doc/next: added release notes for goroutine leak profiles

doc/next/4-runtime.md

@@ -1 +1,62 @@
 ## Runtime {#runtime}
+
+### Goroutine leak profiles {#goroutineleak-profiles}
+
+We introduce a new profile type for goroutine leaks. With the experimental
+flag set to `GOEXPERIMENT=goroutineleakprofile`, it becomes accessible
+through `pprof` under the name `"goroutineleak"`.
+
+The following snippet showcases a common, but erroneous pattern
+that leads to goroutine leaks:
+```go
+type result struct{
+		res workResult
+		err error
+}
+
+func processWorkItems(ws []workItem) ([]workResult, error) {
+	// Process work items in parallel, aggregating results in ch.
+	ch := make(chan result)
+	for _, w := range ws {
+		go func() {
+			res, err := processWorkItem(w)
+			ch <- result{res, err}
+		}()
+	}
+
+	// Collect the results from ch, or return an error if one is found.
+	var results []workResult
+	for range len(ws) {
+		r := <-ch
+		if r.err != nil {
+			// This premature return may cause goroutine leaks
+			return nil, r.err
+		}
+		results = append(results, r.res)
+	}
+	return results, nil
+}
+```
+Because `ch` is unbuffered, if `processWorkItems` returns early due to an error,
+all remaining work item goroutines will leak.
+However, also note that, soon after the leak occurs, `ch` is inaccessible
+to any other goroutine, except those involved in the leak.
+
+To generalize, a goroutine is leaked if it is blocked by concurrency
+primitives (specifically channels, and `sync` primitives such as mutex) that
+are only referenced by the blocked goroutine itself, or other leaked goroutines.
+The Go runtime is now equipped to reveal leaked goroutines by recording their stacks in
+goroutine leak profiles.
+In the example above, the stacks of work item goroutines point to the culprit channel send
+operation.
+
+Note that, while goroutine leak profiles only include true positives, goroutine leaks may be
+missed when caused by concurrency primitives that are accessible globally, or referenced
+by runnable goroutines.
+
+Special thanks to Vlad Saioc at Uber for contributing this work.
+The underlying theory is presented in detail by Saioc et al. in [this publication](https://dl.acm.org/doi/pdf/10.1145/3676641.3715990).
+
+<!-- More details about the implementation are presented in the [design document](https://github.com/golang/proposal/blob/master/design/74609-goroutine-leak-detection-gc.md). -->
+We encourage users to experiment with the new feature in different environments
+(tests, CI, production), and welcome feedback on the [proposal issue](https://github.com/golang/go/issues/74609).