runtime: run syscall/js finalizers on wasm without a manual GC (#5545)

* runtime: run syscall/js finalizers on wasm without a manual GC

* runtime: address review feedback on finalizer idle GC

* runtime: clear a finished task's args pointer so its arguments are collectable

* runtime: skip the finalizer scan with a per-block registration bit

* runtime: guard the finalizer registration bitmap with gcLock

* testdata: cover finalizer invariants on every scheduler

* main_test: limit the finalizer scheduler variants to linux and darwin

* testdata: wait for the finalizer queue to drain before asserting

* testdata: make the finalizer counters atomic and wait for a known drain count

* runtime: add finalizer bookkeeping asserts under runtime_asserts

* runtime: address finalizer GC review feedback

* testdata: strengthen blocked stack finalizer test

* runtime: fix finalizer cleanup edge cases

* runtime: decouple wasm export scheduling from finalizers

* runtime: avoid redundant wakeups for re-entrant wasm exports

* runtime: simplify finalizer comments

compileopts/finalizer_coverage_test.go

@@ -0,0 +1,65 @@
+package compileopts
+
+import (
+	"go/build/constraint"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestFinalizerRunnerSchedulerCoverage verifies that each scheduler selects one runner file.
+// It uses validSchedulerOptions so new schedulers are included.
+func TestFinalizerRunnerSchedulerCoverage(t *testing.T) {
+	files := []string{
+		"gc_finalizer_sched.go",
+		"gc_finalizer_sched_none.go",
+		"gc_finalizer_sched_other.go",
+	}
+	exprs := make([]constraint.Expr, len(files))
+	for i, name := range files {
+		exprs[i] = readBuildConstraint(t, filepath.Join("..", "src", "runtime", name))
+	}
+
+	for _, sched := range validSchedulerOptions {
+		// The finalizer table exists under block GCs.
+		// gc.conservative satisfies the GC condition in every constraint.
+		tags := map[string]bool{
+			"gc.conservative":    true,
+			"scheduler." + sched: true,
+		}
+		var matched []string
+		for i, expr := range exprs {
+			if expr.Eval(func(tag string) bool { return tags[tag] }) {
+				matched = append(matched, files[i])
+			}
+		}
+		if len(matched) != 1 {
+			t.Errorf("scheduler.%s: spawnFinalizerRunner defined in %d files %v, want exactly 1",
+				sched, len(matched), matched)
+		}
+	}
+}
+
+func readBuildConstraint(t *testing.T, path string) constraint.Expr {
+	t.Helper()
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if constraint.IsGoBuild(line) {
+			expr, err := constraint.Parse(line)
+			if err != nil {
+				t.Fatalf("%s: %v", path, err)
+			}
+			return expr
+		}
+		if line != "" && !strings.HasPrefix(line, "//") {
+			break // reached code before any //go:build line
+		}
+	}
+	t.Fatalf("%s: no //go:build line found", path)
+	return nil
+}