rust build support: Find Cargo.lock file upwards in tree

Cargo, the build tool for the rust ecosystem, provides a feature named
"workspaces". In workspaces, multiple library- and binary-crates can be
bundled together in one repository/project.

The workspace functionality also provides functionality where
dependencies are built once for several crates in the workspace. Thus
the "Cargo.lock" file (as well as the ./target folder where the final
binary lives) are placed in the top-level directory of the repository,
while the `cargo build` command may be called somewhere in the working
tree.

Because of this we need to make sure that we find the `Cargo.lock` file
by not only searching the current directory but also the directories
above the `${sourceRoot}`.

This change introduces a little helper to walk up the file tree and
check for a file, starting with the current directory.

pkgs/build-support/rust/fetch-cargo-deps

@@ -50,7 +50,18 @@ EOF
  export CARGO_HOME=$out
  cd $src
 
- if [[ ! -f Cargo.lock ]]; then
+ find_upwards() {
+ [[ "$(realpath "$1")" == "/" ]] && return 1
+ if [ -f "$1/$2" ]; then
+ echo "$1/$2"
+ else
+ find_upwards "../$1" "$2"
+ fi
+ }
+
+ cargo_lock=$(find_upwards . "Cargo.lock")
+
+ if [[ -z "$cargo_lock" ]]; then
  echo
  echo "ERROR: The Cargo.lock file doesn't exist"
  echo