[PATCH] generic/129: add a safeguard against media wearout

[Alexander Patrakov <patrakov@xxxxxxxxx>]

_require_sparse_files is implemented as a list of filesystems known not to
support sparse files, and so misses some cases.

If sparse files do not really work (as it is the case with CIFS and a
Windows Server 2022), this test writes to the disk all the zeros that
would normally be free due to sparse files. This amounts to many
terabytes and presents a significant media wearout concern.

Mitigate this by doing a small-scale test first and checking if the
resulting file ends up being not sufficiently sparse.

Signed-off-by: Alexander Patrakov <patrakov@xxxxxxxxx>
---
 src/looptest.c    | 9 +++++++--
 tests/generic/129 | 8 ++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/looptest.c b/src/looptest.c
index 7194a97d..2e620a60 100644
--- a/src/looptest.c
+++ b/src/looptest.c
@@ -56,6 +56,7 @@ enum {
     FLAG_FLUSH      = 64,
     FLAG_DELETE     = 128,
     FLAG_DIRECT     = 256,
+    FLAG_LEAVE      = 512,
 };
 
 void
@@ -74,6 +75,7 @@ usage(char *argv0)
         "              -s           = sequential\n"
         "              -f           = flush\n"
         "              -D           = direct-IO\n"
+        "              -L           = skip the final truncation\n"
 	"              -h           = usage\n",
             argv0);
 }
@@ -94,7 +96,7 @@ main(int argc, char *argv[])
     char                *buf            = NULL;
     int64_t 		seek_to 	= 0;
         
-    while ((c = getopt(argc, argv, "i:orwb:svthfFDd?")) != EOF) {
+    while ((c = getopt(argc, argv, "i:orwb:svthfFDLd?")) != EOF) {
 	switch (c) {
 	    case 'i':
 		count = atoi(optarg);
@@ -126,6 +128,9 @@ main(int argc, char *argv[])
             case 'D':
                 flags |= FLAG_DIRECT;
                 break;
+            case 'L':
+                flags |= FLAG_LEAVE;
+                break;
             case 'd':
                 flags |= FLAG_DELETE;
                 break;
@@ -230,7 +235,7 @@ main(int argc, char *argv[])
             }
         }
         
-        if (flags & FLAG_TRUNCATE) {
+        if ((flags & FLAG_TRUNCATE) && !((flags & FLAG_LEAVE) && (i == count - 1))) {
             if (flags & FLAG_VERBOSE)
                 printf("seek 0\n");
 
diff --git a/tests/generic/129 b/tests/generic/129
index 3d3a42a2..d1149b65 100755
--- a/tests/generic/129
+++ b/tests/generic/129
@@ -29,6 +29,14 @@ _scratch_mount "-o nosuid"
 
 mkdir $SCRATCH_MNT/looptest
 
+# looptest2, if the missing sparse file support is not detected, writes more
+# than 4 TB to the device. There was a complaint about media wearout, so do a
+# small-scale test first.
+$here/src/looptest -i 10 -L -t -r -w -s -b 102400 $SCRATCH_MNT/looptest/looptest0.tst
+resulting_file_size_kb=$( du -sk $SCRATCH_MNT/looptest/looptest0.tst | cut -f 1 )
+rm -f $SCRATCH_MNT/looptest/looptest0.tst
+[ $resulting_file_size_kb -gt 256 ] && _notrun "Test skipped due to media wearout concerns - sparse files do not work"
+
 $here/src/looptest -i 100000 -r -w -b 8192 -s $SCRATCH_MNT/looptest/looptest1.tst
 $here/src/looptest -i 10000 -t -r -w -s -b 102400 $SCRATCH_MNT/looptest/looptest2.tst
 $here/src/looptest -i 50000 -r -w -b 256 -s $SCRATCH_MNT/looptest/looptest3.tst
-- 
2.43.0