ci: use Swatinem/rust-cache for the Rust workspace job (reliability) (#925)

The Rust Workspace Tests job manually cached the whole `v2/target` via
actions/cache@v4. For a 38-crate workspace that dir is multi-GB, and several
CI runs this cycle intermittently died at the cache/setup step (after
toolchain install, before "Run Rust tests"), each needing a rerun.

Swatinem/rust-cache@v2 is the de-facto standard Rust CI cache: it caches the
cargo registry/git + a pruned target, evicts stale dependencies, and restores
large workspaces far more reliably and faster than a naive whole-target cache.
`workspaces: v2` points it at the v2/ cargo workspace.

Reliability/speed change — verified by observing subsequent main runs.

.github/workflows/ci.yml

@@ -108,16 +108,18 @@ jobs:
     - name: Install Rust toolchain
       uses: dtolnay/rust-toolchain@stable
 
-    - name: Cache cargo
-      uses: actions/cache@v4
+    # Swatinem/rust-cache replaces a naive `actions/cache` of the whole
+    # `v2/target`. That manual cache of a 38-crate target dir (multi-GB) was an
+    # intermittent failure source — several CI runs this cycle died at the
+    # cache/setup step (after toolchain install, before "Run Rust tests"),
+    # needing a rerun. rust-cache is purpose-built for Rust: it caches the
+    # registry + git + a pruned target, evicts stale deps, and restores far more
+    # reliably (and faster) on large workspaces. `workspaces: v2` points it at
+    # the v2/ cargo workspace (keys on v2/Cargo.lock, caches v2/target).
+    - name: Cache cargo (Swatinem/rust-cache)
+      uses: Swatinem/rust-cache@v2
       with:
-        path: |
-          ~/.cargo/registry
-          ~/.cargo/git
-          v2/target
-        key: ${{ runner.os }}-cargo-${{ hashFiles('v2/Cargo.lock') }}
-        restore-keys: |
-          ${{ runner.os }}-cargo-
+        workspaces: v2
 
     - name: Run Rust tests
       working-directory: v2

docs/user-guide.md

@@ -1048,7 +1048,7 @@ The Rust sensing server binary accepts the following flags:
 | `--dataset` | (none) | Path to dataset directory (MM-Fi or Wi-Pose) |
 | `--dataset-type` | `mmfi` | Dataset format: `mmfi` or `wipose` |
 | `--epochs` | `100` | Training epochs |
-| `--export-rvf` | (none) | Export RVF model container and exit |
+| `--export-rvf` | (none) | Export a **placeholder** RVF container-format demo and exit — **not a trained model**. For a real model use `--train` (+ `--save-rvf`) or download a pretrained encoder. |
 | `--save-rvf` | (none) | Save model state to RVF on shutdown |
 | `--model` | (none) | Load a trained `.rvf` model for inference |
 | `--load-rvf` | (none) | Load model config from RVF container |
@@ -1359,7 +1359,7 @@ docker run --rm \
   -v $(pwd)/output:/output \
   --entrypoint /app/sensing-server \
   ruvnet/wifi-densepose:latest \
-  --train --dataset /data --epochs 100 --export-rvf /output/model.rvf
+  --train --dataset /data --epochs 100 --save-rvf /output/model.rvf
 ```
 
 The pipeline runs 10 phases:

v2/crates/wifi-densepose-sensing-server/src/main.rs

@@ -5619,6 +5619,16 @@ fn diagnose_model_load_error(path: &std::path::Path, data: &[u8], err: &str) ->
     )
 }
 
+/// Whether `--export-rvf` should emit the placeholder container-format demo.
+///
+/// It must only do so **standalone**. Combined with `--train`/`--pretrain` the
+/// real model is produced by the training pipeline, so short-circuiting here
+/// would silently skip training and write placeholder weights — the #894 bug
+/// where the documented `--train … --export-rvf` workflow produced a fake model.
+fn export_emits_placeholder_demo(export_set: bool, train: bool, pretrain: bool) -> bool {
+    export_set && !train && !pretrain
+}
+
 // ── Main ─────────────────────────────────────────────────────────────────────
 
 /// If `--ui-path` points nowhere (wrong cwd), try common repo layouts relative to cwd.
@@ -5662,9 +5672,24 @@ async fn main() {
         return;
     }
 
-    // Handle --export-rvf mode: build an RVF container package and exit
-    if let Some(ref rvf_path) = args.export_rvf {
-        eprintln!("Exporting RVF container package...");
+    // Handle --export-rvf: writes a CONTAINER-FORMAT DEMO with placeholder
+    // weights — it is NOT a trained model. Only short-circuit when standalone:
+    // combined with --train/--pretrain the real model is exported by the
+    // training pipeline, and short-circuiting here would silently skip training
+    // and write placeholder weights (#894 — the documented `--train …
+    // --export-rvf` workflow produced a placeholder and never trained).
+    if export_emits_placeholder_demo(args.export_rvf.is_some(), args.train, args.pretrain) {
+        let rvf_path = args
+            .export_rvf
+            .as_ref()
+            .expect("export_emits_placeholder_demo implies export_rvf is set");
+        eprintln!(
+            "WARNING: --export-rvf writes a CONTAINER-FORMAT DEMO with placeholder \
+             weights — it is NOT a trained model. Train one with \
+             `--train --dataset <DIR>` (which exports a calibrated .rvf to the \
+             models/ directory), or download a pretrained encoder. See issue #894."
+        );
+        eprintln!("Exporting RVF container package (placeholder weights)...");
         use rvf_pipeline::RvfModelBuilder;
 
         let mut builder = RvfModelBuilder::new("wifi-densepose", "1.0.0");
@@ -5713,6 +5738,13 @@ async fn main() {
             }
         }
         return;
+    } else if args.export_rvf.is_some() {
+        // --export-rvf alongside --train/--pretrain: don't emit a placeholder.
+        // Fall through so training runs; it exports the real calibrated model.
+        eprintln!(
+            "Note: --export-rvf is ignored in training mode — the trained model \
+             is exported by the training pipeline to the models/ directory."
+        );
     }
 
     // Handle --pretrain mode: self-supervised contrastive pretraining (ADR-024)
@@ -7310,3 +7342,29 @@ mod model_load_diagnostic_tests {
         assert!(msg.contains("wifi-densepose-train"), "{msg}");
     }
 }
+
+#[cfg(test)]
+mod export_rvf_mode_tests {
+    use super::export_emits_placeholder_demo;
+
+    #[test]
+    fn standalone_export_emits_placeholder() {
+        // --export-rvf alone → the container-format demo (placeholder weights).
+        assert!(export_emits_placeholder_demo(true, false, false));
+    }
+
+    #[test]
+    fn export_with_train_does_not_short_circuit() {
+        // #894: `--train --export-rvf` must NOT emit a placeholder + skip
+        // training — it must fall through to the real training pipeline.
+        assert!(!export_emits_placeholder_demo(true, true, false));
+        assert!(!export_emits_placeholder_demo(true, false, true));
+        assert!(!export_emits_placeholder_demo(true, true, true));
+    }
+
+    #[test]
+    fn no_export_flag_never_emits() {
+        assert!(!export_emits_placeholder_demo(false, false, false));
+        assert!(!export_emits_placeholder_demo(false, true, false));
+    }
+}