Add workaround for broken Linux pipes

Linux has an unresolved hang if you resize a pipe with bytes
in it.

Since there's no obvious way to detect this happening, added a
workaround to disable resizing the pipe buffer if you set an
environment variable.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rich Ercolani <[email protected]>
Closes openzfs#13309

lib/libzfs/os/linux/libzfs_sendrecv_os.c

@@ -35,6 +35,22 @@
 void
 libzfs_set_pipe_max(int infd)
 {
+#if __linux__
+	/*
+	 * Sadly, Linux has an unfixed deadlock if you do SETPIPE_SZ on a pipe
+	 * with data in it.
+	 * cf. #13232, https://bugzilla.kernel.org/show_bug.cgi?id=212295
+	 *
+	 * And since the problem is in waking up the writer, there's nothing
+	 * we can do about it from here.
+	 *
+	 * So if people want to, they can set this, but they
+	 * may regret it...
+	 */
+	if (getenv("ZFS_SET_PIPE_MAX") == NULL)
+		return;
+#endif
+
 	FILE *procf = fopen("/proc/sys/fs/pipe-max-size", "re");
 
 	if (procf != NULL) {

man/man8/zfs.8

@@ -713,6 +713,14 @@ to use
 to mount ZFS datasets.
 This option is provided for backwards compatibility with older ZFS versions.
 .El
+.Bl -tag -width "ZFS_SET_PIPE_MAX"
+.It Sy ZFS_SET_PIPE_MAX
+Tells
+.Nm zfs
+to set the maximum pipe size for sends/recieves.
+Disabled by default on Linux
+due to an unfixed deadlock in Linux's pipe size handling code.
+.El
 .
 .Sh INTERFACE STABILITY
 .Sy Committed .