From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtpng3.m.smailru.net (smtpng3.m.smailru.net [94.100.177.149]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dev.tarantool.org (Postfix) with ESMTPS id C561346970E for ; Tue, 14 Jan 2020 23:16:15 +0300 (MSK) From: Nikita Pettik Date: Tue, 14 Jan 2020 23:16:10 +0300 Message-Id: <3f49ddc14c2e07f4ac96fc71f3912fd0c5663882.1579032293.git.korablev@tarantool.org> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Subject: [Tarantool-patches] [PATCH] box: rfc for stacked diagnostic area List-Id: Tarantool development patches List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: tarantool-patches@dev.tarantool.org From: Kirill Shcherbatov 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@tarantool.org, + Pettik Nikita korablev@tarantool.org +* **Issues**: [#1148](https://github.com/tarantool//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: +1. diag_set(code = 1) +Errors: NULL> +2. diag_add(code = 2) +Errors: e2(code = 2) -> NULL> +3. diag_set(code = 3) +Errors: 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: NULL> +2. diag_add(code = 2) +Errors: e2(code = 2) -> NULL> +-- End of execution + +-- Processing of request #2 +1. diag_add(code = 1) +Errors: 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