[attribute] When updating attribution, zero out the anchor/sentinel prior to placing the new value in

This ensures that attribution codes that are shorter than the anchor/sentinel will not be mangled.

dmg/attribution.c

@@ -274,6 +274,12 @@ int updateAttribution(AbstractFile* abstractIn, AbstractFile* abstractOut, const
   printf("anchorOffset: 0x%llx\n", anchorOffset);
 
   ASSERT(rawAnchor + dataLen <= rawBuffer + attributionResource->rawLength, "data too long!");
+  // Zero out the anchor area, in case the data is shorter than the anchor
+  // Note that we're taking the strlen of `rawAnchor` and not the `anchor`
+  // passed in. This ensures that the entire anchor is zero'ed, even if only
+  // a prefix of it was provided.
+  memset((void *)rawAnchor, 0, strlen(rawAnchor));
+  // Copy the new data into the same spot the anchor was in
   memcpy((void *)rawAnchor, data, dataLen);
 
   // Write the new block.

test/attribution_shorter_than_sentinel.t

@@ -0,0 +1,33 @@
+Make sure we have a fresh build:
+
+  $ export BUILDDIR=$TESTDIR/../build
+  $ cd $BUILDDIR
+  $ make 2> /dev/null >/dev/null
+  $ cd $CRAMTMP
+
+Prepare content and the attribution code:
+  $ mkdir stagedir
+  $ echo "content-a" >> stagedir/a
+  $ sentinel="__MOZILLA__attribution-code"
+  $ attribution_code="short"
+
+Create the filesystem with the sentinel value in the attribute:
+
+  $ mkdir output
+  $ cp $TESTDIR/empty.hfs output/short.hfs
+  $ $BUILDDIR/hfs/hfsplus output/short.hfs add stagedir/a a
+  $ $BUILDDIR/hfs/hfsplus output/short.hfs setattr a attr-key "$sentinel"
+
+Build a DMG:
+
+  $ $BUILDDIR/dmg/dmg build output/short.hfs output/short.dmg "$sentinel" >/dev/null
+
+Now attribute:
+
+  $ $BUILDDIR/dmg/dmg attribute output/short.dmg output/short-attributed.dmg "__MOZILLA__" "$attribution_code" >/dev/null
+
+Extract the HFS from the attributed DMG, and check to see if the code is correct:
+
+  $ $BUILDDIR/dmg/dmg extract output/short-attributed.dmg output/short-attributed.hfs >/dev/null
+  $ $BUILDDIR/hfs/hfsplus output/short-attributed.hfs getattr a attr-key | tr -d '\0'
+  short (no-eol)