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RSDK-10643: condition var refresh thread #428

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RSDK-10643: condition var refresh thread #428

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05b21d8
condition var refresh thread
lia-viam May 1, 2025
a2c8bf2
include cond var
lia-viam May 1, 2025
a78291f
concise refresh
lia-viam May 1, 2025
7198e15
const lock guard
lia-viam May 2, 2025
4fd7e13
Merge branch 'main' of github.com:viamrobotics/viam-cpp-sdk into robo…
lia-viam May 5, 2025
1a81c32
Merge branch 'robot-client-cond-var' of github.com:lia-viam/viam-cpp-…
lia-viam May 5, 2025
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28 changes: 19 additions & 9 deletions src/viam/sdk/robot/client.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -143,7 +143,11 @@ RobotClient::~RobotClient() {
}

void RobotClient::close() {
should_refresh_.store(false);
if (should_refresh_) {
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If should_refresh_ is changing state, it should do so under the lock I think.

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I think it is not changing state-- should_refresh_ is only mutated in this method and in the named constructor which sets it in the first place, so there may be a concurrent read happening here but there's no way for it to be read while being mutated

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I'll argue that this is one of those cases where even if it is safe in practice, it is still better to do the check under the lock. It'll be annoying if someday we stand up TSAN and it whines about a leak here, and this isn't a hot path, so trying to eliminate the lock doesn't buy much of anything.

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I mean race of course, not leak, since I'm talking about TSAN not ASAN.

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done!

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I'd even move the read of should_referesh_ under the lock, or even eliminate the check entirely. It should be fine in close to just unconditionally set should_refresh_ = false (under the lock), and then notify. If there is no refresh thread, it is just a big no-op.

const std::unique_lock<std::mutex> lk{refresh_lock_};
should_refresh_ = false;
refresh_cv_.notify_one();
}

if (refresh_thread_.joinable()) {
refresh_thread_.join();
Expand Down Expand Up @@ -220,16 +224,22 @@ void RobotClient::refresh() {
}

void RobotClient::refresh_every() {
while (should_refresh_.load()) {
try {
std::this_thread::sleep_for(refresh_interval_);
refresh();
std::unique_lock<std::mutex> lk{refresh_lock_};

} catch (std::exception&) {
break;
refresh_cv_.wait_for(lk, refresh_interval_, [this] {
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I think this is going to terminate after one timeout, but tbh I always get this stuff wrong, so don't let me mislead you if you have thought it through and this actually does what is intended.

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you were right, classic case of a solution that is simple, elegant, and wrong 🙂

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I'm generally skeptical of the predicate taking condition things because they are so easy to get wrong, and require too much thinking.

Here is how I think I would write this loop unrolled:

std::unique_lock<std::mutex> lk{refresh_lock_};
// Terminate if we ever see `should_refresh` as `false` while holding the lock
while (should_refresh_) {
    // Compute the absolute time on which the next refresh should happen
    const auto wake_time = std::chrono::steady_clock::now + refresh_interval_;
    // Loop until deadline is reached or `should_refresh_` becomes `false`
    while (should_refresh_) {
        const auto wait_result = refresh_cv.wait_until(lk, wake_time);
        if (wait_result == cv_status::timeout && should_refresh_) {
            // We hit the deadline: do a refresh
            try {
                refresh();
            } catch (...) { ... }
            // Break out of inner loop to compute next deadline
            break;
        } else {
            // If this was a spurious wake, then `should_refresh_` is still `true`, and
            // `wait_until` will be reinvoked with existing deadline. If this was not
            // a spurious wake, then we were notified, and both the inner and outer loop
            // will terminate.
        }
    }
}

But it is possible that the predicate version is more or less equivalent to that, in which case it should be fine.

if (should_refresh_) {
try {
refresh();
} catch (const std::exception& e) {
VIAM_SDK_LOG(warn) << "Refresh thread got exception " << e.what();
// TODO: maybe recoverable
return true;
}
}
}
};

return !should_refresh_;
});
}

RobotClient::RobotClient(ViamChannel channel)
: viam_channel_(std::move(channel)),
Expand Down
6 changes: 4 additions & 2 deletions src/viam/sdk/robot/client.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
/// @brief gRPC client implementation for a `robot`.
#pragma once

#include <atomic>
#include <condition_variable>
#include <string>
#include <thread>

Expand Down Expand Up @@ -186,7 +186,9 @@ class RobotClient {
void refresh_every();

std::thread refresh_thread_;
std::atomic<bool> should_refresh_;
std::mutex refresh_lock_;
std::condition_variable refresh_cv_;
bool should_refresh_;
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Might be possible to eliminate the bool by making refresh_interval_ an optional.

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i think you're right, we're storing a T data; bool valid; which is basically just what an optional is under the hood.

(we could just have refresh_interval_ alone do all the work, but it's a little less self-documenting, although there are compact_optional types that allow this sort of thing)

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I think it is probably worth doing. There's just less state to manage, which means less risk of things decohering. It'd also simplify RobotClient::with_channel:

From

    robot->refresh_interval_ = std::chrono::seconds{options.refresh_interval()};
    robot->should_refresh_ = (robot->refresh_interval_ > std::chrono::seconds{0});
    if (robot->should_refresh_) {
        robot->refresh_thread_ = std::thread{&RobotClient::refresh_every, robot.get()};
    }

To

    if (options.refresh_interval() != 0) {
        robot->refresh_interval_.reset(options.refresh_interval());
        robot->refresh_thread_ = std::thread{&RobotClient::refresh_every, robot.get()}
    }

std::chrono::seconds refresh_interval_;

ViamChannel viam_channel_;
Expand Down