[PATCH] generic/747: handle ENOSPC gracefully during write/delete cycles

[Leo Martins <loemra.dev@gmail.com>]

generic/747 consistently fails on btrfs in my fstests setup, with an
~88% failure rate across multiple runs on kernels ranging from v6.9 to
v7.0-rc5. This is not a regression but a pre-existing issue since the
test was added.

The test fills a filesystem to 95% then does mixed write/delete cycles,
using statfs to decide whether to write or delete. However, statfs
f_bavail may overestimate the actual available space. On btrfs, the
statfs implementation documents its estimate as "a close approximation"
(fs/btrfs/super.c). At high fill levels the discrepancy between what
statfs reports and what the filesystem can actually allocate becomes
significant, causing dd to hit ENOSPC even though statfs indicated
there was room.

This is not a filesystem bug. The filesystem correctly rejects the
write when it cannot reserve space. The test's purpose is to stress
garbage collection through write/delete churn, not to validate space
accounting.

Handle ENOSPC by cleaning up the partial file and making room:

In _direct_fillup: break out of the fill loop (we're full enough).
In _mixed_write_delete: delete a file to free space and retry. If
writes fail 10 consecutive times, _fail the test as that indicates a
real filesystem issue rather than a transient statfs discrepancy.

Redirect dd stderr to seqres.full so errors are preserved for
debugging without polluting the expected output.

Signed-off-by: Leo Martins <loemra.dev@gmail.com>
---
 tests/generic/747 | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/tests/generic/747 b/tests/generic/747
index 44834186..35de3ccb 100755
--- a/tests/generic/747
+++ b/tests/generic/747
@@ -35,11 +35,7 @@ _create_file() {
 
 	POSIXLY_CORRECT=yes dd if=/dev/zero of=${file_name} \
 		bs=${bs} count=$(( $file_sz / ${bs} )) \
-		status=none $dd_extra  2>&1
-
-	if [ $? -ne 0 ]; then
-		_fail "Failed writing $file_name"
-	fi
+		status=none $dd_extra  2>>$seqres.full
 }
 
 _total_M() {
@@ -69,7 +65,10 @@ _direct_fillup () {
 	while [ $(_used_percent) -lt $fill_percent ]; do
 		local fsz=$(_get_random_fsz)
 
-		_create_file $testseq $fsz "oflag=direct conv=fsync"
+		if ! _create_file $testseq $fsz "oflag=direct conv=fsync"; then
+			rm ${SCRATCH_MNT}/data_${testseq}
+			break
+		fi
 		testseq=$((${testseq} + 1))
 	done
 }
@@ -79,14 +78,25 @@ _mixed_write_delete() {
 	local total_M=$(_total_M)
 	local to_write_M=$(( ${overwrite_percentage} * ${total_M} / 100 ))
 	local written_M=0
+	local enospc_retries=0
+	local max_enospc_retries=10
 
 	while [ $written_M -lt $to_write_M ]; do
 		if [ $(_used_percent) -lt $fill_percent ]; then
 			local fsz=$(_get_random_fsz)
 
-			_create_file $testseq $fsz "$dd_extra"
-			written_M=$((${written_M} + ${fsz}/${M}))
-			testseq=$((${testseq} + 1))
+			if ! _create_file $testseq $fsz "$dd_extra"; then
+				rm ${SCRATCH_MNT}/data_${testseq}
+				_delete_random_file
+				enospc_retries=$((enospc_retries + 1))
+				if [ $enospc_retries -ge $max_enospc_retries ]; then
+					_fail "failed to write after $max_enospc_retries consecutive ENOSPC attempts"
+				fi
+			else
+				written_M=$((${written_M} + ${fsz}/${M}))
+				testseq=$((${testseq} + 1))
+				enospc_retries=0
+			fi
 		else
 			_delete_random_file
 		fi
-- 
2.52.0