginning the transaction, this method validates that the handle
/// has not expired, whilst also refreshing the expiry and taking a lock
/// on the row, to prevent it being expired by another thread whilst this
/// transaction is in progress.
///
/// The callback is passed two arguments: a reference to the connection,
/// and the integer ID of this handle. Currently this is the only way to
/// access the handle's ID as it is not recommended to use this ID outside
/// of a transaction.
pub fn transaction<R, E, F>(&mut self, f: F) -> Result<R, E>
where
E: From<PleaseError<P::Error>>,
F: FnOnce(&P::Connection, i32) -> Result<R, E>
{
use self::schema::*;
Self::transaction_internal(&self.provider, |conn| {
// We simultaneously refresh the ID, *and* validate that
// it hasn't expired.
let num_rows = diesel::update(
please_ids::table.filter(please_ids::id.eq(self.id))
).set(
please_ids::refresh_count.eq(please_ids::refresh_count + 1)
).execute(conn).map_err(PleaseError::Query)?;
if num_rows == 1 {
// One row was updated, we're good
f(conn, self.id)
} else {
// Presumably the row has expired and been removed
Err(PleaseError::Expired.into())
}
})
}
/// Refreshes the handle, resetting the time before it will be automatically closed.
///
/// This is equivalent to running an empty transaction.
pub fn refresh(&mut self) -> PleaseResult<(), P::Error> {
// Just do an empty transaction
self.transaction(|_conn, _id| Ok(()))
}
/// Expire the handle. Future operations on this handle will fail with the error `Expired`.
///
/// Useful for testing.
pub fn expire(&mut self) -> PleaseResult<ExpiredId, P::Error> {
use self::schema::*;
Self::transaction_internal(&self.provider, |conn| {
diesel::delete(
please_ids::table.filter(please_ids::id.eq(self.id))
)
.get_result(conn)
.optional()?
.ok_or(PleaseError::Expired)
}).map(ExpiredId)
}
/// Close the handle, allowing any errors to be handled.
///
/// This is automatically called when a handle is dropped,
/// but in that case errors are silently ignored.
pub fn close(mut self) -> PleaseResult<(), P::Error> {
self.expire()?;
self.id = -1;
Ok(())
}
/// Get the ID of this handle.
///
/// A good rule of thumb is to never use this outside of a transaction,
/// as in that case it may not have been recently validated.
pub fn id(&self) -> i32 { self.id }
}
impl<P: ConnectionProvider> Drop for PleaseHandle<P> {
/// Closes the handle, ignoring any errors that might
/// have occurred.
fn drop(&mut self) {
// Ignore errors here
if self.id != -1 {
let _ = self.expire();
}
}
}
#[cfg(test)]
mod tests {
extern crate dotenv;
use super::*;
use std::env;
#[derive(Copy, Clone, Debug)]
struct TestConnectionProvider;
impl ConnectionProvider for TestConnectionProvider {
type Connection = PgConnection;
type Error = diesel::ConnectionError;
fn get(&self) -> Result<PgConnection, Self::Error> {
dotenv::dotenv().ok();
PgConnection::establish(&env::var("DATABASE_URL").unwrap())
}
}
fn new_handle(name: &str) -> PleaseHandle<TestConnectionProvider> {
PleaseHandle::new(TestConnectionProvider, name)
.expect("Failed to create handle")
}
fn new_handle_with_connection(name: &str) -> PleaseHandle<TestConnectionProvider> {
let conn = TestConnectionProvider.get()
.expect("Failed to get connection");
conn.transaction(|| {
PleaseHandle::new_with_connection(TestConnectionProvider, name, &conn)
}).expect("Failed to run transaction")
}
#[test]
fn smoke() {
let mut handle = new_handle("smoke");
handle.transaction(|_conn, _id| Ok::<(), PleaseError<_>>(()))
.expect("Failed to run no-op transaction");
handle.close()
.expect("Failed to close handle");
}
#[test]
fn smoke_with_connection() {
let mut handle = new_handle_with_connection("smoke");
handle.transaction(|_conn, _id| Ok::<(), PleaseError<_>>(()))
.expect("Failed to run no-op transaction");
handle.close()
.expect("Failed to close handle");
}
#[test]
fn two_handles() {
let mut handle1 = new_handle("two_handles::1");
let mut handle2 = new_handle("two_handles::2");
handle1.transaction(|_conn1, id1| {
handle2.transaction(|_conn2, id2| {
assert_ne!(id1, id2);
Ok::<(), PleaseError<_>>(())
})
}).expect("Failed to run transactions");
handle1.close()
.expect("Failed to close handle 1");
handle2.close()
.expect("Failed to close handle 2");
}
#[test]
fn expiry() {
let mut handle = new_handle("smoke");
handle.expire()
.expect("Failed to expire handle");
let err = handle.transaction(|_conn, _id| Ok::<(), PleaseError<_>>(()))
.expect_err("Transaction should fail on expired handle");
assert_eq!(err, PleaseError::Expired);
}
}