build(pd): fail if asset zipfiles aren't complete

Updates the build.rs file for pd to error out if the frontend asset
bundles are incomplete, by checking for a small filesize. That isn't a
super durable method, but it's a cheap check and easy to update in the
future.

Closes #4703.

crates/bin/pd/build.rs

@@ -1,12 +1,50 @@
+use std::io::Read;
 use std::path::Path;
 
 use anyhow::Context;
 
 fn main() -> anyhow::Result<()> {
+    check_frontend_asset_zipfiles()?;
     setup_testnet_config()?;
     Ok(())
 }
 
+// Check that the zip files for bundled frontend code are functional.
+// If git-lfs is not configured on the build host, the zip files will
+// be plaintext lfs pointer files.
+fn check_frontend_asset_zipfiles() -> anyhow::Result<()> {
+    // Declare a minimum filesize, below which we'll assume the zip file is
+    // actually a git-lfs pointer.
+    const MINIMUM_FILESIZE_BYTES: usize = 500;
+    // Build paths to the zip files in the local build env.
+    let zipfiles = vec![
+        Path::new(env!("CARGO_MANIFEST_DIR")).join("../../../assets/minifront.zip"),
+        Path::new(env!("CARGO_MANIFEST_DIR")).join("../../../assets/node-status.zip"),
+    ];
+    for zipfile in zipfiles {
+        let mut bytes = Vec::new();
+        let f = std::fs::File::open(&zipfile).context(format!(
+            "failed to open zip file of frontend code: {}",
+            &zipfile.display()
+        ))?;
+        let mut reader = std::io::BufReader::new(f);
+        reader.read_to_end(&mut bytes).context(format!(
+            "failed to read zip file of frontend code: {}",
+            zipfile.display()
+        ))?;
+        if bytes.len() < MINIMUM_FILESIZE_BYTES {
+            anyhow::bail!(
+                format!(
+                    "asset zip file {} is smaller than {} bytes; install git-lfs, run 'git lfs pull', and retry the build",
+                    zipfile.display(),
+                    MINIMUM_FILESIZE_BYTES
+                )
+                );
+        }
+    }
+    Ok(())
+}
+
 // Set build-time environment variables to point to the latest testnet's config files.
 fn setup_testnet_config() -> anyhow::Result<()> {
     // Get the path to the testnets directory, in a platform-agnostic manner