runtime,internal/poll,loader: wasip2 pollable poll-integration + TCP I/O

Mirrors PR #5386's wasip1 work for wasip2. The cooperative scheduler's
idle path now calls wasi:io/poll.Poll over a combined list of (clock
pollable, registered pollables) instead of blocking the wasm module on
a single monotonic-clock subscription, so goroutines doing TCP I/O can
park while the scheduler runs other goroutines.

Plumbing components:

- runtime/netpoll_wasip2.go: pollable-keyed pollDesc registry; pollIO
  builds one combined wasi:io/poll.Poll call (clock pollable + active
  pollables). Linkname-exposed runtime_netpoll_addpollable_wasip2 /
  done / pdfired / wake for internal/poll and future net.
- runtime/scheduler_idle_wasip2.go + scheduler_idle_wasip2_none.go:
  cooperative-variant sleepTicks / waitForEvents that route through
  pollIO; non-coop fallback uses monotonicclock.Block. Mirrors the
  wasip1 structure introduced in 7000e7b.
- runtime/runtime_wasip2.go: sleepTicks moved out to the
  scheduler_idle_wasip2*.go files.
- runtime/wait_other.go: build tag tightened to exclude wasip2.

internal/poll surface:

- internal/poll/fd_wasip2.go: WasipNFD wraps a (TcpSocket, InputStream,
  OutputStream) triple. DialTCPWasip2, ListenTCPWasip2, Accept, Read,
  Write, Close, SetDeadline*. Each blocking op tries the wasi call,
  on would-block subscribes, parks, retries — same pattern as the
  wasip1 internal/poll.FD but pollable-keyed. Linkname-friendly
  Wasip2TCP{Listen,Dial,Accept,Read,Write,Close,SetDeadline} wrappers
  for test / future net callers.
- internal/poll/errors_wasip.go: ErrFileClosing / ErrNetClosing /
  ErrDeadlineExceeded / ErrNoDeadline extracted from fd_wasip1.go to
  a wasip1||wasip2 shared file.

Loader change:

- loader/goroot.go: listGorootMergeLinks now filters TinyGo files by
  //go:build constraints (via go/build.Context.MatchFile) before
  deciding "TinyGo owns this directory". Files that don't match the
  current target no longer cause upstream Go files at the same level
  to be dropped. Unblocks per-target overrides in directories like
  src/net/ for future net.wasip2 work without disturbing wasip1.

End-to-end verification:

  $ wasmtime run -Sinherit-network -Stcp ./tcpecho_wasip2.wasm &
  listening on 127.0.0.1:9999
  tick 1
  tick 2
  tick 3
  $ echo hello | nc 127.0.0.1 9999
  hello                                    # echoed by the wasm
  $ # two concurrent clients echo cleanly while ticker keeps ticking

The test program (not shipped) uses //go:linkname to drive the
internal/poll TCP helpers directly, since TinyGo doesn't yet have a
net.Listen / net.Dial path on wasip2 (upstream Go's net doesn't build
for wasip2 due to cgo_linux.go reaching for Linux headers). The
src/net/ wasip2 wrappers are out of scope for this PR and tracked as
follow-up — once they land, callers will use net.Listen / Dial
directly and the linkname wrappers can drop.

Wasip1 regression sweep: tcpecho.wasm still passes; time.Sleep / parkfile
/ parksynth unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: achille-roussel

loader/goroot.go

@@ -16,6 +16,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"errors"
+	"go/build"
 	"io"
 	"io/fs"
 	"os"
@@ -48,7 +49,7 @@ func GetCachedGoroot(config *compileopts.Config) (string, error) {
 	overrides := pathsToOverride(config.GoMinorVersion, needsSyscallPackage(config.BuildTags()))
 
 	// Resolve the merge links within the goroot.
-	merge, err := listGorootMergeLinks(goroot, tinygoroot, overrides)
+	merge, err := listGorootMergeLinks(goroot, tinygoroot, overrides, config)
 	if err != nil {
 		return "", err
 	}
@@ -143,10 +144,22 @@ func GetCachedGoroot(config *compileopts.Config) (string, error) {
 }
 
 // listGorootMergeLinks searches goroot and tinygoroot for all symlinks that must be created within the merged goroot.
-func listGorootMergeLinks(goroot, tinygoroot string, overrides map[string]bool) (map[string]string, error) {
+func listGorootMergeLinks(goroot, tinygoroot string, overrides map[string]bool, config *compileopts.Config) (map[string]string, error) {
 	goSrc := filepath.Join(goroot, "src")
 	tinygoSrc := filepath.Join(tinygoroot, "src")
 	merges := make(map[string]string)
+
+	// buildContext is used to evaluate //go:build constraints on TinyGo
+	// source files. A TinyGo file that doesn't match the current target
+	// (e.g. a *_wasip2.go file in a wasip1 build) must not be treated as
+	// "TinyGo owns this directory" — otherwise wasip1 builds would lose
+	// upstream Go files at the same level.
+	bctx := build.Default
+	bctx.GOOS = config.GOOS()
+	bctx.GOARCH = config.GOARCH()
+	bctx.BuildTags = config.BuildTags()
+	bctx.Compiler = "gc"
+
 	for dir, merge := range overrides {
 		if !merge {
 			// Use the TinyGo version.
@@ -168,6 +181,14 @@ func listGorootMergeLinks(goroot, tinygoroot string, overrides map[string]bool)
 
 			// Link this file.
 			name := e.Name()
+			// Only count this file as a TinyGo override of the directory
+			// when its build tags match the current target. Files with a
+			// non-matching //go:build are invisible to this build anyway
+			// (the compiler would skip them), so they shouldn't cause
+			// upstream files to be dropped from the merge.
+			if matched, _ := bctx.MatchFile(tinygoDir, name); !matched {
+				continue
+			}
 			merges[filepath.Join("src", dir, name)] = filepath.Join(tinygoDir, name)
 
 			hasTinyGoFiles = true

src/internal/poll/errors_wasip.go

@@ -0,0 +1,24 @@
+//go:build wasip1 || wasip2
+
+// Shared error sentinels for the wasip1 and wasip2 internal/poll
+// implementations. The error values are part of the public API surface
+// upstream net relies on; sharing them keeps fd_wasip1.go and
+// fd_wasip2.go free of redundant declarations.
+
+package poll
+
+import "errors"
+
+// ErrFileClosing is returned when a Read or Write is started on a closed FD.
+var ErrFileClosing = errors.New("use of closed file")
+
+// ErrNetClosing is returned for network operations on a closed FD.
+var ErrNetClosing = errors.New("use of closed network connection")
+
+// ErrDeadlineExceeded is returned by Read/Write when a deadline expired.
+// Matches the error returned by os.IsTimeout-style helpers.
+var ErrDeadlineExceeded = errors.New("i/o timeout")
+
+// ErrNoDeadline is returned if SetDeadline is called on an FD whose
+// underlying type does not support deadlines.
+var ErrNoDeadline = errors.New("file type does not support deadline")

src/internal/poll/fd_wasip1.go

@@ -20,27 +20,12 @@
 package poll
 
 import (
-	"errors"
 	"internal/task"
 	"syscall"
 	"time"
 	"unsafe"
 )
 
-// ErrFileClosing is returned when a Read or Write is started on a closed FD.
-var ErrFileClosing = errors.New("use of closed file")
-
-// ErrNetClosing is returned for network operations on a closed FD.
-var ErrNetClosing = errors.New("use of closed network connection")
-
-// ErrDeadlineExceeded is returned by Read/Write when a deadline expired.
-// Matches the error returned by os.IsTimeout-style helpers.
-var ErrDeadlineExceeded = errors.New("i/o timeout")
-
-// ErrNoDeadline is returned if SetDeadline is called on an FD whose
-// underlying type does not support deadlines.
-var ErrNoDeadline = errors.New("file type does not support deadline")
-
 // pollMode constants must mirror runtime/netpoll_wasip1.go's pollRead/
 // pollWrite values.
 const (