# Stop Exceptions capturing object references for trace arguments - Thread: https://externals.io/message/100189 - Original archive: https://news-web.php.net/php.internals/100189 - Started by Rowan Collins on 2017-08-11T21:03:59+00:00 - Messages: 7 - Reply by email to internals@lists.php.net - More for agents: search with https://externals.io/search.md?q={query} · JSON API at https://externals.io/api · documentation at https://externals.io/llms.txt --- ## #100189 — Rowan Collins — 2017-08-11T21:03:59+00:00 URL: https://externals.io/message/100189#100189 Hi All, This topic has been discussed a couple of times before: - http://marc.info/?t=138118341600002&r=1&w=2 - http://marc.info/?t=142708828500001&r=1&w=2 - http://marc.info/?t=143798102800002&r=1&w=2 I was inspired to revive the topic by this bug report: https://bugs.php.net/bug.php?id=75056&edit=3 Currently, the backtrace of an Exception stores the following: - function: string - line: integer - file: string - class: string - type: string - args: array This is effectively the same as `debug_backtrace()` with neither `DEBUG_BACKTRACE_PROVIDE_OBJECT` nor `DEBUG_BACKTRACE_IGNORE_ARGS` set. The "args" part of this contains full object references to anything that happens to have been a function argument in the stack, and causes two problems: - Serializing an exception can fail, because some of these referenced objects may be non-serializable (e.g. SimpleXMLElement, PDO). - Destructors for these objects do not fire until the Exception is destructed, causing resources to be held open unexpectedly after the stack has been unwound. (The only other place I know of destructors being unreliable in PHP is if there are circular references, in which case the destructor will only fire when the GC detects the cycle.) Both effects are almost entirely unpredictable, because it depends if the object happens to have been a parameter in the current stack, rather than a local variable or $this reference. The simplest solution would be to simply remove this 'args' key from the backtraces, as when `DEBUG_BACKTRACE_IGNORE_ARGS` is set on `debug_backtrace()`. This would mean that any normal exception would hold no object references, so would not extend any lifetimes, and would not trigger any serialization errors. Obviously, this has the downside of breaking any code that makes use of this array; if anyone has examples of practical uses for it I would be grateful to see them, and whether a compromise such as storing the class name but not the object would be useful. Regards, -- Rowan Collins [IMSoP] --- ## #100192 — Stanislav Malyshev — 2017-08-12T03:25:56+00:00 URL: https://externals.io/message/100189#100192 · In reply to: #100189 Hi! > The "args" part of this contains full object references to anything that > happens to have been a function argument in the stack, and causes two > problems: I think it makes sense to make exception not to collect args. In fact, I think this may also be one of rare cases where new ini setting would be appropriate, where the default could be the old way (at least for now, for BC), and recommended production setting would be off. -- Stas Malyshev smalyshev@gmail.com --- ## #100193 — Rowan Collins — 2017-08-12T14:37:08+00:00 URL: https://externals.io/message/100189#100193 · In reply to: #100192 > Hi! > > > […] > I think it makes sense to make exception not to collect args. In fact, I > think this may also be one of rare cases where new ini setting would be > appropriate, where the default could be the old way (at least for now, > for BC), and recommended production setting would be off. > My only concern is if anyone is using this data for anything other than debugging - e.g. if they were somehow extracting extra context about the exception by traversing the backtrace to a particular point and grabbing the arguments. That feels like a horrible hack to me, but that doesn't mean someone isn't relying on it. An interesting side effect of an ini setting, though, is that it could in turn be made to raise deprecation warnings at some point, whereas just accessing the 'args' element of an array is pretty hard to detect. So for instance, we could theoretically have: 7.3, add ini setting defaulting to on; 7.4, change default to off and raise deprecation notice if it's turned on; 8.0, remove feature, error if ini setting is not set to on (ignore if set to off for smoother upgrades). Regards, -- Rowan Collins [IMSoP] --- ## #100194 — Stanislav Malyshev — 2017-08-12T15:54:25+00:00 URL: https://externals.io/message/100189#100194 · In reply to: #100193 Hi! > My only concern is if anyone is using this data for anything other than > debugging - e.g. if they were somehow extracting extra context about the > exception by traversing the backtrace to a particular point and grabbing > the arguments. That feels like a horrible hack to me, but that doesn't > mean someone isn't relying on it. Sure, that's why I propose to keep the option. If you use the horrible hack, I think it wouldn't be too much to ask to set an INI variable. Since it's rather local, it can be `INI_ALL` so you could do it in the script even. > An interesting side effect of an ini setting, though, is that it could > in turn be made to raise deprecation warnings at some point, whereas > just accessing the 'args' element of an array is pretty hard to detect. > So for instance, we could theoretically have: 7.3, add ini setting > defaulting to on; 7.4, change default to off and raise deprecation > notice if it's turned on; 8.0, remove feature, error if ini setting is > not set to on (ignore if set to off for smoother upgrades). Yep, sounds like a plan. Not sure if we need to drop it for 8.0 even, but we can vote on that. Anybody to make an RFC for this? :) -- Stas Malyshev smalyshev@gmail.com --- ## #100198 — Niklas Keller — 2017-08-13T14:31:26+00:00 URL: https://externals.io/message/100189#100198 · In reply to: #100194 2017-08-12 17:54 GMT+02:00 Stanislav Malyshev : > Hi! > > > […] > > Sure, that's why I propose to keep the option. If you use the horrible > hack, I think it wouldn't be too much to ask to set an INI variable. > Since it's rather local, it can be `INI_ALL` so you could do it in the > script even. > > > […] > > Yep, sounds like a plan. Not sure if we need to drop it for 8.0 even, > but we can vote on that. Anybody to make an RFC for this? :) > If it's explicitly needed, someone could still just call `debug_backtrace()` and manually store the args in the exception constructor, no? Would the args still be available in the stringified version? Because they're quite useful for debugging there. Regards, Niklas --- ## #100207 — Rowan Collins — 2017-08-13T20:44:32+00:00 URL: https://externals.io/message/100189#100207 · In reply to: #100198 > > If it's explicitly needed, someone could still just call > `debug_backtrace()` and manually store the args in the exception > constructor, no? That depends exactly how it's being used, but yes if you control the throw site or Exception constructor you could grab the backtrace manually. > > Would the args still be available in the stringified version? Because > they're quite useful for debugging there. Hm, I guess we'd need to store the string representation (the class name, basically) somewhere so we could keep that. Doesn't seem like it ought to be too hard, unless there's some hidden complexity or performance impact there. Regards, -- Rowan Collins [IMSoP] --- ## #100352 — Niklas Keller — 2017-09-02T11:52:21+00:00 URL: https://externals.io/message/100189#100352 · In reply to: #100207 > > Would the args still be available in the stringified version? Because > > […] > > Hm, I guess we'd need to store the string representation (the class name, > basically) somewhere so we could keep that. Doesn't seem like it ought to > be too hard, unless there's some hidden complexity or performance impact > there. Not sure whether there will be a noticeable performance impact. Formatting doesn't have to take place on collection, but a replacement of object references to some representation that can later be formatted. A refcount increase can be saved, but that's probably totally trivial performance wise. Related issue for ReactPHP's promises: https://github.com/reactphp/promise/issues/46#issuecomment-326739299 Regards, Niklas