per-function `Exception` handling (#457)
This PR replaces the global strict exception mode with an option to handle `Exception` per function while at the same time enabling strict exception checking globally by default as has been planned for v4. `handleException` mode raises `AsyncExceptionError` to distinguish it from `ValueError` which may originate from user code. * remove obsolete 1.2 config options
This commit is contained in:
Jacek Sieka
GitHub
parent
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commit
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105
README.md
105
README.md
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@ -9,12 +9,12 @@
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Chronos is an efficient [async/await](https://en.wikipedia.org/wiki/Async/await) framework for Nim. Features include:
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* Efficient dispatch pipeline for asynchronous execution
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* Asynchronous socket and process I/O
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* HTTP server with SSL/TLS support out of the box (no OpenSSL needed)
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* Cancellation support
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* Synchronization primitivies like queues, events and locks
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* FIFO processing order of dispatch queue
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* Minimal exception effect support (see [exception effects](#exception-effects))
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* Cancellation
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* Efficient dispatch pipeline with excellent multi-platform support
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* Exception effect support (see [exception effects](#exception-effects))
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## Installation
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@ -152,16 +152,13 @@ feet, in a certain section, is to not use `await` in it.
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### Error handling
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Exceptions inheriting from `CatchableError` are caught by hidden `try` blocks
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and placed in the `Future.error` field, changing the future's status to
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`Failed`.
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Exceptions inheriting from [`CatchableError`](https://nim-lang.org/docs/system.html#CatchableError)
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interrupt execution of the `async` procedure. The exception is placed in the
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`Future.error` field while changing the status of the `Future` to `Failed`
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and callbacks are scheduled.
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When a future is awaited, that exception is re-raised only to be caught again
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by a hidden `try` block in the calling async procedure. That's how these
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exceptions move up the async chain.
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A failed future's callbacks will still be scheduled, but it's not possible to
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resume execution from the point an exception was raised.
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When a future is awaited, the exception is re-raised, traversing the `async`
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execution chain until handled.
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```nim
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proc p1() {.async.} =
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@ -206,11 +203,11 @@ proc p3() {.async.} =
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await fut2
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```
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Chronos does not allow that future continuations and other callbacks raise
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`CatchableError` - as such, calls to `poll` will never raise exceptions caused
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originating from tasks on the dispatcher queue. It is however possible that
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`Defect` that happen in tasks bubble up through `poll` as these are not caught
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by the transformation.
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Because `chronos` ensures that all exceptions are re-routed to the `Future`,
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`poll` will not itself raise exceptions.
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`poll` may still panic / raise `Defect` if such are raised in user code due to
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undefined behavior.
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#### Checked exceptions
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@ -230,6 +227,53 @@ proc p2(): Future[void] {.async, (raises: [IOError]).} =
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Under the hood, the return type of `p1` will be rewritten to an internal type
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which will convey raises informations to `await`.
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#### The `Exception` type
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Exceptions deriving from `Exception` are not caught by default as these may
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include `Defect` and other forms undefined or uncatchable behavior.
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Because exception effect tracking is turned on for `async` functions, this may
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sometimes lead to compile errors around forward declarations, methods and
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closures as Nim conservatively asssumes that any `Exception` might be raised
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from those.
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Make sure to excplicitly annotate these with `{.raises.}`:
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```nim
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# Forward declarations need to explicitly include a raises list:
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proc myfunction() {.raises: [ValueError].}
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# ... as do `proc` types
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type MyClosure = proc() {.raises: [ValueError].}
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proc myfunction() =
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raise (ref ValueError)(msg: "Implementation here")
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let closure: MyClosure = myfunction
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```
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For compatibility, `async` functions can be instructed to handle `Exception` as
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well, specifying `handleException: true`. `Exception` that is not a `Defect` and
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not a `CatchableError` will then be caught and remapped to
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`AsyncExceptionError`:
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```nim
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proc raiseException() {.async: (handleException: true, raises: [AsyncExceptionError]).} =
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raise (ref Exception)(msg: "Raising Exception is UB")
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proc callRaiseException() {.async: (raises: []).} =
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try:
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raiseException()
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except AsyncExceptionError as exc:
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# The original Exception is available from the `parent` field
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echo exc.parent.msg
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```
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This mode can be enabled globally with `-d:chronosHandleException` as a help
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when porting code to `chronos` but should generally be avoided as global
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configuration settings may interfere with libraries that use `chronos` leading
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to unexpected behavior.
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### Raw functions
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Raw functions are those that interact with `chronos` via the `Future` type but
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@ -302,27 +346,6 @@ annotated as raising `CatchableError` only raise on _some_ platforms - in order
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to work on all platforms, calling code must assume that they will raise even
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when they don't seem to do so on one platform.
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### Strict exception mode
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`chronos` currently offers minimal support for exception effects and `raises`
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annotations. In general, during the `async` transformation, a generic
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`except CatchableError` handler is added around the entire function being
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transformed, in order to catch any exceptions and transfer them to the `Future`.
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Because of this, the effect system thinks no exceptions are "leaking" because in
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fact, exception _handling_ is deferred to when the future is being read.
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Effectively, this means that while code can be compiled with
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`{.push raises: []}`, the intended effect propagation and checking is
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**disabled** for `async` functions.
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To enable checking exception effects in `async` code, enable strict mode with
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`-d:chronosStrictException`.
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In the strict mode, `async` functions are checked such that they only raise
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`CatchableError` and thus must make sure to explicitly specify exception
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effects on forward declarations, callbacks and methods using
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`{.raises: [CatchableError].}` (or more strict) annotations.
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### Cancellation support
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Any running `Future` can be cancelled. This can be used for timeouts,
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@ -379,9 +402,9 @@ waitFor(cancellationExample())
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Even if cancellation is initiated, it is not guaranteed that
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the operation gets cancelled - the future might still be completed
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or fail depending on the ordering of events and the specifics of
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the operation.
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the operation.
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If the future indeed gets cancelled, `await` will raise a
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If the future indeed gets cancelled, `await` will raise a
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`CancelledError` as is likely to happen in the following example:
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```nim
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proc c1 {.async.} =
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@ -11,100 +11,77 @@
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## `chronosDebug` can be defined to enable several debugging helpers that come
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## with a runtime cost - it is recommeneded to not enable these in production
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## code.
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when (NimMajor, NimMinor) >= (1, 4):
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const
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chronosStrictException* {.booldefine.}: bool = defined(chronosPreviewV4)
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## Require that `async` code raises only derivatives of `CatchableError`
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## and not `Exception` - forward declarations, methods and `proc` types
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## used from within `async` code may need to be be explicitly annotated
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## with `raises: [CatchableError]` when this mode is enabled.
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const
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chronosHandleException* {.booldefine.}: bool = false
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## Remap `Exception` to `AsyncExceptionError` for all `async` functions.
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##
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## This modes provides backwards compatibility when using functions with
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## inaccurate `{.raises.}` effects such as unannotated forward declarations,
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## methods and `proc` types - it is recommened to annotate such code
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## explicitly as the `Exception` handling mode may introduce surprising
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## behavior in exception handlers, should `Exception` actually be raised.
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##
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## The setting provides the default for the per-function-based
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## `handleException` parameter which has precedence over this global setting.
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##
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## `Exception` handling may be removed in future chronos versions.
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chronosStrictFutureAccess* {.booldefine.}: bool = defined(chronosPreviewV4)
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chronosStrictFutureAccess* {.booldefine.}: bool = defined(chronosPreviewV4)
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chronosStackTrace* {.booldefine.}: bool = defined(chronosDebug)
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## Include stack traces in futures for creation and completion points
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chronosStackTrace* {.booldefine.}: bool = defined(chronosDebug)
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## Include stack traces in futures for creation and completion points
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chronosFutureId* {.booldefine.}: bool = defined(chronosDebug)
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## Generate a unique `id` for every future - when disabled, the address of
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## the future will be used instead
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chronosFutureId* {.booldefine.}: bool = defined(chronosDebug)
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## Generate a unique `id` for every future - when disabled, the address of
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## the future will be used instead
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chronosFutureTracking* {.booldefine.}: bool = defined(chronosDebug)
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## Keep track of all pending futures and allow iterating over them -
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## useful for detecting hung tasks
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chronosFutureTracking* {.booldefine.}: bool = defined(chronosDebug)
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## Keep track of all pending futures and allow iterating over them -
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## useful for detecting hung tasks
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chronosDumpAsync* {.booldefine.}: bool = defined(nimDumpAsync)
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## Print code generated by {.async.} transformation
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chronosDumpAsync* {.booldefine.}: bool = defined(nimDumpAsync)
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## Print code generated by {.async.} transformation
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chronosProcShell* {.strdefine.}: string =
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when defined(windows):
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"cmd.exe"
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chronosProcShell* {.strdefine.}: string =
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when defined(windows):
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"cmd.exe"
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else:
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when defined(android):
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"/system/bin/sh"
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else:
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when defined(android):
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"/system/bin/sh"
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else:
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"/bin/sh"
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## Default shell binary path.
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##
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## The shell is used as command for command line when process started
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## using `AsyncProcessOption.EvalCommand` and API calls such as
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## ``execCommand(command)`` and ``execCommandEx(command)``.
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"/bin/sh"
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## Default shell binary path.
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##
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## The shell is used as command for command line when process started
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## using `AsyncProcessOption.EvalCommand` and API calls such as
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## ``execCommand(command)`` and ``execCommandEx(command)``.
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chronosEventsCount* {.intdefine.} = 64
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## Number of OS poll events retrieved by syscall (epoll, kqueue, poll).
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chronosEventsCount* {.intdefine.} = 64
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## Number of OS poll events retrieved by syscall (epoll, kqueue, poll).
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chronosInitialSize* {.intdefine.} = 64
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## Initial size of Selector[T]'s array of file descriptors.
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chronosInitialSize* {.intdefine.} = 64
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## Initial size of Selector[T]'s array of file descriptors.
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chronosEventEngine* {.strdefine.}: string =
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when defined(linux) and not(defined(android) or defined(emscripten)):
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"epoll"
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elif defined(macosx) or defined(macos) or defined(ios) or
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defined(freebsd) or defined(netbsd) or defined(openbsd) or
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defined(dragonfly):
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"kqueue"
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elif defined(android) or defined(emscripten):
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"poll"
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elif defined(posix):
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"poll"
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else:
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""
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## OS polling engine type which is going to be used by chronos.
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chronosEventEngine* {.strdefine.}: string =
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when defined(linux) and not(defined(android) or defined(emscripten)):
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"epoll"
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elif defined(macosx) or defined(macos) or defined(ios) or
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defined(freebsd) or defined(netbsd) or defined(openbsd) or
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defined(dragonfly):
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"kqueue"
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elif defined(android) or defined(emscripten):
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"poll"
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elif defined(posix):
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"poll"
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else:
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""
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## OS polling engine type which is going to be used by chronos.
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else:
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# 1.2 doesn't support `booldefine` in `when` properly
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const
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chronosStrictException*: bool =
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defined(chronosPreviewV4) or defined(chronosStrictException)
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chronosStrictFutureAccess*: bool =
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defined(chronosPreviewV4) or defined(chronosStrictFutureAccess)
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chronosStackTrace*: bool = defined(chronosDebug) or defined(chronosStackTrace)
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chronosFutureId*: bool = defined(chronosDebug) or defined(chronosFutureId)
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chronosFutureTracking*: bool =
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defined(chronosDebug) or defined(chronosFutureTracking)
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chronosDumpAsync*: bool = defined(nimDumpAsync)
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chronosProcShell* {.strdefine.}: string =
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when defined(windows):
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"cmd.exe"
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else:
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when defined(android):
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"/system/bin/sh"
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else:
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"/bin/sh"
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chronosEventsCount*: int = 64
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chronosInitialSize*: int = 64
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chronosEventEngine* {.strdefine.}: string =
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when defined(linux) and not(defined(android) or defined(emscripten)):
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"epoll"
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elif defined(macosx) or defined(macos) or defined(ios) or
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defined(freebsd) or defined(netbsd) or defined(openbsd) or
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defined(dragonfly):
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"kqueue"
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elif defined(android) or defined(emscripten):
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"poll"
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elif defined(posix):
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"poll"
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else:
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""
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when defined(chronosStrictException):
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{.warning: "-d:chronosStrictException has been deprecated in favor of handleException".}
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# In chronos v3, this setting was used as the opposite of
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# `chronosHandleException` - the setting is deprecated to encourage
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# migration to the new mode.
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when defined(debug) or defined(chronosConfig):
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import std/macros
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@ -113,7 +90,7 @@ when defined(debug) or defined(chronosConfig):
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hint("Chronos configuration:")
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template printOption(name: string, value: untyped) =
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hint(name & ": " & $value)
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printOption("chronosStrictException", chronosStrictException)
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printOption("chronosHandleException", chronosHandleException)
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printOption("chronosStackTrace", chronosStackTrace)
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printOption("chronosFutureId", chronosFutureId)
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printOption("chronosFutureTracking", chronosFutureTracking)
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@ -33,7 +33,9 @@ proc processBody(node, setResultSym, baseType: NimNode): NimNode {.compileTime.}
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node[i] = processBody(node[i], setResultSym, baseType)
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node
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proc wrapInTryFinally(fut, baseType, body, raises: NimNode): NimNode {.compileTime.} =
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proc wrapInTryFinally(
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fut, baseType, body, raises: NimNode,
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handleException: bool): NimNode {.compileTime.} =
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# creates:
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# try: `body`
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# [for raise in raises]:
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@ -92,15 +94,15 @@ proc wrapInTryFinally(fut, baseType, body, raises: NimNode): NimNode {.compileTi
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newCall(ident "fail", fut, excName)
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))
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let raises = if raises == nil:
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const defaultException =
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when defined(chronosStrictException): "CatchableError"
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else: "Exception"
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nnkTupleConstr.newTree(ident(defaultException))
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var raises = if raises == nil:
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nnkTupleConstr.newTree(ident"CatchableError")
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elif isNoRaises(raises):
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nnkTupleConstr.newTree()
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else:
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raises
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raises.copyNimTree()
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if handleException:
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raises.add(ident"Exception")
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for exc in raises:
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if exc.eqIdent("Exception"):
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@ -115,7 +117,9 @@ proc wrapInTryFinally(fut, baseType, body, raises: NimNode): NimNode {.compileTi
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newCall(ident "fail", fut,
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nnkStmtList.newTree(
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nnkAsgn.newTree(closureSucceeded, ident"false"),
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quote do: (ref ValueError)(msg: `excName`.msg, parent: `excName`)))
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quote do:
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(ref AsyncExceptionError)(
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msg: `excName`.msg, parent: `excName`)))
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)
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elif exc.eqIdent("CancelledError"):
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addCancelledError
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|
@ -132,6 +136,8 @@ proc wrapInTryFinally(fut, baseType, body, raises: NimNode): NimNode {.compileTi
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newCall(ident "fail", fut, excName)
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))
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addDefect # Must not complete future on defect
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nTry.add nnkFinally.newTree(
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nnkIfStmt.newTree(
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nnkElifBranch.newTree(
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|
@ -193,7 +199,13 @@ proc cleanupOpenSymChoice(node: NimNode): NimNode {.compileTime.} =
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for child in node:
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result.add(cleanupOpenSymChoice(child))
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proc decodeParams(params: NimNode): tuple[raw: bool, raises: NimNode] =
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type
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AsyncParams = tuple
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raw: bool
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raises: NimNode
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handleException: bool
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proc decodeParams(params: NimNode): AsyncParams =
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# decodes the parameter tuple given in `async: (name: value, ...)` to its
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# recognised parts
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params.expectKind(nnkTupleConstr)
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@ -201,6 +213,7 @@ proc decodeParams(params: NimNode): tuple[raw: bool, raises: NimNode] =
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var
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raw = false
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raises: NimNode = nil
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handleException = chronosHandleException
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for param in params:
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param.expectKind(nnkExprColonExpr)
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@ -216,10 +229,12 @@ proc decodeParams(params: NimNode): tuple[raw: bool, raises: NimNode] =
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elif param[0].eqIdent("raw"):
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# boolVal doesn't work in untyped macros it seems..
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raw = param[1].eqIdent("true")
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elif param[0].eqIdent("handleException"):
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handleException = param[1].eqIdent("true")
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else:
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warning("Unrecognised async parameter: " & repr(param[0]), param)
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(raw, raises)
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(raw, raises, handleException)
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proc isEmpty(n: NimNode): bool {.compileTime.} =
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# true iff node recursively contains only comments or empties
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|
@ -261,7 +276,7 @@ proc asyncSingleProc(prc, params: NimNode): NimNode {.compileTime.} =
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let
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baseTypeIsVoid = baseType.eqIdent("void")
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(raw, raises) = decodeParams(params)
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(raw, raises, handleException) = decodeParams(params)
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internalFutureType =
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if baseTypeIsVoid:
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newNimNode(nnkBracketExpr, prc).
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|
@ -406,7 +421,8 @@ proc asyncSingleProc(prc, params: NimNode): NimNode {.compileTime.} =
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castFutureSym, baseType,
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if baseTypeIsVoid: procBody # shortcut for non-generic `void`
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else: newCall(setResultSym, procBody),
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raises
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raises,
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handleException
|
||||
)
|
||||
|
||||
closureBody = newStmtList(resultDecl, setResultDecl, completeDecl)
|
||||
|
|
|
|||
|
|
@ -3,3 +3,7 @@ type
|
|||
## Generic async exception
|
||||
AsyncTimeoutError* = object of AsyncError
|
||||
## Timeout exception
|
||||
|
||||
AsyncExceptionError* = object of AsyncError
|
||||
## Error raised in `handleException` mode - the original exception is
|
||||
## available from the `parent` field.
|
||||
|
|
|
|||
|
|
@ -533,3 +533,14 @@ suite "Exceptions tracking":
|
|||
|
||||
expect(Defect): f.fail((ref CatchableError)(), warn = false)
|
||||
check: not f.finished()
|
||||
|
||||
test "handleException behavior":
|
||||
proc raiseException() {.
|
||||
async: (handleException: true, raises: [AsyncExceptionError]).} =
|
||||
raise (ref Exception)(msg: "Raising Exception is UB and support for it may change in the future")
|
||||
|
||||
proc callCatchAll() {.async: (raises: []).} =
|
||||
expect(AsyncExceptionError):
|
||||
await raiseException()
|
||||
|
||||
waitFor(callCatchAll())
|
||||
|
|
|
|||
Loading…
Reference in New Issue