[PATCH] btrfs-progs: balance: check for full-balance before background fork

[Vladimir Panteleev <git@vladimir.panteleev.md>]

Move the full-balance warning to before the fork, so that the user can
see and react to it.

Notes on test:

- Don't use grep -q, as it causes a SIGPIPE during the countdown, and
  the balance thus doesn't start.

- The "balance cancel" is superfluous as the last command, but it
  provides some idempotence and allows adding more tests below it.

Fixes: https://github.com/kdave/btrfs-progs/issues/168

Signed-off-by: Vladimir Panteleev <git@vladimir.panteleev.md>
---
 cmds/balance.c                                | 36 +++++++++----------
 .../002-balance-full-no-filters/test.sh       |  5 +++
 2 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/cmds/balance.c b/cmds/balance.c
index 6f2d4803..32830002 100644
--- a/cmds/balance.c
+++ b/cmds/balance.c
@@ -437,24 +437,6 @@ static int do_balance(const char *path, struct btrfs_ioctl_balance_args *args,
 	if (fd < 0)
 		return 1;
 
-	if (!(flags & BALANCE_START_FILTERS) && !(flags & BALANCE_START_NOWARN)) {
-		int delay = 10;
-
-		printf("WARNING:\n\n");
-		printf("\tFull balance without filters requested. This operation is very\n");
-		printf("\tintense and takes potentially very long. It is recommended to\n");
-		printf("\tuse the balance filters to narrow down the scope of balance.\n");
-		printf("\tUse 'btrfs balance start --full-balance' option to skip this\n");
-		printf("\twarning. The operation will start in %d seconds.\n", delay);
-		printf("\tUse Ctrl-C to stop it.\n");
-		while (delay) {
-			printf("%2d", delay--);
-			fflush(stdout);
-			sleep(1);
-		}
-		printf("\nStarting balance without any filters.\n");
-	}
-
 	ret = ioctl(fd, BTRFS_IOC_BALANCE_V2, args);
 	if (ret < 0) {
 		/*
@@ -634,6 +616,24 @@ static int cmd_balance_start(const struct cmd_struct *cmd,
 		}
 	}
 
+	if (!(start_flags & BALANCE_START_FILTERS) && !(start_flags & BALANCE_START_NOWARN)) {
+		int delay = 10;
+
+		printf("WARNING:\n\n");
+		printf("\tFull balance without filters requested. This operation is very\n");
+		printf("\tintense and takes potentially very long. It is recommended to\n");
+		printf("\tuse the balance filters to narrow down the scope of balance.\n");
+		printf("\tUse 'btrfs balance start --full-balance' option to skip this\n");
+		printf("\twarning. The operation will start in %d seconds.\n", delay);
+		printf("\tUse Ctrl-C to stop it.\n");
+		while (delay) {
+			printf("%2d", delay--);
+			fflush(stdout);
+			sleep(1);
+		}
+		printf("\nStarting balance without any filters.\n");
+	}
+
 	if (force)
 		args.flags |= BTRFS_BALANCE_FORCE;
 	if (verbose)
diff --git a/tests/cli-tests/002-balance-full-no-filters/test.sh b/tests/cli-tests/002-balance-full-no-filters/test.sh
index 9c31dd6f..daadcc44 100755
--- a/tests/cli-tests/002-balance-full-no-filters/test.sh
+++ b/tests/cli-tests/002-balance-full-no-filters/test.sh
@@ -18,4 +18,9 @@ run_check $SUDO_HELPER "$TOP/btrfs" balance start "$TEST_MNT"
 run_check $SUDO_HELPER "$TOP/btrfs" balance --full-balance "$TEST_MNT"
 run_check $SUDO_HELPER "$TOP/btrfs" balance "$TEST_MNT"
 
+run_check_stdout $SUDO_HELPER "$TOP/btrfs" balance start --background "$TEST_MNT" |
+	grep -F "Full balance without filters requested." ||
+	_fail "full balance warning not in the output"
+run_mayfail $SUDO_HELPER "$TOP/btrfs" balance cancel "$TEST_MNT"
+
 run_check_umount_test_dev
-- 
2.22.0