[PATCH] btrfs-progs: add dev stats returncode option

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

Currently, `btrfs device stats` returns non-zero only when there was an
error getting the counter values.  This is fine for when it gets run by a
user directly, but is a serious pain when trying to use it in a script or
for monitoring since you need to parse the (not at all machine friendly)
output to check the counter values.

This patch adds an option ('-s') which causes `btrfs device stats`
to set bit 7 in the return code if any of the counters are non-zero.
This greatly simplifies checking from a script or monitoring software
if any errors have been recorded.  In the event that this switch is
passed and an error occurs reading the stats, the return code will have
bit 0 set (so if there are errors reading counters, and the counters
which were read were non-zero, the return value will be 129).

Signed-off-by: Austin S. Hemmelgarn <ahferroin7@gmail.com>
---
Tested on multiple filesystems with various values of error counters
(all manually set with a hex-editor)

Both the flag letter and the bit being set were picked rather arbitrarily
(-s intended to be short for status, bit 7 just seemed reasonable).
I have no issue changing either, but would prefer to avoid bikeshedding
about stuff like this since this helps out with an area where BTRFS is
severely lacking right now (monitoring).

 Documentation/btrfs-device.asciidoc |  8 +++++++-
 cmds-device.c                       | 39 ++++++++++++++++++++++++++++++-------
 2 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/Documentation/btrfs-device.asciidoc b/Documentation/btrfs-device.asciidoc
index 239c99b..97a2ed6 100644
--- a/Documentation/btrfs-device.asciidoc
+++ b/Documentation/btrfs-device.asciidoc
@@ -98,7 +98,7 @@ remain as such. Reloading the kernel module will drop this information. There's
 an alternative way of mounting multiple-device filesystem without the need for
 prior scanning. See the mount option 'device'.
 
-*stats* [-z] <path>|<device>::
+*stats* [-zs] <path>|<device>::
 Read and print the device IO error statistics for all devices of the given
 filesystem identified by <path> or for a single <device>. See section *DEVICE
 STATS* for more information.
@@ -108,6 +108,9 @@ STATS* for more information.
 -z::::
 Print the stats and reset the values to zero afterwards.
 
+-s::::
+Set the high bit of the return-code if any error statistics are non-zero.
+
 *usage* [options] <path> [<path>...]::
 Show detailed information about internal allocations in devices.
 +
@@ -231,6 +234,9 @@ EXIT STATUS
 *btrfs device* returns a zero exit status if it succeeds. Non zero is
 returned in case of failure.
 
+If the '-s' option is used, *btrfs device stats* will add 128 to the
+exit status if any of the error counters is non-zero.
+
 AVAILABILITY
 ------------
 *btrfs* is part of btrfs-progs.
diff --git a/cmds-device.c b/cmds-device.c
index fa0830f..3fa3018 100644
--- a/cmds-device.c
+++ b/cmds-device.c
@@ -376,6 +376,7 @@ static const char * const cmd_device_stats_usage[] = {
 	"Show current device IO stats.",
 	"",
 	"-z                     show current stats and reset values to zero",
+	"-s                     return non-zero if any stat counter is not zero",
 	NULL
 };
 
@@ -389,14 +390,18 @@ static int cmd_device_stats(int argc, char **argv)
 	int i;
 	int c;
 	int err = 0;
+	int status = 0;
 	__u64 flags = 0;
 	DIR *dirstream = NULL;
 
-	while ((c = getopt(argc, argv, "z")) != -1) {
+	while ((c = getopt(argc, argv, "zs")) != -1) {
 		switch (c) {
 		case 'z':
 			flags = BTRFS_DEV_STATS_RESET;
 			break;
+		case 's':
+			status = 1;
+			break;
 		case '?':
 		default:
 			usage(cmd_device_stats_usage);
@@ -440,7 +445,7 @@ static int cmd_device_stats(int argc, char **argv)
 		if (ioctl(fdmnt, BTRFS_IOC_GET_DEV_STATS, &args) < 0) {
 			error("DEV_STATS ioctl failed on %s: %s",
 			      path, strerror(errno));
-			err = 1;
+			err |= 1;
 		} else {
 			char *canonical_path;
 
@@ -457,31 +462,51 @@ static int cmd_device_stats(int argc, char **argv)
 					 "devid:%llu", args.devid);
 			}
 
-			if (args.nr_items >= BTRFS_DEV_STAT_WRITE_ERRS + 1)
+			if (args.nr_items >= BTRFS_DEV_STAT_WRITE_ERRS + 1) {
 				printf("[%s].write_io_errs   %llu\n",
 				       canonical_path,
 				       (unsigned long long) args.values[
 					BTRFS_DEV_STAT_WRITE_ERRS]);
-			if (args.nr_items >= BTRFS_DEV_STAT_READ_ERRS + 1)
+				if ((status == 1) && (args.values[BTRFS_DEV_STAT_WRITE_ERRS] > 0)) {
+					err |= 128;
+				}
+			}
+			if (args.nr_items >= BTRFS_DEV_STAT_READ_ERRS + 1) {
 				printf("[%s].read_io_errs    %llu\n",
 				       canonical_path,
 				       (unsigned long long) args.values[
 					BTRFS_DEV_STAT_READ_ERRS]);
-			if (args.nr_items >= BTRFS_DEV_STAT_FLUSH_ERRS + 1)
+				if ((status == 1) && (args.values[BTRFS_DEV_STAT_READ_ERRS] > 0)) {
+					err |= 128;
+				}
+			}
+			if (args.nr_items >= BTRFS_DEV_STAT_FLUSH_ERRS + 1) {
 				printf("[%s].flush_io_errs   %llu\n",
 				       canonical_path,
 				       (unsigned long long) args.values[
 					BTRFS_DEV_STAT_FLUSH_ERRS]);
-			if (args.nr_items >= BTRFS_DEV_STAT_CORRUPTION_ERRS + 1)
+				if ((status == 1) && (args.values[BTRFS_DEV_STAT_FLUSH_ERRS] > 0)) {
+					err |= 128;
+				}
+			}
+			if (args.nr_items >= BTRFS_DEV_STAT_CORRUPTION_ERRS + 1) {
 				printf("[%s].corruption_errs %llu\n",
 				       canonical_path,
 				       (unsigned long long) args.values[
 					BTRFS_DEV_STAT_CORRUPTION_ERRS]);
-			if (args.nr_items >= BTRFS_DEV_STAT_GENERATION_ERRS + 1)
+				if ((status == 1) && (args.values[BTRFS_DEV_STAT_CORRUPTION_ERRS] > 0)) {
+					err |= 128;
+				}
+			}
+			if (args.nr_items >= BTRFS_DEV_STAT_GENERATION_ERRS + 1) {
 				printf("[%s].generation_errs %llu\n",
 				       canonical_path,
 				       (unsigned long long) args.values[
 					BTRFS_DEV_STAT_GENERATION_ERRS]);
+				if ((status == 1) && (args.values[BTRFS_DEV_STAT_GENERATION_ERRS] > 0)) {
+					err |= 128;
+				}
+			}
 
 			free(canonical_path);
 		}
-- 
2.11.0