e2fsck: don't flush the FS unless it's actually dirty

ext2fs_flush2() unconditionally writes the block group descriptors to
disk even if the underlying FS isn't marked dirty.  This causes the
following error message on a fsck -n run:

e2fsck 1.43-WIP (09-Jul-2014)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Error writing block 2 (Attempt to write block to filesystem resulted in short write).  Ignore error? no

Error writing block 2 (Attempt to write block to filesystem resulted in short write).  Ignore error? no

Error writing file system info: Attempt to write block to filesystem resulted in short write

Since ext2fs_close2() only calls flush if the dirty flag is set,
modify e2fsck to exhibit the same behavior so that we don't spit out
write errors for a read only check.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

e2fsck/unix.c

@@ -1726,9 +1726,11 @@ no_journal:
 	}
 
 	e2fsck_write_bitmaps(ctx);
-	pctx.errcode = ext2fs_flush(ctx->fs);
-	if (pctx.errcode)
-		fix_problem(ctx, PR_6_FLUSH_FILESYSTEM, &pctx);
+	if (fs->flags & EXT2_FLAG_DIRTY) {
+		pctx.errcode = ext2fs_flush(ctx->fs);
+		if (pctx.errcode)
+			fix_problem(ctx, PR_6_FLUSH_FILESYSTEM, &pctx);
+	}
 	pctx.errcode = io_channel_flush(ctx->fs->io);
 	if (pctx.errcode)
 		fix_problem(ctx, PR_6_IO_FLUSH, &pctx);

tests/f_readonly_fsck/expect

@@ -0,0 +1,11 @@
+Pass 1: Checking inodes, blocks, and sizes
+Pass 2: Checking directory structure
+Pass 3: Checking directory connectivity
+Pass 4: Checking reference counts
+Pass 5: Checking group summary information
+Free blocks count wrong (4294968254, counted=958).
+Fix? no
+
+test_filesys: 11/256 files (9.1% non-contiguous), 18446744069414585410/2048 blocks
+Exit status is 0
+crc did not change.  2466215161

tests/f_readonly_fsck/name

@@ -0,0 +1 @@
+ensure that a readonly check doesn't modify the fs

tests/f_readonly_fsck/script

@@ -0,0 +1,46 @@
+FSCK_OPT=-fn
+OUT=$test_name.log
+if [ -f $test_dir/expect.gz ]; then
+	EXP=$test_name.tmp
+	gunzip < $test_dir/expect.gz > $EXP1
+else
+	EXP=$test_dir/expect
+fi
+
+cp /dev/null $OUT
+
+dd if=/dev/zero of=$TMPFILE bs=1k count=512 > /dev/null 2>&1
+
+gzip -d < $test_dir/image.gz > $TMPFILE
+
+old="$($CRCSUM < $TMPFILE)"
+
+$FSCK $FSCK_OPT -N test_filesys $TMPFILE > $OUT.new 2>&1
+status=$?
+echo Exit status is $status >> $OUT.new
+sed -f $cmd_dir/filter.sed -e "s;$TMPFILE;test.img;" $OUT.new >> $OUT
+rm -f $OUT.new
+
+new="$($CRCSUM < $TMPFILE)"
+
+if [ "${old}" != "${new}" ]; then
+	echo "ERROR: crc mismatch!  ${old} ${new}" >> $OUT
+else
+	echo "crc did not change.  ${old}" >> $OUT
+fi
+
+rm -f $TMPFILE
+
+cmp -s $OUT $EXP
+status=$?
+
+if [ "$status" = 0 ] ; then
+	echo "$test_name: $test_description: ok"
+	touch $test_name.ok
+else
+	echo "$test_name: $test_description: failed"
+	diff $DIFF_OPTS $EXP $OUT > $test_name.failed
+	rm -f $test_name.tmp
+fi
+
+unset IMAGE FSCK_OPT OUT EXP old new