fix(build) :: make frontend asset urls idempotent if nothing changed

Frontend assets were hashed via gzip's output which includes a nonidempotent/deterministc timestamp from libflate. This changes the hash on each build.

Now we hash over the bundled file contents in `frontend/dist`.

Author: 81reap

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## unreleased
 
 - Big number cards with a `color` once again display their main value in that color in light and dark themes, including values with a link. Their compact vertical padding has also been restored.
+- Frontend asset urls are now idempotent between builds if nothing changed in the frontend. Now browsers will only re-download frontend assets when there has been a code change in them.
 - `cargo install sqlpage` or any build from the crates.io tarball only needs Rust and no longer needs internet access. Pre-built binaries and the Docker image are unaffected.
 - The browser assets are now bundled as a whole, so SQLPage's own scripts are minified along with their dependencies and are no longer readable in browser devtools.
 

build.rs

@@ -40,8 +40,12 @@ fn track_dist_changes() {
     }
 }
 
+fn dist_path(name: &str) -> PathBuf {
+    Path::new(DIST).join(name)
+}
+
 fn open_dist(name: &str) -> File {
-    let path = Path::new(DIST).join(name);
+    let path = dist_path(name);
     File::open(&path).unwrap_or_else(|err| {
         panic!(
             "Unable to read the browser asset {}: {err}\n\
@@ -61,7 +65,7 @@ fn embed_served_asset(name: &str) {
         .expect("Unable to write compressed frontend asset");
     std::fs::write(
         format!("{}.filename.txt", built.display()),
-        hashed_filename(&built),
+        hashed_filename(&dist_path(name)),
     )
     .unwrap();
 }