[Spice-devel] [PATCH v2 22/24] Throw exception in case of write failure
Christophe de Dinechin
cdupontd at redhat.com
Thu Feb 22 14:09:24 UTC 2018
> On 22 Feb 2018, at 14:12, Lukáš Hrázký <lhrazky at redhat.com> wrote:
>
> Apart from what's below, I'm also thinking if it wouldn't be better to
> leave this addition of exception classes for later and concentrate on
> finishing what's already present in this big series. Adding more stuff
> to it doesn't really speed things up :/
>
> I think write_all() throwing a runtime_error as of now is surely no biggie.
It is a biggie. I hoped that your interest in writing unit tests would make it obvious.
It’s big enough to be E13 in the C++ Core Guidelines, https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Re-exception-types
"The standard-library classes derived from exception should be used only as base classes or for exceptions that require only "generic" handling. Like built-in types, their use could clash with other people's use of them."
Let me stand my ground here. I will not throw runtime_error in application code I write. I personally consider this “lazy”. I strongly invite you to not do it for the reasons already exposed, and if you don’t, I will propose patches to fix what I see as a design bug.
> On Thu, 2018-02-22 at 12:27 +0100, Christophe de Dinechin wrote:
>>> On 22 Feb 2018, at 10:42, Lukáš Hrázký <lhrazky at redhat.com> wrote:
>>>
>>> On Wed, 2018-02-21 at 18:46 +0100, Christophe de Dinechin wrote:
>>>> From: Christophe de Dinechin <dinechin at redhat.com>
>>>>
>>>> This also introduces the spice::streaming_error::Error class, which we can reuse
>>>> later as a base class for all agent-specific errors. This class provides a
>>>> formatted 'message()' class that returns a string, making it easier to format
>>>> errors without allocating memory at throw-time.
>>>
>>> I am not at all convinced this approach has any advantages.
>>
>> I realize that I only shared the benefits in a private message, not on-list. So here:
>>
>> The benefit I see in this approach are
>>
>> 1. there is no alloc at the throw point,
>
> I don't see this as a benefit in itself, are there any implications I’m missing?
At the throw point, the maximum amount of resources is allocated, and throwing the wrong exception will do the maximum amount of damage In particular, if you want to retry on a write error or do some specific cleanup and rethrow, getting a bad_alloc will kill these possibilities.
At the catch point, you have already discriminated what you could deal with, you were able to securely do retries or anything you wanted to do with exceptions you caught, and if after everything else fails, you get an error while printing the error message, that’s the minimum amount of possible damage.
But as I already wrote, if that’s so much of a concern, we can rewrite the message-generation code so that it never allocates.
>
>> 2. the catch point can extract a specific exception type and its args (e.g. for unit testing)
>
> This is true, but as I said, I consider this useful only for specific cases. Often you don't care that much to differentiante.
This “you” is not me. I certainly do care that I can discriminate my exceptions.
> In unit tests, you can also check the error message.
You could, does not make it a good idea. IMO, it’s about as bad as
throw “That did not work”;
where you “could” catch const char * and do strcmp on the message. You can, the standard allows it, but it’s bad.
> I certainly would not introduce specific error classes only for the sake of the tests.
Sorry, straw-man. I gave four reasons, I could list quite a few more if you insist on that misguided suggestion of throwing runtime_error. So it’s definitely not “only for the sake of the tests”.
>
>> 3. formatting is in the class, rather than ad-hoc for every single throw
>
> I find this partially benefitial for certain classes of exceptions.
We have a good example here, where instead of having to strerror(errno) at every place, we let the class do it.
As usual, it’s only one of many. This approach will only get better over time…
> By partially I mean, that while it puts the formatting in one place, it adds the boilerplate of the class around it, so LOC for LOC, it's not that great for small number of throws.
LOC is not my reference metric. As far as typing goes, this discussion is already well beyond the code in the patch :-)
>
>> 4. We can distinguish errors we “expect” (e.g. Error and derived) from those we expect “less” (e.g. runtime_error).
>
> I think this ir more or less 2.?
No, not at all. We expect write errors, we can do something about it, e.g. close the channel, reopen, etc.
We should never *expect* a runtime_error, it’s the standard library telling us something bad happened, e.g. you misused one of the standard containers. It’s more the result of a stray pointer in your code or unchecked input arguments than from bogus user input.
In other words, WriteError is a “normal” behavior of your program under external stress. Getting runtime_error should never be considered a normal behavior, it should indicate a bug you need to fix.
Writing this made me realize that Error should not derive from runtime_error, but from exception. We don’t want to allocate an std::string, for one thing.
>
>> Which ones do you disagree with?
>
> As you can see, I don't disagree in a black and white fashion. I think I just want to avoid having 10+ different error classes with all the noise it brings,
Sorry, straw-man. I introduced two classes (Error and WriteError). There will be a couple more coming (IOError, ReadError), each time extremely simple.
The claim of “noise” is FUD. The error classes describe an intent and facilitate changes we know we have to do later anyway. Almost, but not quite, anything but noise :-)
In terms of code efficiency, constructing one class with three fields is certain much more efficient than calling string::operator+ a couple of times, constructing a string, etc.
> it seems negatives outweight positives for me. Yes, been there, done that.
The C++ Core Guidelines disagree with you, so do I.
That being said, I’m interested in the horror story you seem to imply behind your “been there, done that”. There is probably a lesson to learn from it, though I’m pretty positive it is not to throw runtime_error.
>
>>> How exactly makes it easier to format messages?
>>
>> Consider for example if you wanted to use a printf-style interface for formatting the message.
>>
>> Two use cases:
>> 1. Localization (printf is better than appending strings, allows for reordering)
>> 2. Flight recorder
>
> If we have such a need, then sure, let's do it.
We have such a need, we have not written the code yet. So my writing the code that way is planning ahead.
>
>>> This means we would need classes
>>> for all kinds of errors throughout the agent (unless you still want to
>>> use runtime_errors in some places).
>>
>> You don’t “need to”, you “want to”. It’s important to be able to discriminate between classes of error. For exceptions, the exception class plays the same role as errno values. If you would return the same errno, you can throw the same class. Otherwise, you shouldn’t.
>
> At current state of things, I don't want :) I only want to distinguish error classes when I need to. Otherwise, spare me the churn :)
That’s not where the churn is.
>
>>> While, as I said, it makes sense to me in some places, in others I find it unnecessary work and code bloat (for lack of better word to express myself).
>>
>> There are cases were we can reuse the same exception class, but using runtime_error is not that class.
>>
>>>
>>>> Signed-off-by: Christophe de Dinechin <dinechin at redhat.com>
>>>> ---
>>>> src/spice-streaming-agent.cpp | 65 ++++++++++++++++++++++++-------------------
>>>> 1 file changed, 36 insertions(+), 29 deletions(-)
>>>>
>>>> diff --git a/src/spice-streaming-agent.cpp b/src/spice-streaming-agent.cpp
>>>> index 4aae2cb..a789aad 100644
>>>> --- a/src/spice-streaming-agent.cpp
>>>> +++ b/src/spice-streaming-agent.cpp
>>>> @@ -59,15 +59,30 @@ static uint64_t get_time(void)
>>>>
>>>> }
>>>>
>>>> +class Error : public std::runtime_error
>>>> +{
>>>> +public:
>>>> + Error(const char *msg): runtime_error(msg) {}
>>>> + virtual std::string message() { return what(); }
>>>> +};
>>>> +
>>>
>>> I think introducing this class would warrant a separate commit.
>>
>> I don’t like introducing a class without its use case, you can’t test it.
>
> True that.
>
>>> Also, it could go in it's own file straight away.
>>
>> Yes.
>>
>>>
>>>> class Stream
>>>> {
>>>> typedef std::set<SpiceVideoCodecType> codecs_t;
>>>>
>>>> public:
>>>> - class WriteError : public std::runtime_error
>>>> + class WriteError final : public Error
>>>> {
>>>> public:
>>>> - WriteError(const char *msg): runtime_error(msg) {}
>>>> + WriteError(const char *msg, const char *operation, int saved_errno)
>>>
>>> The class is called WriteError, but this signature is apparently meant for an arbitrary operation that uses errno.
>>
>> No. It’s for a write error as the name indicates. I could have a ReadError with the same signature, and if I really have both, have an IOError base with two trivial derived classes.
>>
>> If at some point we want to deal with such errors, how we reconnect is likely to differ for read and write errors. Same for unit testing.
>
> Ok, I would still add the classes only when/if we need them and not
> prepare for that in advance... The change can be done anytime.
You asked me to throw from write, which I did. But I won’t throw runtime_error. It’s a spice streaming agent write error, not a runtime error.
>
>>
>>>
>>>> + : Error(msg), operation(operation), saved_errno(saved_errno) {}
>>>> + std::string message()
>>>> + {
>>>> + return Error::message() + " in " + operation + ": " + strerror(saved_errno);
>>>
>>> Ok, but isn't this actually creating the problem you described in an earlier email?
>>
>> It is, which is, as I pointed out, the reason the standard stuck with const char *.
>>
>> But in our application, at the point I’m causing the potential problem, cleanup already happened. In other words, I’m moving allocations at the place where they are most likely to succeed.
>
> That's right, fair enough.
>
>> BTW, if we are so out of memory that this will cause a bad_alloc, then iostream itself may also throw it, so short of going the long way and using ‘write’, we can’t do much better.
>>
>>> You construct the error string during the exception handling, thus if this throws an exception (as it can), you get a terminate?
>>
>> The (only safe) alternative is to format the message in a pre-allocated static buffer using snprintf, and to print it with ::write to avoid anything that can allocate buffers. A bit overkill IMO.
>>
>>>
>>> Lukas
>>>
>>>> + }
>>>> + private:
>>>> + const char *operation;
>>>> + int saved_errno;
>>>> };
>>>>
>>>> public:
>>>> @@ -95,18 +110,11 @@ public:
>>>> int read_command(bool blocking);
>>>>
>>>> template <typename Message, typename ...PayloadArgs>
>>>> - bool send(PayloadArgs... payload)
>>>> + void send(PayloadArgs... payload)
>>>> {
>>>> Message message(payload...);
>>>> std::lock_guard<std::mutex> stream_guard(mutex);
>>>> - size_t expected = message.size(payload...);
>>>> - size_t written = message.write(*this, payload...);
>>>> - bool result = written == expected;
>>>> - if (!result) {
>>>> - syslog(LOG_ERR, "sent only %zu bytes out of %zu", written, expected);
>>>> - throw WriteError("Unable to write complete packet");
>>>> - }
>>>> - return result;
>>>> + message.write(*this, payload...);
>>>> }
>>>>
>>>> size_t write_all(const char *what, const void *buf, const size_t len);
>>>> @@ -151,9 +159,9 @@ struct FormatMessage : Message<StreamMsgFormat, FormatMessage>
>>>> {
>>>> return StreamMsgFormat{ .width = w, .height = h, .codec = c, .padding1 = {} };
>>>> }
>>>> - size_t write(Stream &stream, unsigned w, unsigned h, uint8_t c)
>>>> + void write(Stream &stream, unsigned w, unsigned h, uint8_t c)
>>>> {
>>>> - return stream.write_all("FormatMessage", this, sizeof(message_t));
>>>> + stream.write_all("FormatMessage", this, sizeof(message_t));
>>>> }
>>>> };
>>>>
>>>> @@ -170,10 +178,10 @@ struct FrameMessage : Message<StreamMsgData, FrameMessage>
>>>> {
>>>> return StreamMsgData();
>>>> }
>>>> - size_t write(Stream &stream, const void *frame, size_t length)
>>>> + void write(Stream &stream, const void *frame, size_t length)
>>>> {
>>>> - return stream.write_all("FrameMessage header", this, sizeof(message_t))
>>>> - + stream.write_all("FrameMessage frame", frame, length);
>>>> + stream.write_all("FrameMessage header", this, sizeof(message_t));
>>>> + stream.write_all("FrameMessage frame", frame, length);
>>>> }
>>>> };
>>>>
>>>> @@ -208,11 +216,11 @@ struct X11CursorMessage : Message<StreamMsgCursorSet, X11CursorMessage>
>>>> .data = { }
>>>> };
>>>> }
>>>> - size_t write(Stream &stream, XFixesCursorImage *cursor)
>>>> + void write(Stream &stream, XFixesCursorImage *cursor)
>>>> {
>>>> unsigned pixel_size = pixel_count(cursor) * sizeof(uint32_t);
>>>> - return stream.write_all("X11CursorMessage header", this, sizeof(message_t))
>>>> - + stream.write_all("X11CursorMessage pixels", pixels.get(), pixel_size);
>>>> + stream.write_all("X11CursorMessage header", this, sizeof(message_t));
>>>> + stream.write_all("X11CursorMessage pixels", pixels.get(), pixel_size);
>>>> }
>>>> void fill_pixels(XFixesCursorImage *cursor)
>>>> {
>>>> @@ -329,9 +337,7 @@ void X11CursorThread::cursor_changes()
>>>> }
>>>>
>>>> last_serial = cursor->cursor_serial;
>>>> - if (!stream.send<X11CursorMessage>(cursor)) {
>>>> - syslog(LOG_WARNING, "FAILED to send cursor\n");
>>>> - }
>>>> + stream.send<X11CursorMessage>(cursor);
>>>> }
>>>> }
>>>>
>>>> @@ -462,7 +468,7 @@ size_t Stream::write_all(const char *what, const void *buf, const size_t len)
>>>> continue;
>>>> }
>>>> syslog(LOG_ERR, "write %s failed - %m", what);
>>>> - return l;
>>>> + throw WriteError("streaming agent write failed", what, errno);
>>>> }
>>>> written += l;
>>>> }
>>>> @@ -553,16 +559,13 @@ void ConcreteAgent::CaptureLoop(Stream &stream, FrameLog &frame_log)
>>>>
>>>> syslog(LOG_DEBUG, "wXh %uX%u codec=%u\n", width, height, codec);
>>>>
>>>> - if (!stream.send<FormatMessage>(width, height, codec))
>>>> - throw std::runtime_error("FAILED to send format message");
>>>> + stream.send<FormatMessage>(width, height, codec);
>>>> }
>>>> if (frame_log) {
>>>> frame_log.dump(frame.buffer, frame.buffer_size);
>>>> }
>>>> - if (!stream.send<FrameMessage>(frame.buffer, frame.buffer_size)) {
>>>> - syslog(LOG_ERR, "FAILED to send a frame\n");
>>>> - break;
>>>> - }
>>>> + stream.send<FrameMessage>(frame.buffer, frame.buffer_size);
>>>> +
>>>> //usleep(1);
>>>> if (stream.read_command(false) < 0) {
>>>> syslog(LOG_ERR, "FAILED to read command\n");
>>>> @@ -640,6 +643,10 @@ int main(int argc, char* argv[])
>>>> FrameLog frame_log(log_filename, log_binary);
>>>> agent.CaptureLoop(streamfd, frame_log);
>>>> }
>>>> + catch (Error &err) {
>>>> + syslog(LOG_ERR, "%s\n", err.message().c_str());
>>>> + ret = EXIT_FAILURE;
>>>> + }
>>>> catch (std::exception &err) {
>>>> syslog(LOG_ERR, "%s\n", err.what());
>>>> ret = EXIT_FAILURE;
>>>
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>>
>>
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