From 37b302046a62ddfe0524f7bd8c725d4ddc5184b1 Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Tue, 26 May 2020 17:21:04 +0200
Subject: [PATCH] lib/blkdev: add support for --lock and LOCK_BLOCK_DEVICE

All simple function to parse --lock <mode> and $LOCK_BLOCK_DEVICE,
and to flock the fd.

The supported <mode> is:

	"1" or "yes"	- LOCK_EX
	"0" or "no"	- do nothing
	"nonblock"	- LOCK_EX | LOCK_NB

The function tries LOCK_NB before the solo LOCK_EX and prints
inform user that it will wait, for example:

session A:
	# sfdisk --lock /dev/sdc

session B:
	# sfdisk --lock /dev/sdc
	sfdisk: /dev/sdc: device already locked, waiting to get lock ...
	^C

	# sfdisk --lock=nonblock /dev/sdc
	sfdisk: /dev/sdc: device already locked

Addresses: https://github.com/karelzak/util-linux/issues/921
Signed-off-by: Karel Zak <kzak@redhat.com>
---
 include/blkdev.h |  1 +
 lib/blkdev.c     | 52 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/include/blkdev.h b/include/blkdev.h
index 62eda82af..6cbecbb65 100644
--- a/include/blkdev.h
+++ b/include/blkdev.h
@@ -146,5 +146,6 @@ int blkdev_get_geometry(int fd, unsigned int *h, unsigned int *s);
 /* convert scsi type code to name */
 const char *blkdev_scsi_type_to_name(int type);
 
+int blkdev_lock(int fd, const char *devname, const char *lockmode);
 
 #endif /* BLKDEV_H */
diff --git a/lib/blkdev.c b/lib/blkdev.c
index c71550e63..c22853ddc 100644
--- a/lib/blkdev.c
+++ b/lib/blkdev.c
@@ -6,6 +6,7 @@
  */
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <sys/file.h>
 #include <sys/ioctl.h>
 #include <unistd.h>
 #include <stdint.h>
@@ -361,6 +362,57 @@ const char *blkdev_scsi_type_to_name(int type)
 	return NULL;
 }
 
+/* return 0 on success */
+int blkdev_lock(int fd, const char *devname, const char *lockmode)
+{
+	int oper, rc, msg = 0;
+
+	if (!lockmode)
+		lockmode = getenv("LOCK_BLOCK_DEVICE");
+	if (!lockmode)
+		return 0;
+
+	if (strcasecmp(lockmode, "yes") == 0 ||
+	    strcmp(lockmode, "1") == 0)
+		oper = LOCK_EX;
+
+	else if (strcasecmp(lockmode, "nonblock") == 0)
+		oper = LOCK_EX | LOCK_NB;
+
+	else if (strcasecmp(lockmode, "no") == 0 ||
+		 strcmp(lockmode, "0") == 0)
+		return 0;
+	else {
+		warnx(_("unsupported lock mode: %s"), lockmode);
+		return -EINVAL;
+	}
+
+	if (!(oper & LOCK_NB)) {
+		/* Try non-block first to provide message */
+		rc = flock(fd, oper | LOCK_NB);
+		if (rc == 0)
+			return 0;
+		if (rc != 0 && errno == EWOULDBLOCK) {
+			fprintf(stderr, _("%s: %s: device already locked, waiting to get lock ... "),
+					program_invocation_short_name, devname);
+			msg = 1;
+		}
+	}
+	rc = flock(fd, oper);
+	if (rc != 0) {
+		switch (errno) {
+		case EWOULDBLOCK: /* LOCK_NB */
+			warnx(_("%s: device already locked"), devname);
+			break;
+		default:
+			warn(_("%s: failed to get lock"), devname);
+		}
+	} else if (msg)
+		fprintf(stderr, _("OK\n"));
+	return rc;
+}
+
+
 #ifdef TEST_PROGRAM_BLKDEV
 #include <stdio.h>
 #include <stdlib.h>