os: only employ sendfile(3ext) on illumos when target is regular file

Fixes #68863

Change-Id: I0ca324137d1f7510bd0b245791fef07d3b5b401a
Reviewed-on: https://go-review.googlesource.com/c/go/+/605355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/os/readfrom_unix_test.go

@@ -198,7 +198,7 @@ func TestCopyFile(t *testing.T) {
 				}
 				switch runtime.GOOS {
 				case "illumos", "solaris":
-					// On SunOS, We rely on File.Stat to get the size of the file,
+					// On SunOS, We rely on File.Stat to get the size of the source file,
 					// which doesn't work for pipe.
 					if hook.called {
 						t.Fatalf("%s: shouldn't have called the hook with a source of pipe", testName)
@@ -267,7 +267,7 @@ func TestCopyFile(t *testing.T) {
 				}
 				switch runtime.GOOS {
 				case "illumos", "solaris":
-					// On SunOS, We rely on File.Stat to get the size of the file,
+					// On SunOS, We rely on File.Stat to get the size of the source file,
 					// which doesn't work for pipe.
 					if hook.called {
 						t.Fatalf("%s: shouldn't have called the hook with a source of pipe", testName)

src/os/zero_copy_solaris.go

@@ -7,6 +7,7 @@ package os
 import (
 	"internal/poll"
 	"io"
+	"runtime"
 	"syscall"
 )
 
@@ -55,6 +56,24 @@ func (f *File) readFrom(r io.Reader) (written int64, handled bool, err error) {
 		return 0, false, nil
 	}
 
+	// sendfile() on illumos seems to incur intermittent failures when the
+	// target file is a standard stream (stdout/stderr), we hereby skip any
+	// character devices conservatively and leave them to generic copy.
+	// Check out https://go.dev/issue/68863 for more details.
+	if runtime.GOOS == "illumos" {
+		fi, err := f.Stat()
+		if err != nil {
+			return 0, false, nil
+		}
+		st, ok := fi.Sys().(*syscall.Stat_t)
+		if !ok {
+			return 0, false, nil
+		}
+		if typ := st.Mode & syscall.S_IFMT; typ == syscall.S_IFCHR || typ == syscall.S_IFBLK {
+			return 0, false, nil
+		}
+	}
+
 	if remain == 0 {
 		fi, err := src.Stat()
 		if err != nil {