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

[Xen patchbot -unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User emellor@xxxxxxxxxxxxxxxxxxxxxx
# Node ID 871f768aadc621cf5e44e090c30293074aa27033
# Parent  1a1e3dcbbf19401bb13ff5f304bd2ab65f0af720
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 1a1e3dcbbf19 -r 871f768aadc6 tools/examples/block
--- a/tools/examples/block      Thu Mar  2 20:45:13 2006
+++ b/tools/examples/block      Fri Mar  3 14:32:42 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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