fix: build Rust parts from source if available

Old workflow:

 - Checkout submodule
 - Try to download precompiled assets
   - If downloading fails, compile the checked out submoduke

New workflow:

 - Download precompiled assets if submodule directory is empty
   - If download fails checkout the submodule
   - Compile the submodule
 - If the submodule directory is not empty, compile what is in there

The new workflow makes it easy to have a custom version of the submodule
checked out in the directory and getting everything built correctly.

install-rust-fil-sector-builder

@@ -7,42 +7,50 @@ source "install-shared.bash"
 
 subm_dir="rust-fil-sector-builder"
 
-git submodule update --init --recursive $subm_dir
-
-if download_release_tarball tarball_path "${subm_dir}"; then
-    tmp_dir=$(mktemp -d)
-    tar -C "$tmp_dir" -xzf "$tarball_path"
-
-    cp "${tmp_dir}/include/sector_builder_ffi.h" .
-    cp "${tmp_dir}/lib/libsector_builder_ffi.a" .
-    cp "${tmp_dir}/lib/pkgconfig/sector_builder_ffi.pc" .
-
-    cp "${tmp_dir}/bin/paramcache" .
-else
-    (>&2 echo "failed to find or obtain precompiled assets for ${subm_dir} - falling back to local build")
-    build_from_source "${subm_dir}"
+# If the directory is empty, try to download the pre-built tarballs or
+# checkout the source code as fallback
+if [ -z "$(ls -A $subm_dir)" ]; then
+    if download_release_tarball tarball_path "${subm_dir}"; then
+        tmp_dir=$(mktemp -d)
+        tar -C "$tmp_dir" -xzf "$tarball_path"
+
+        cp "${tmp_dir}/include/sector_builder_ffi.h" .
+        cp "${tmp_dir}/lib/libsector_builder_ffi.a" .
+        cp "${tmp_dir}/lib/pkgconfig/sector_builder_ffi.pc" .
+
+        cp "${tmp_dir}/bin/paramcache" .
+
+        exit 0
+    else
+        (>&2 echo "failed to find or obtain precompiled assets for ${subm_dir} - falling back to local build")
+        git submodule update --init --recursive $subm_dir
+    fi
+fi
 
-    mkdir -p include
-    mkdir -p lib/pkgconfig
+# Directory is not empty, hence compiling whatever source is in there
+(>&2 echo "building ${subm_dir} from source")
+build_from_source "${subm_dir}"
 
-    find "${subm_dir}/target/release" -type f -name sector_builder_ffi.h -exec cp -- "{}" . \;
-    find "${subm_dir}/target/release" -type f -name libsector_builder_ffi.a -exec cp -- "{}" . \;
-    find "${subm_dir}" -type f -name sector_builder_ffi.pc -exec cp -- "{}" . \;
+mkdir -p include
+mkdir -p lib/pkgconfig
 
-    if [[ ! -f "./sector_builder_ffi.h" ]]; then
-        (>&2 echo "failed to install sector_builder_ffi.h")
-        exit 1
-    fi
+find "${subm_dir}/target/release" -type f -name sector_builder_ffi.h -exec cp -- "{}" . \;
+find "${subm_dir}/target/release" -type f -name libsector_builder_ffi.a -exec cp -- "{}" . \;
+find "${subm_dir}" -type f -name sector_builder_ffi.pc -exec cp -- "{}" . \;
 
-    if [[ ! -f "./libsector_builder_ffi.a" ]]; then
-        (>&2 echo "failed to install libsector_builder_ffi.a")
-        exit 1
-    fi
+if [[ ! -f "./sector_builder_ffi.h" ]]; then
+    (>&2 echo "failed to install sector_builder_ffi.h")
+    exit 1
+fi
 
-    if [[ ! -f "./sector_builder_ffi.pc" ]]; then
-        (>&2 echo "failed to install sector_builder_ffi.pc")
-        exit 1
-    fi
+if [[ ! -f "./libsector_builder_ffi.a" ]]; then
+    (>&2 echo "failed to install libsector_builder_ffi.a")
+    exit 1
+fi
 
-    (>&2 echo "WARNING: paramcache was not installed - you may wish to 'cargo install' it")
+if [[ ! -f "./sector_builder_ffi.pc" ]]; then
+    (>&2 echo "failed to install sector_builder_ffi.pc")
+    exit 1
 fi
+
+(>&2 echo "WARNING: paramcache was not installed - you may wish to 'cargo install' it")