os: disable the use of netpoll on regular files on *BSDs.

The kqueue based netpoller always registers file descriptors with EVFILT_READ and EVFILT_WRITE.
However only EVFILT_READ notification is supported for regular files.
On FreeBSD a regular file is always reported as ready for writing, resulting in a busy wait.
On Darwin, Dragonfly, NetBSD and OpenBSD, a regular file is reported as ready for both reading and writing only once.

Updates #19093

Change-Id: If284341f60c6c2332fb5499637d4cfa7a4e26b7b
Reviewed-on: https://go-review.googlesource.com/c/156379
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/file_unix.go

@@ -117,23 +117,29 @@ func newFile(fd uintptr, name string, kind newFileKind) *File {
 
 	pollable := kind == kindOpenFile || kind == kindPipe || kind == kindNonBlock
 
-	// Don't try to use kqueue with regular files on FreeBSD.
-	// It crashes the system unpredictably while running all.bash.
-	// Issue 19093.
 	// If the caller passed a non-blocking filedes (kindNonBlock),
 	// we assume they know what they are doing so we allow it to be
 	// used with kqueue.
-	if runtime.GOOS == "freebsd" && kind == kindOpenFile {
-		pollable = false
-	}
-
-	// On Darwin, kqueue does not work properly with fifos:
-	// closing the last writer does not cause a kqueue event
-	// for any readers. See issue #24164.
-	if runtime.GOOS == "darwin" && kind == kindOpenFile {
+	if kind == kindOpenFile {
 		var st syscall.Stat_t
-		if err := syscall.Fstat(fdi, &st); err == nil && st.Mode&syscall.S_IFMT == syscall.S_IFIFO {
-			pollable = false
+		switch runtime.GOOS {
+		// Don't try to use kqueue with regular files on *BSDs.
+		// on FreeBSD with older kernels it used to crash the system unpredictably while running all.bash.
+		// while with newer kernels a regular file is always reported as ready for writing.
+		// on Dragonfly, NetBSD and OpenBSD the fd is signaled only once as ready (both read and write).
+		// Issue 19093.
+		case "dragonfly", "freebsd", "netbsd", "openbsd":
+			if err := syscall.Fstat(fdi, &st); err == nil && st.Mode&syscall.S_IFMT == syscall.S_IFREG {
+				pollable = false
+			}
+		case "darwin":
+			// In addition to the behavior described above for regular files,
+			// on Darwin, kqueue does not work properly with fifos:
+			// closing the last writer does not cause a kqueue event
+			// for any readers. See issue #24164.
+			if err := syscall.Fstat(fdi, &st); err == nil && (st.Mode&syscall.S_IFMT == syscall.S_IFIFO || st.Mode&syscall.S_IFMT == syscall.S_IFREG) {
+				pollable = false
+			}
 		}
 	}