Ensure that the rpath is prepended instead of overwritten

deploy/linux-post-link.sh

@@ -12,7 +12,12 @@
 # but they might be a different version. It "might" work, it might cause subtle
 # bugs, or it might not work at all.
 #
-# To patch the libraries, a tool called `patchelf` is used.
+# It's possible there's no current rpath for a particular library. It's also
+# possible that the library has other dependencies in its existing rpath. So
+# updating the rpath is non-trivial and it's a real shame that qmake doesn't
+# do this for us.
+#
+# To patch the libraries `readelf` and `patchelf` tools are used.
 #
 # If the libraries' rpath isn't set correctly then LD_LIBRARY_PATH would need
 # to be used, like such:
@@ -50,16 +55,40 @@ find "${QTDIR}" \
     -executable \
     2>/dev/null |
 while IFS='' read -r library; do
+    # Get the library's current RPATH (RUNPATH)
+    # Example output of `readelf readelf -d ./build/build-qgroundcontrol-Desktop_Qt_5_15_2_GCC_64bit-Debug/staging/QGroundControl`:
+    #  0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/Qt/libs:/home/kbennett/storage/Qt/5.15.2/gcc_64/lib]
+    #
+    # If you find a better way to do this, please fix.
+    set +o pipefail
+    current_rpath="$(
+        # read the library, parsing its header
+        # search for the RUNPATH field
+        # filter out the human-readable text to leave only the RUNPATH value
+        readelf -d "${library}" |
+        grep -P '^ 0x[0-9a-f]+ +\(RUNPATH\) ' |
+        sed -r 's/^ 0x[0-9a-f]+ +\(RUNPATH\) +Library runpath: \[(.*)\]$/\1/g'
+    )"
+    set -o pipefail
+
     # Get the directory containing the library
     library_dir="$(dirname "${library}")"
 
     # Get the relative path from the library's directory to the Qt/libs directory.
-    library_rpath="$(realpath --relative-to "${library_dir}" "${RPATHDIR}")"
+    our_rpath="$(realpath --relative-to "${library_dir}" "${RPATHDIR}")"
 
-    # patch the library's rpath to point to the Qt/libs directory.
+    # Calculate a new rpath with our library's rpath prefixed.
     # Note: '$ORIGIN' must not be expanded by the shell!
     # shellcheck disable=SC2016
-    patchelf --set-rpath '$ORIGIN/'"${library_rpath}" "${library}"
+    new_rpath='$ORIGIN/'"${our_rpath}"
+
+    # If the library already had an rpath, then prefix ours to it.
+    if [ -n "${current_rpath}" ]; then
+        new_rpath="${new_rpath}:${current_rpath}"
+    fi
+
+    # patch the library's rpath
+    patchelf --set-rpath "${new_rpath}" "${library}"
 done
 
 # Create a qt.conf file