[Xen-changelog] Cope when the filename returned by losetup for an existing loop device is not

[Xen patchbot -3.0-testing <patchbot-3.0-testing@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User emellor@xxxxxxxxxxxxxxxxxxxxxx
# Node ID bac71019f6aacc8393eddfb7f28a3972d253ea50
# Parent  d47438cf9fcb8514b12f890ca161f77597f058a8
Cope when the filename returned by losetup for an existing loop device is not
fully qualified.  In this case, the readlink -f call to canonicalise that name
fails.

Closes bug #553.

Signed-off-by: Ewan Mellor <ewan@xxxxxxxxxxxxx>

diff -r d47438cf9fcb -r bac71019f6aa tools/examples/block
--- a/tools/examples/block      Wed Mar  8 22:57:27 2006
+++ b/tools/examples/block      Wed Mar  8 22:57:34 2006
@@ -321,7 +321,28 @@
               fi
             fi
 
-            f=$(readlink -f "$f")
+            # Canonicalise the filename for the comparison.
+
+            # I have seen this readlink fails because the filename given by
+            # losetup is only the basename.  This cannot happen when the loop
+            # device is set up through this script, because file is
+            # canonicalised above, but it may happen when loop devices are set
+            # up some other way.  This readlink may also conceivably fail if
+            # the file backing this loop device has been removed.
+
+            # For maximum safety, in the case that $f does not resolve, we
+            # assume that $file and $f are in the same directory.
+
+            # If you create a loopback filesystem, remove it and continue to
+            # run on it, and then create another file with the same name, then
+            # this check will block that -- don't do that.
+
+            # If you create loop devices through some other mechanism, use
+            # relative filenames, and then use the same filename through this
+            # script, then this check will block that -- don't do that either.
+
+            f=$(readlink -f "$f" || echo $(dirname "$file")/$(basename "$f"))
+
 
             if [ "$f" == "$file" ]
             then

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