From 53f583e8fc82854a2f77d4740c3b6a0273c0286e Mon Sep 17 00:00:00 2001
From: Gabor Retvari <retvari@tmit.bme.hu>
Date: Fri, 30 Aug 2024 18:06:38 +0200
Subject: [PATCH] Nested goroutines+context exercise

---
 22-goroutines/06-context/.README.md        | 40 +++++++++++++++++
 22-goroutines/06-context/.exercise_test.go | 50 ++++++++++++++++++++++
 22-goroutines/06-context/README.md         |  1 +
 22-goroutines/06-context/exercise.go       |  6 +++
 22-goroutines/06-context/exercise.yaml     |  1 +
 22-goroutines/06-context/exercise_test.go  |  1 +
 6 files changed, 99 insertions(+)
 create mode 100644 22-goroutines/06-context/.README.md
 create mode 100644 22-goroutines/06-context/.exercise_test.go
 create mode 100644 22-goroutines/06-context/README.md
 create mode 100644 22-goroutines/06-context/exercise.go
 create mode 100644 22-goroutines/06-context/exercise.yaml
 create mode 100644 22-goroutines/06-context/exercise_test.go

diff --git a/22-goroutines/06-context/.README.md b/22-goroutines/06-context/.README.md
new file mode 100644
index 0000000..e21fbd2
--- /dev/null
+++ b/22-goroutines/06-context/.README.md
@@ -0,0 +1,40 @@
+# Nested Goroutines
+
+In this exercise, you'll practice using contexts to manage nested goroutines in Go. Your task is to
+implement two functions: one that starts a long-running operation in a separate goroutine, and
+another that performs the actual long-running operation. You'll use contexts to manage the lifetime
+of these goroutines and implement a cancellation mechanism that allows the caller to stop both
+goroutines.
+
+The two function signatures:
+
+```go
+func StartTask(ctx context.Context) (result string, err error)
+func SubTask(ctx context.Context) (result string, err error)
+```
+
+Requirements:
+
+1. `StartTask` should:
+   - Create a new context with a 1-second timeout, derived from the input context.
+   - Start `SubTask` in a new goroutine using the derived context.
+   - If the main context is canceled, cancel the subtask and return with an empty string and the
+     error provided by the main context (use [`ctx.Err()`](https://pkg.go.dev/context#Context)).
+   - If `SubTask` finishes with a non-empty error, return that error with an empty string.
+   - Otherwise return whatever string `SubTask` returns prepended with the string `"Main task
+     status:"` and a `nil` error.
+
+2. `SubTask` should:
+   - Simulate a long-running task by attempting to run for 200 milliseconds.
+   - If the provided context is canceled before the task completes, return immediately with the
+     error provided by the context (use [`ctx.Err()`](https://pkg.go.dev/context#Context)).
+   - Otherwise, return the string `"Subtask completed successfully"` with an empty error.
+
+3. Use channels to communicate the subtask result and errors between the goroutines. Make sure to
+   close the channels in the goroutine that actually writes the channels otherwise you will see
+   ugly race conditions and random panics.
+
+4. Ensure proper resource cleanup by using `defer` statements where appropriate.
+
+Insert the code into the file `exercise.go` at the placeholder `// INSERT YOUR CODE HERE`.
+
diff --git a/22-goroutines/06-context/.exercise_test.go b/22-goroutines/06-context/.exercise_test.go
new file mode 100644
index 0000000..6032d87
--- /dev/null
+++ b/22-goroutines/06-context/.exercise_test.go
@@ -0,0 +1,50 @@
+package subtask
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+func TestStartLongRunningTask(t *testing.T) {
+	t.Run("Successful completion", func(t *testing.T) {
+		ctx := context.Background()
+		result, err := StartTask(ctx)
+		if err != nil {
+			t.Errorf("Expected no error, got %v", err)
+		}
+		if result != "Main task status: Subtask completed successfully" &&
+			result != "Main task status:Subtask completed successfully" {
+			t.Errorf("Expected 'Main task status:Subtask completed successfully', got %s", result)
+		}
+	})
+
+	t.Run("Cancelled by main context", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		go func() {
+			time.Sleep(100 * time.Millisecond)
+			cancel()
+		}()
+
+		result, err := StartTask(ctx)
+		if err == nil {
+			t.Error("Expected an error, got nil")
+		}
+		if result != "" {
+			t.Errorf("Expected empty result, got %s", result)
+		}
+	})
+
+	t.Run("Immediate cancellation", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel() // Cancel immediately
+
+		result, err := StartTask(ctx)
+		if err == nil {
+			t.Error("Expected an error, got nil")
+		}
+		if result != "" {
+			t.Errorf("Expected empty result, got %s", result)
+		}
+	})
+}
diff --git a/22-goroutines/06-context/README.md b/22-goroutines/06-context/README.md
new file mode 100644
index 0000000..90c149f
--- /dev/null
+++ b/22-goroutines/06-context/README.md
@@ -0,0 +1 @@
+# PLEASE RUN make generate
diff --git a/22-goroutines/06-context/exercise.go b/22-goroutines/06-context/exercise.go
new file mode 100644
index 0000000..2425899
--- /dev/null
+++ b/22-goroutines/06-context/exercise.go
@@ -0,0 +1,6 @@
+package subtask
+
+// DO NOT REMOVE THIS COMMENT
+//go:generate go run ../../exercises-cli.go -student-id=$STUDENT_ID generate
+
+// INSERT YOUR CODE HERE
diff --git a/22-goroutines/06-context/exercise.yaml b/22-goroutines/06-context/exercise.yaml
new file mode 100644
index 0000000..d8255d7
--- /dev/null
+++ b/22-goroutines/06-context/exercise.yaml
@@ -0,0 +1 @@
+name: subtask
diff --git a/22-goroutines/06-context/exercise_test.go b/22-goroutines/06-context/exercise_test.go
new file mode 100644
index 0000000..4cf09ae
--- /dev/null
+++ b/22-goroutines/06-context/exercise_test.go
@@ -0,0 +1 @@
+// PLEASE RUN make generate