[PATCH V2] filefrag: count 0 extents properly when verbose

[Eric Sandeen <sandeen@redhat.com>]

# rm -f a; touch a; filefrag a
/boot/a: 0 extents found

works properly, but

# rm -f a; touch a; filefrag -v a
Filesystem type is: ef53
Filesystem cylinder groups is approximately 61
File size of a is 0 (0 blocks, blocksize 1024)
 ext logical physical expected length flags
a: 1 extent found

yields 1 extent when it should be 0.

Fix this up by special-casing no extents returned in verbose
mode; skip printing the header for the columns too, since there
are no columns to print.

Also, in nonverbose mode we can set fm_extent_count to 0
so that FIEMAP will just query the extent count without gathering
details; clarify this with a comment.

Addresses-redhat-bugzilla: 653234
Signed-off-by: Eric Sandeen <sandeen@redhat.com>

diff --git a/misc/filefrag.c b/misc/filefrag.c
index d604b6c..2795e15 100644
--- a/misc/filefrag.c
+++ b/misc/filefrag.c
@@ -180,6 +180,10 @@ static int filefrag_fiemap(int fd, int blk_shift, int *num_extents)
 
 	memset(fiemap, 0, sizeof(struct fiemap));
 
+	/*
+	 * If count (and therefore fm_extent_count) == 0, FIEMAP
+	 * returns count of extents found without filling in details.
+	 */
 	if (!verbose)
 		count = 0;
 
@@ -204,6 +208,14 @@ static int filefrag_fiemap(int fd, int blk_shift, int *num_extents)
 		}
 
 		if (verbose && !fiemap_header_printed) {
+			/*
+			 * No extents on first call?
+			 * Skip header and show 0 extents.
+			 */
+			if (fiemap->fm_mapped_extents == 0) {
+				*num_extents = 0;
+				goto out;
+			}
 			printf(" ext %*s %*s %*s length flags\n", logical_width,
 			       "logical", physical_width, "physical",
 			       physical_width, "expected");