How Can a Go Multiplexer Ensure Fair and Safe Access Across Multiple Input Channels?

A Channel Multiplexer with Equal Access

This Go multiplexer aims to merge the outputs of multiple channels into one, ensuring that each input channel has equal rights to the output channel. However, the provided test gives unexpected results.

Analysis of the Issue

The critical issue lies in the goroutines spawned from the Mux function. The channel parameter c, intended to represent each input channel, is being updated on each iteration of the loop. This means that all goroutines end up pulling from the same channel instead of their intended individual channels.

Solution

To resolve this issue, modify the goroutine creation loop to pass the correct channel to each goroutine:

for _, c := range channels {
    go func(c <-chan big.Int) {
        ...
    }(c)
}

By doing this, each goroutine captures the value of the channel when it is created, eliminating the issue and producing the desired results.

Improving Concurrency Safety

In addition to ensuring equal access to the output channel, it is crucial to ensure concurrency safety. The initial code uses an int variable n to track the closing of input channels. However, with GOMAXPROCS greater than 1, it is possible for multiple goroutines to access n simultaneously, potentially causing race conditions.

A safer approach is to use a sync.WaitGroup object, which allows goroutines to wait for each other and ensures that n is updated safely. The revised code using a sync.WaitGroup:

import (
    "math/big"
    "sync"
)

// ... other code ...

// The channel to output to.
ch := make(chan big.Int, len(channels))

var wg sync.WaitGroup
wg.Add(len(channels))

// ... other code ...

// Close the channel when the pumping is finished.
go func() {
    // Wait for everyone to be done.
    wg.Wait()
    // Close.
    close(ch)
}()

With these modifications, the multiplexer now operates correctly and safely, ensuring that all input channels have equal access to the output channel and avoiding race conditions.

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