Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

07-fan-in/main.go

@@ -0,0 +1,49 @@
+// Exercise 07 — Fan-in / Merge Channels
+//
+// ASSIGNMENT
+// ----------
+// Implement Merge(ctx, chs...):
+//
+//  1. Returns a single channel carrying every value from every input channel.
+//  2. The output channel is CLOSED once all input channels are closed and
+//     drained.
+//  3. If ctx is cancelled, Merge stops forwarding, and all its internal
+//     goroutines exit even if inputs never close and even if nobody reads
+//     the output anymore. (No goroutine leaks — the tests count goroutines.)
+//  4. Order across inputs is unspecified.
+//
+// Run the tests:   go test -race ./07-fan-in/
+package main
+
+import (
+	"context"
+	"fmt"
+)
+
+// Merge fans in all input channels into one output channel.
+// TODO: implement me.
+func Merge[T any](ctx context.Context, chs ...<-chan T) <-chan T {
+	out := make(chan T)
+	// TODO:
+	//   - one forwarder goroutine per input channel
+	//   - each forward is a select: {out <- v} vs {<-ctx.Done()}
+	//   - reading the input must ALSO select on ctx.Done()
+	//   - closer goroutine: wg.Wait(); close(out)
+	return out
+}
+
+func main() {
+	gen := func(vals ...int) <-chan int {
+		ch := make(chan int)
+		go func() {
+			defer close(ch)
+			for _, v := range vals {
+				ch <- v
+			}
+		}()
+		return ch
+	}
+	for v := range Merge(context.Background(), gen(1, 2, 3), gen(10, 20), gen(100)) {
+		fmt.Println(v)
+	}
+}