1 files changed, 29 insertions, 17 deletions

diff --git a/src/remake/api.rs b/src/remake/api.rs
index 826c81e..3530423 100644
--- a/src/remake/api.rs
+++ b/src/remake/api.rs
@@ -17,7 +17,8 @@
 //!   can't commit it any more anyway (`coordinator`). `DELETE /admin/remakes` does the
 //!   same for the undecided remake of the local epoch, without having to know its ID.
 //!
-//! Request body for POST (all optional):
+//! Request body for POST (all optional; `baseHeads` not for `compact`, whose base is
+//! fixed when it commits):
 //! `{ "kind": "compact", "spec": null, "baseHeads": ["<hex>", …],
 //!    "replayWindow": {"epochs": 1, "seconds": 604800},
 //!    "archiveRetention": {"epochs": 2, "seconds": 2592000},
@@ -395,6 +396,15 @@ async fn start_kind<H: Host>(
         }
     };
 
+    if kind.base_at_commit() && request.base_heads.is_some() {
+        return error_response(
+            StatusCode::BAD_REQUEST,
+            format!(
+                "a {kind_name} remake fixes its base when it commits; baseHeads is not accepted"
+            ),
+            None,
+        );
+    }
     let base = match &request.base_heads {
         None => None,
         Some(heads) => {
@@ -454,7 +464,8 @@ async fn start_kind<H: Host>(
     let mut proposal = Proposal {
         coordinator: host.node_id(),
         from_epoch: host.current_epoch().into(),
-        base_heads: Vec::new(),
+        // Filled in when the proposal is written, unless the kind fixes it at the commit.
+        base_heads: None,
         policy: Policy {
             replay_window: request
                 .replay_window
@@ -598,6 +609,7 @@ async fn propose<H: Host>(
     let electorate_host = host.clone();
     let me = host.node_id();
     let started = host.remake().started_at() + grace;
+    let base_at_commit = kind::by_name(&proposal.kind).is_some_and(|kind| kind.base_at_commit());
     let record = host
         .commit_doc(move |doc| -> Result<Record, StartError> {
             let mut all = records::load_all(doc);
@@ -656,22 +668,22 @@ async fn propose<H: Host>(
             }
 
             // The base is the requested heads, or everything before the proposal (the
-            // proposal itself is never in it).
-            let base = match base {
-                Some(base) => {
-                    if let Some(unknown) = base
-                        .iter()
-                        .find(|head| doc.get_change_by_hash(head).is_none())
-                    {
-                        return Err(StartError::UnknownBase(unknown.to_string()));
+            // proposal itself is never in it); or, for a compaction, left to the commit.
+            if !base_at_commit {
+                let base = match base {
+                    Some(base) => {
+                        if let Some(unknown) = base
+                            .iter()
+                            .find(|head| doc.get_change_by_hash(head).is_none())
+                        {
+                            return Err(StartError::UnknownBase(unknown.to_string()));
+                        }
+                        base
                     }
-                    base
-                }
-                None => doc.get_heads(),
-            };
-            let mut heads: Vec<String> = base.iter().map(|h| h.to_string()).collect();
-            heads.sort();
-            proposal.base_heads = heads;
+                    None => doc.get_heads(),
+                };
+                proposal.base_heads = Some(records::encode_heads(&base));
+            }
 
             // We vote for our own remake, so we are always in the electorate.
             let mut electorate = electorate_host.electorate(doc, now);