Deadline:

This work should be completed before the exercise on Thursday 1st April.

Instructions

For instructions on how to do and submit the assignment, please see the assignments section of the course instructions.

Homework

Study the following course literature:

• Read the following from the Fundamentals of Concurrent Programming
  • Channels
  • Select
  • Data races
  • Detecting data races
  • Deadlock

Task 1 - Debugging Concurrent Programs

In this task, you will be provided with two buggy programs. For each program, you should:

1. Explain what is wrong with the code.
2. Fix the bug.
3. Explain why your solution fixes the bug.

Buggy program 1

src/bug01.go

Buggy program 2

src/bug02.go

Assistant's requirement: You are not allowed to move or remove the time.Sleep(10 * time.Millisecond) statement in the Print function. A deterministic solution (e.g. using a channel or WaitGroup) is required and removing the sleep is only a quick fix.

Task 2 - Many Senders; Many Receivers

The program many2many.go contains four producers that together send 32 strings over a channel. At the other end there are two consumers that receive the strings. Describe what happens, and explain why it happens, if you make the following changes in the program. Try first to reason your way through, and then test your hypothesis by changing and running the program.

• What happens if you switch the order of the statements wgp.Wait() and close(ch) in the end of the main function?
• What happens if you move the close(ch) from the main function and instead close the channel in the end of the function Produce?
• What happens if you remove the statement close(ch) completely?
• What happens if you increase the number of consumers from 2 to 4?
• Can you be sure that all strings are printed before the program stops?

Finally, modify the code by adding a new WaitGroup that waits for all consumers to finish.

Task 3 - Pythia, the Oracle of Delphi

The code in oracle.go contains the outline for a program that will answer 'questions'. Complete the Oracle function. You should not modify the main function or other function signatures. Note that answers should not appear immediately; instead there should be a delay or pause for thought. Also, the Oracle will still print helpful predictions even if there are not any questions. You may structure your solution into multiple functions.

Your program should contain two channels: One channel for questions, and one for answers and predictions. In the Oracle function you should start three indefinite go-routines.

• A go-routine that receives all questions, and for each incoming question, creates a separate go-routine that answers that question
• A go-routine that generates predictions
• A go-routine that receives all answers and predictions, and prints then to stdout

Whilst the Oracle function is the most important of the assignment, you may also want to improve the answer-algorithm.

• The strings and regex packages may be of some help
• The program can seem more human if the Oracle prints it answers one character at a time
• Take a look at the story of ELIZA

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Please commit any written answers or diagrams to the "docs" folder as a PDF (or Markdown) document, and commit any code developed to the "src" folder of your KTH Github repo. Remember to push to KTH Github before your exercise.