zfs: pipe size: default to value of /proc/sys/fs/pipe-max-siz

Addition by @problame: move getPipeCapacityHint() into platform-specific
code. This has the added benefit of not recognizing the envvar as an
envconst on platform that do not support resizing pipes.  => won't show
up in (zrepl status --raw).Global.Envconst

fixes #424
cloes #449

zfs/zfs_pipe_linux.go

@@ -3,11 +3,33 @@ package zfs
 import (
 	"errors"
 	"fmt"
+	"io/ioutil"
 	"os"
+	"strconv"
+	"strings"
 
 	"golang.org/x/sys/unix"
+
+	"github.com/zrepl/zrepl/util/envconst"
 )
 
+func getPipeCapacityHint(envvar string) int {
+	var capacity int64 = 1 << 25
+
+	// Work around a race condition in Linux >= 5.8 related to pipe resizing.
+	// https://github.com/zrepl/zrepl/issues/424#issuecomment-800370928
+	// https://bugzilla.kernel.org/show_bug.cgi?id=212295
+	if _, err := os.Stat("/proc/sys/fs/pipe-max-size"); err == nil {
+		if dat, err := ioutil.ReadFile("/proc/sys/fs/pipe-max-size"); err == nil {
+			if capacity, err = strconv.ParseInt(strings.TrimSpace(string(dat)), 10, 64); err != nil {
+				capacity = 1 << 25
+			}
+		}
+	}
+
+	return int(envconst.Int64(envvar, capacity))
+}
+
 func trySetPipeCapacity(p *os.File, capacity int) {
 	res, err := unix.FcntlInt(p.Fd(), unix.F_SETPIPE_SZ, capacity)
 	if err != nil {