runtime,syscall,internal/poll,os: wasip1 poll_oneoff scheduler integration + net.FileListener (#5386)

* runtime,syscall,internal/poll,os: wasip1 poll_oneoff scheduler integration + net.FileListener

On wasip1 today every syscall.Read/Write blocks the entire wasm module — the
cooperative scheduler invokes poll_oneoff only for sleep/timer wakeups, and
there's no path from the net package to a working TCP server. This change
fixes both: it threads poll_oneoff through the scheduler's idle path so a
goroutine doing FD I/O parks instead of blocking the module, and it provides
enough internal/poll / os / syscall surface that upstream Go's
net.FileListener / net.FileConn works on a host-pre-opened TCP socket.

* runtime: keep scheduler_cooperative idle-wait calls direct

TestBinarySize/hifive1b/examples/echo regressed by 32 bytes after the
previous commit routed the scheduler's idle wait through a
schedulerIdleWait helper. The extra call frame + branch landed on every
non-wasip1 cooperative target, where the original direct sleepTicks /
waitForEvents calls compile to a single inlined call.

src/os/file_unix.go

@@ -37,9 +37,19 @@ func rename(oldname, newname string) error {
 // can overwrite this data, which could cause the finalizer
 // to close the wrong file descriptor.
 type file struct {
-	handle     FileHandle
-	name       string
-	dirinfo    *dirInfo // nil unless directory being read
+	handle  FileHandle
+	name    string
+	dirinfo *dirInfo // nil unless directory being read
+
+	// pfd is set on wasip1 by (*File).PollFD to a *poll.FD that wraps
+	// the underlying syscall FD. When set, Close routes through it so
+	// the refcount semantics shared with net.FileListener / net.FileConn
+	// are honoured. On non-wasip1 builds pfd is a literal empty struct
+	// (see pollfd_other.go) and stays at zero bytes — provided it's not
+	// the last field of this struct, which is why it lives here above
+	// appendMode rather than at the end.
+	pfd pollFD
+
 	appendMode bool
 }
 
@@ -48,6 +58,9 @@ func (f *file) close() (err error) {
 		f.dirinfo.close()
 		f.dirinfo = nil
 	}
+	if f.pfd.Exist() {
+		return f.pfd.Close()
+	}
 	return f.handle.Close()
 }
 

src/os/file_wasip1.go

@@ -0,0 +1,38 @@
+//go:build wasip1
+
+package os
+
+import "internal/poll"
+
+// PollFD returns the *poll.FD wrapping this file's underlying syscall
+// FD. The first call lazily allocates and caches the *poll.FD on the
+// File; subsequent calls return the same pointer so that refcount
+// semantics shared with net.FileListener / net.FileConn (via
+// poll.FD.Copy) work correctly:
+//
+//   - net.FileListener(f) calls f.PollFD().Copy(); the Copy increments
+//     the refcount via the cached *poll.FD's SysFile.
+//   - f.Close() routes through the cached *poll.FD's Close (see
+//     file_unix.go's file.close), which decrements the refcount and
+//     only releases the syscall FD when the count reaches zero.
+//   - The eventual Listener.Close / Conn.Close decrements the refcount
+//     from the other side.
+//
+// PollFD is intended for use by upstream Go's net/file_wasip1.go (which
+// reaches it via a //go:linkname-style type assertion in this package).
+func (f *File) PollFD() *poll.FD {
+	if f.handle == nil {
+		return nil
+	}
+	if f.pfd != nil {
+		return f.pfd
+	}
+	pfd := &poll.FD{
+		Sysfd:    int(f.handle.(interface{ Fd() uintptr }).Fd()),
+		IsStream: true,
+	}
+	pfd.SysFile.RefCount = 1
+	pfd.SysFile.RefCountPtr = &pfd.SysFile.RefCount
+	f.pfd = pfd
+	return pfd
+}

src/os/pollfd_other.go

@@ -0,0 +1,12 @@
+//go:build !wasip1
+
+package os
+
+// pollFD is a literal empty struct on non-wasip1 targets. As long as the
+// field is not the last in its containing struct, Go gives a zero-sized
+// non-trailing field a true zero byte layout — file then occupies exactly
+// the same space as it would without the field at all.
+type pollFD struct{}
+
+func (pollFD) Close() error { return nil }
+func (pollFD) Exist() bool  { return false }

src/os/pollfd_wasip1.go

@@ -0,0 +1,12 @@
+//go:build wasip1
+
+package os
+
+import "internal/poll"
+
+// pollFD on wasip1 is the *poll.FD that backs net.FileListener /
+// net.FileConn handoffs. The alias makes file.pfd directly typed as
+// *poll.FD so PollFD reads/writes need no type conversion. The Exist
+// method on *poll.FD (defined in internal/poll) absorbs the nil-check
+// that file.close needs on the shared code path.
+type pollFD = *poll.FD