runtime/metrics: fix panic in Read with empty slice

Calling Read with a nil or empty slice previously caused a panic with
"index out of range" because the function unconditionally accessed the
first element of the slice (via &m[0]) to pass the pointer to the
runtime.

This change adds a check for len(m) == 0 to return early, preventing
the panic when no samples are provided.

Fixes #77231

Change-Id: I442635f5c61de432883c8d0efae9cc6aa1363cc9
GitHub-Last-Rev: 6f24f67b18
GitHub-Pull-Request: golang/go#77233
Reviewed-on: https://go-review.googlesource.com/c/go/+/737380
Reviewed-by: Amol Yadav <amolyadav6125@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/runtime/metrics/description_test.go

@@ -156,3 +156,10 @@ func TestDocs(t *testing.T) {
 		fmt.Fprintf(os.Stderr, "go test -generate: doc.go already up-to-date\n")
 	}
 }
+
+func TestReadEmptySlice(t *testing.T) {
+	// Test that Read does not panic when given an empty slice.
+	// This should be a no-op.
+	metrics.Read(nil)
+	metrics.Read([]metrics.Sample{})
+}

src/runtime/metrics/sample.go

@@ -43,5 +43,8 @@ func runtime_readMetrics(unsafe.Pointer, int, int)
 // Sample values with names not appearing in [All] will have their Value populated
 // as KindBad to indicate that the name is unknown.
 func Read(m []Sample) {
+	if len(m) == 0 {
+		return
+	}
 	runtime_readMetrics(unsafe.Pointer(&m[0]), len(m), cap(m))
 }