internal/poll: better panic for invalid write return value

For #61060

Change-Id: I13cd73b4062cb7bd248d2a4afae06dfa29ac0203
Reviewed-on: https://go-review.googlesource.com/c/go/+/577955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/internal/poll/fd_unix.go

@@ -7,6 +7,7 @@
 package poll
 
 import (
+	"internal/itoa"
 	"internal/syscall/unix"
 	"io"
 	"sync/atomic"
@@ -379,6 +380,14 @@ func (fd *FD) Write(p []byte) (int, error) {
 		}
 		n, err := ignoringEINTRIO(syscall.Write, fd.Sysfd, p[nn:max])
 		if n > 0 {
+			if n > max-nn {
+				// This can reportedly happen when using
+				// some VPN software. Issue #61060.
+				// If we don't check this we will panic
+				// with slice bounds out of range.
+				// Use a more informative panic.
+				panic("invalid return from write: got " + itoa.Itoa(n) + " from a write of " + itoa.Itoa(max-nn))
+			}
 			nn += n
 		}
 		if nn == len(p) {