[Tarantool-patches] [PATCH] box: rfc for stacked diagnostic area
Nikita Pettik
korablev at tarantool.org
Wed Jan 22 16:54:09 MSK 2020
On 14 Jan 23:16, Nikita Pettik wrote:
> From: Kirill Shcherbatov <kshcherbatov at tarantool.org>
Hi guys,
Does anybody want to look at this RFC? It's quite similar to the one which
got LGTM from Konstantin, but it is even more simplified. I have to get
formal approvement to start fixing code itself. Thanks.
> Part of #1148
> ---
> rfc in human-readable format: https://github.com/tarantool/tarantool/blob/np/gh-1148-stacked-diag/doc/rfc/1148-stacked-diagnostics.md
> Previous version of rfc: https://github.com/tarantool/tarantool/commit/c2a7e1a10732fdb231780ac04d1a4bc1618c0468
>
> Changes in this version:
> - All savepoint routines have been removed as meaningless;
> - Removed SQL warnings mentions since they are unrelated to the subject of rfc;
> - Removed mentions about exposing box.error.new() with filename and
> line parameters (again, it is barely related to the subject of rfc);
> - Described in details difference between diag_set() in diag_add()
> functions as a part of C interface;
> - Other minor clean-ups.
>
> Now RFC looks quite brief and straightforward in its implementation;
> it attempts at solving only #1148 issue.
>
> doc/rfc/1148-stacked-diagnostics.md | 214 ++++++++++++++++++++++++++++++++++++
> 1 file changed, 214 insertions(+)
> create mode 100644 doc/rfc/1148-stacked-diagnostics.md
>
> diff --git a/doc/rfc/1148-stacked-diagnostics.md b/doc/rfc/1148-stacked-diagnostics.md
> new file mode 100644
> index 000000000..68fa6d21a
> --- /dev/null
> +++ b/doc/rfc/1148-stacked-diagnostics.md
> @@ -0,0 +1,214 @@
> +# Stacked Diagnostics
> +
> +* **Status**: In progress
> +* **Start date**: 30-07-2019
> +* **Authors**: Kirill Shcherbatov @kshcherbatov kshcherbatov at tarantool.org,
> + Pettik Nikita korablev at tarantool.org
> +* **Issues**: [#1148](https://github.com/tarantool/<repository\>/issues/1148)
> +
> +## Background and motivation
> +
> +Support stacked diagnostics for Tarantool allows to accumulate all errors
> +occurred during request processing. It allows to better understand what
> +happened, and handle errors appropriately.
> +
> +### Current error diagnostics
> +
> +Currently Tarantool has `diag_set()` mechanism to set a diagnostic error.
> +Object representing error featuring following properties:
> + - type (string) error’s C++ class;
> + - code (number) error’s number;
> + - message (string) error’s message;
> + - file (string) Tarantool source file;
> + - line (number) line number in the Tarantool source file.
> +
> +The last error raised is exported with `box.error.last()` function.
> +
> +Type of error is represented by a few C++ classes (all are inherited from
> +Exception class):
> +```
> +ClientError
> + | LoggedError
> + | AccessDeniedError
> + | UnsupportedIndexFeature
> +
> +XlogError
> + | XlogGapError
> +
> +SystemError
> + | SocketError
> + | OutOfMemory
> + | TimedOut
> +
> +ChannelIsClosed
> +FiberIsCancelled
> +LuajitError
> +IllegalParams
> +CollationError
> +SwimError
> +CryptoError
> +```
> +
> +All codes and names of ClientError class are available in box.error.
> +User is able to create a new error instance of predefined type using
> +box.error.new() function. For example:
> +```
> +tarantool> t = box.error.new(box.error.CREATE_SPACE, "myspace", "just cause")
> +tarantool> t:unpack()
> +---
> +- type: ClientError
> + code: 9
> + message: 'Failed to create space ''myspace'': just because'
> + trace:
> + - file: '[string "t = box.error.new(box.error.CREATE_SPACE, "my..."]'
> + line: 1
> +```
> +
> +User is also capable of defining own errors with any code by means of:
> +```
> +box.error.new({code = user_code, reason = user_error_msg})
> +```
> +For instance:
> +```
> +e = box.error.new({code = 500, reason = 'just cause'})
> +```
> +
> +Error cdata object has `:unpack()`, `:raise()`, `:match(...)`, `:__serialize()`
> +methods and `.type`, `.message` and `.trace` fields.
> +
> +## Proposal
> +
> +In some cases a diagnostic information should be more complicated than
> +one last raised error. Consider following example: persistent Lua function
> +referenced by functional index has a bug in it's definition, Lua handler sets
> +an diag message. Then functional index extractor code setups an own, more
> +specialized error. Without stacked diagnostic area, only last error is
> +delivired to user. One way to deal with this problem is to introduce stack
> +accumulating all errors happened during request processing.
> +
> +### C API
> +
> +Let's keep existent `diag_set()` method as is. It is supposed to replace the
> +last error in diagnostic area with a new one. To add new error at the top of
> +existing one, let's introduce new method `diag_add()`. It is assumed to keep
> +an existent error message in diagnostic area (if any) and sets it as a reason
> +error for a recently-constructed error object. Note that `diag_set()` is not
> +going to preserve pointer to previous error which is held in error to be
> +substituted. To illustrate last point consider example:
> +
> +```
> +0. Errors: <NULL>
> +1. diag_set(code = 1)
> +Errors: <e1(code = 1) -> NULL>
> +2. diag_add(code = 2)
> +Errors: <e1(code = 1) -> e2(code = 2) -> NULL>
> +3. diag_set(code = 3)
> +Errors: <e3(code = 3) -> NULL>
> +```
> +
> +Hence, developer takes responsibility of placing `diag_set()` where the most
> +basic error should be raised. For instance, if during request processing
> +`diag_add()` is called before `diag_set()` then it will result in inheritance
> +of all errors from previous error raise:
> +
> +```
> +-- Processing of request #1
> +1. diag_set(code = 1)
> +Errors: <e1(code = 1) -> NULL>
> +2. diag_add(code = 2)
> +Errors: <e1(code = 1) -> e2(code = 2) -> NULL>
> +-- End of execution
> +
> +-- Processing of request #2
> +1. diag_add(code = 1)
> +Errors: <e1(code = 1) -> e2(code = 2) -> e3(code = 1) -> NULL>
> +-- End of execution
> +```
> +
> +As a result, at the end of execution fo second request, three errors in
> +stack are reported instead of one.
> +
> +Another way to resolve this issue is to erase diagnostic area before
> +request processing. However, it breaks current user-visible behaviour
> +since box.error.last() will preserve last occurred error only until execution
> +of the next request.
> +
> +The diagnostic area (now) contains (nothing but) pointer to the top error:
> +```
> +struct diag {
> + struct error *last;
> +};
> +
> +```
> +
> +To organize errors in a list let's extend error structure with pointer to
> +the previous element. Or alternatively, add member of any data structure
> +providing list properties (struct rlist, struct stailq or whatever):
> +```
> +struct diag {
> + struct stailq *errors;
> +};
> +
> +struct error {
> + ...
> + struct stailq_entry *in_errors;
> +};
> +```
> +
> +
> +### Lua API
> +
> +Tarantool returns a last-set (diag::last) error as `cdata` object from central
> +diagnostic area to Lua in case of error. User should be unable to modify it
> +(since it is considered to be a bad practice - in fact object doesn't belong
> +to user). On the other hand, user needs an ability to inspect a collected
> +diagnostic information. Hence, let's extend the `box.error` API with a function
> +which provides the way to get the previous error (reason): `:prev()` (and
> +correspondingly `.prev` field).
> +
> +```
> +-- Return a reason error object for given error
> +-- (when exists, nil otherwise).
> +box.error.prev(error) == error.prev
> +```
> +
> +Furthermore, let's extend signature of `box.error.new()` with new (optional)
> +argument - the 'reason' parent error object:
> +
> +```
> +e1 = box.error.new({code = 111, reason = "just cause"})
> +e2 = box.error.new({code = 222, reason = "just cause x2", prev = e1})
> +```
> +
> +### Binary protocol
> +
> +Currently errors are sent as `(IPROTO_ERROR | errcode)` response with an
> +string message containing error details as a payload. There are not so many
> +options to extend current protocol wihtout breaking backward compatibility
> +(which is obviously one of implementation requirements). One way is to extend
> +existent binary protocol with a new key IPROTO_ERROR_STACK (or
> +IPROTO_ERROR_REASON or simply IPROTO_ERROR_V2):
> +```
> +{
> + // backward compatibility
> + IPROTO_ERROR: "the most recent error message",
> + // modern error message
> + IPROTO_ERROR_STACK: {
> + {
> + // the most recent error object
> + IPROTO_ERROR_CODE: error_code_number,
> + IPROTO_ERROR_REASON: error_reason_string,
> + },
> + ...
> + {
> + // the oldest (reason) error object
> + },
> + }
> +}
> +```
> +
> +IPROTO_ERROR is always sent (as in older versions) in case of error.
> +IPROTO_ERROR_STACK is presented in response only if there's at least two
> +elements in diagnostic list. Map which contains error stack can be optimized
> +in terms of space, so that avoid including error which is already encoded
> +in IPROTO_ERROR.
> --
> 2.15.1
>
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