Build one candidate when the construction budget is already spent

docs/vtrees.md

@@ -45,6 +45,12 @@ caller can read which of those happened: `VtreeBuild::limits` lists the builds
 that finished, the builds the budget cut short, the time they spent and the
 candidates never started.
 
+If the budget is already spent when the walk starts and nothing has been built
+yet — which happens when preprocessing used it up, or when earlier components
+did — the first candidate still gets one attempt under a fixed one-second wall,
+and the candidates behind it are reported as never started. That build returns a
+tree rather than failing the construction.
+
 `VtreeBuild::construction_ms` reports the broader end-to-end construction wall
 from the library entry through the finished whole or grafted tree. It includes
 setup, simple constructors and component grafting that are deliberately outside

src/component/mod.rs

@@ -521,11 +521,11 @@ fn build_per_component<O: BuildObserver>(
     // between the components by clause count.
     //
     // A component whose share is already zero starts expired, and portfolio
-    // answers that with a construction error — so there is no separate
-    // deadline check here, and the error propagates straight out of this
-    // loop (`?` below), aborting the whole multi-component build. Tiny
-    // components skip this: minfill takes no deadline and cannot fail this
-    // way.
+    // answers that by giving its first candidate one short attempt and
+    // reporting the rest as never started — so there is no separate deadline
+    // check here, and a budget spent by the earlier components costs the later
+    // ones the rest of their catalog rather than the build. Tiny components
+    // skip this: minfill takes no deadline and never consults one.
     let mut clauses_left: usize = comps.iter().map(|c| c.len()).sum();
     for comp_indices in comps {
         let comp_deadline = limits

src/config/mod.rs

@@ -784,10 +784,9 @@ impl RunConfig {
         }
         if self.construction_budget == (ConstructionBudget::Deterministic { units: 0 }) {
             return Err(VitriError::config(
-                "a deterministic construction budget of 0 work units leaves construction \
-                 nothing to spend, so no vtree could be built — pass the work a construction \
-                 should be allowed to do, which ConstructionBudget::for_wall_ms converts from \
-                 a wall in milliseconds",
+                "a deterministic construction budget of 0 work units asks construction to do \
+                 no work at all — pass the work a construction should be allowed to do, which \
+                 ConstructionBudget::for_wall_ms converts from a wall in milliseconds",
             ));
         }
         if self.candidates == 0 {

src/decompose/portfolio/catalog.rs

@@ -318,11 +318,16 @@ pub(super) struct RunState {
     /// wall it never reaches. The deadline is otherwise consulted only between
     /// entries, which cannot stop the one that has already begun — and that is
     /// the entry which overruns the ceiling.
+    ///
+    /// The one exception is the attempt the driver allows when the deadline is
+    /// already spent and nothing has been built: there the share and the wall
+    /// are both a fixed short number, because what is left is zero or less.
     pub(super) cand_wall_ms: Option<i64>,
-    /// Latched once some entry has overrun its own fair share. Until it latches
-    /// every entry is bounded only by the whole remaining budget; after it
-    /// latches the remaining FlowCutter builds are additionally tightened to the
-    /// fair share, and take the tight search with it (see `fc_time_cap_ms` and
+    /// Latched once some entry has overrun its own fair share, and set outright
+    /// for the one attempt a spent deadline allows. Until it latches every entry
+    /// is bounded only by the whole remaining budget; after it latches the
+    /// remaining FlowCutter builds are additionally tightened to the fair share,
+    /// and take the tight search with it (see `fc_time_cap_ms` and
     /// `fc_cap_mode`).
     pub(super) behind_schedule: bool,
     pub(super) flowcutter_incidence_td_cache: Option<crate::decompose::TreeDecomposition>,
@@ -479,9 +484,10 @@ impl RunState {
     ///
     /// Three sources, and the tightest wins:
     /// - `cand_wall_ms`, the time actually left in the construction budget when
-    ///   this entry started. Under a deadline this is always armed, the first
-    ///   entry included, which is what makes the budget a ceiling rather than a
-    ///   suggestion.
+    ///   this entry started — or the fixed short wall of the one attempt a spent
+    ///   deadline allows, where the time left is zero or less. Under a deadline
+    ///   this is always armed, the first entry included, which is what makes the
+    ///   budget a ceiling rather than a suggestion.
     /// - `cand_cap_ms`, this entry's fair share, once `behind_schedule` has
     ///   latched. That is the scheduling tightening the latch has always
     ///   applied; it no longer decides whether a cap exists at all.
@@ -503,7 +509,8 @@ impl RunState {
     /// Tightness changes what the search considers, not only when it stops (see
     /// [`WallCapMode`]), so it is keyed on the two conditions that mean the
     /// build is already in the regime where finishing beats searching:
-    /// - `behind_schedule` — some entry has already overrun its fair share;
+    /// - `behind_schedule` — some entry has already overrun its fair share, or
+    ///   this is the one attempt a spent deadline allows;
     /// - `flowcutter_cap_ms` — the projected large-component cap, whose whole
     ///   purpose is to cut a grinding `flowcutter-primal` short.
     ///

src/decompose/portfolio/driver.rs

@@ -1,7 +1,9 @@
 //! The portfolio driver: run the catalog, select a winner, publish the result.
 //!
-//! A construction budget that leaves nothing built is a hard error: no
-//! fallback stands between an exhausted budget and
+//! A construction budget that leaves nothing built is a hard error. A budget
+//! already spent when the walk starts is not that case: the entry the walk
+//! stops at gets one attempt under a fixed short wall, and only a budget under
+//! which that attempt also produces nothing reaches
 //! `Err(VitriError::construction(..))`.
 //!
 //! **Determinism:** what a portfolio build produces is a function of the
@@ -48,6 +50,15 @@ use super::catalog::{
     gate_hypergraph_bisect, outspent, work_ms_since,
 };
 
+/// The wall one catalog entry gets when the construction deadline is already
+/// spent and nothing has been built.
+///
+/// It is a fixed number rather than a share of what is left, because what is
+/// left is zero or less. Short enough that a build already over its budget does
+/// not go far past it, and long enough for the first entry — an anytime cutter
+/// under a timed budget — to return a decomposition.
+const LAST_ATTEMPT_MS: i64 = 1_000;
+
 /// One build's wall report: a build that left candidates unstarted is the
 /// truncated one, and a build that walked the whole catalog is the complete
 /// one. Stated here rather than at the call site so the rule can be asked
@@ -326,18 +337,42 @@ pub(crate) fn vtree_from_portfolio(
     }
 
     // A deadline already passed on entry — common once a multi-component build
-    // has spent its budget on earlier components — skips the whole catalog on
-    // the first iteration, so construction fails outright.
+    // has spent its budget on earlier components — would skip the whole catalog
+    // on the first iteration and fail the construction outright. A candidate
+    // that could have been built is worth more than the deadline it misses, so
+    // the entry the loop stopped at gets one attempt under a fixed short wall
+    // when nothing has been built yet; the rest are skipped either way.
     let mut skipped: Vec<&'static str> = Vec::new();
+    let mut last_attempt = false;
     for (i, c) in catalog.iter().enumerate() {
         if inp.out_of_time() {
-            skipped.extend(catalog[i..].iter().map(|c| c.name));
-            break;
+            // Both, because which of the two a built candidate lands in depends
+            // on the mode: plain selection adopts into `best`, projected
+            // selection collects into `cands` and chooses at the end.
+            if run.best.vtree.is_none() && run.cands.is_empty() {
+                diag!(
+                    "[portfolio] deadline spent with nothing built; {} gets {LAST_ATTEMPT_MS}ms",
+                    c.name,
+                );
+                last_attempt = true;
+            } else {
+                skipped.extend(catalog[i..].iter().map(|c| c.name));
+                break;
+            }
+        }
+        if last_attempt {
+            run.cand_cap_ms = Some(LAST_ATTEMPT_MS);
+            run.cand_wall_ms = Some(LAST_ATTEMPT_MS);
+            // The regime where finishing beats searching, which is what this
+            // attempt is: the wall is the whole budget it has.
+            run.behind_schedule = true;
+        } else {
+            run.cand_cap_ms = inp.fair_share_ms(catalog.len() - i);
+            // The hard bound: whatever is still left of the whole construction
+            // budget. `out_of_time` above has already ruled out a non-positive
+            // one.
+            run.cand_wall_ms = inp.remaining_ms().map(|r| r.max(1));
         }
-        run.cand_cap_ms = inp.fair_share_ms(catalog.len() - i);
-        // The hard bound: whatever is still left of the whole construction
-        // budget. `out_of_time` above has already ruled out a non-positive one.
-        run.cand_wall_ms = inp.remaining_ms().map(|r| r.max(1));
         // Where this entry's slice starts, on the construction clock: what the
         // latch below decides — whether the entries behind this one search less
         // patiently — is a decision about which tree comes out, so it is
@@ -354,6 +389,12 @@ pub(crate) fn vtree_from_portfolio(
         if open && let Some(built) = (c.build)(&inp, &mut run) {
             run.fold(&inp, c, built);
         }
+        // One attempt is all a spent deadline buys, whether or not it produced
+        // anything: the entries behind it are skipped.
+        if last_attempt {
+            skipped.extend(catalog[i + 1..].iter().map(|c| c.name));
+            break;
+        }
         if run
             .cand_cap_ms
             .is_some_and(|cap| (work_ms_since(slice_start) as i64) > cap)

src/decompose/portfolio/tests/driver.rs

@@ -138,34 +138,46 @@ fn budget_fixture() -> crate::cnf::CnfFormula {
     crate::tests::circuit_fixture::multiplier()
 }
 
-/// SPENT BUDGET IS A HARD ERROR: a deadline that has ALREADY passed on entry
-/// skips every catalog candidate, and the build fails outright rather than
-/// handing back a degraded vtree.
+/// A SPENT BUDGET STILL BUILDS ONE CANDIDATE: a deadline that has ALREADY
+/// passed on entry leaves no share for anything, but a tree the caller can use
+/// is worth more than the deadline it misses. The first catalog entry runs
+/// under a fixed short wall and every entry behind it is reported as never
+/// started, which is how a caller tells this tree from a complete one.
+///
+/// The spent deadline is constructed, not waited for: an `Instant` already in
+/// the past is past on entry on any machine, so the case under test is reached
+/// without timing anything.
 #[test]
-fn expired_deadline_is_a_construction_error() {
+fn an_expired_deadline_still_builds_the_first_candidate() {
     use std::time::{Duration, Instant};
     let formula = budget_fixture();
     let limits = BuildLimits {
         deadline: Some(Instant::now() - Duration::from_secs(1)),
         ..BuildLimits::default()
     };
-    // Matched by hand rather than `.expect_err()`: `VtreeArtifacts` (the `Ok`
-    // side) carries an `Arc<Vtree>` and does not derive `Debug`, which
-    // `.expect_err()`'s bound would otherwise require adding just for this test.
-    match vtree_from_portfolio(
+    let built = vtree_from_portfolio(
         &formula,
         150_000,
         15,
         Reading::default(),
         &SelectionCtx::plain(),
         &limits,
-    ) {
-        Ok(_) => panic!("an already-spent deadline must fail construction, not build a vtree"),
-        Err(e) => assert!(
-            matches!(e, crate::error::VitriError::Construction { .. }),
-            "expected a construction error, got {e:?}",
-        ),
-    }
+    )
+    .expect("a spent deadline must still hand back a vtree");
+    assert_eq!(
+        built.vtree.num_leaves(),
+        formula.num_vars,
+        "the tree must cover the formula",
+    );
+    assert_eq!(
+        built.limits.truncated_builds, 1,
+        "a build that left catalog entries unstarted is the truncated one",
+    );
+    let behind_the_first: Vec<String> = catalog().iter().skip(1).map(|c| c.name.into()).collect();
+    assert_eq!(
+        built.limits.skipped, behind_the_first,
+        "one attempt is all a spent deadline buys: every entry behind it is never started",
+    );
 }
 
 /// NO BEHAVIOR DRIFT: a deadline far beyond what construction needs must
@@ -213,6 +225,17 @@ fn generous_deadline_matches_no_deadline() {
         unbounded.vtree.to_vtree_text(),
         "a generous budget changed the constructed vtree",
     );
+    // The other side of the fallback above: with time left on entry the walk is
+    // the ordinary fair-share one, so nothing is skipped and no entry is cut
+    // down to the one-attempt wall.
+    assert!(
+        bounded.limits.skipped.is_empty(),
+        "a budget with time left must walk the whole catalog",
+    );
+    assert_eq!(
+        bounded.limits.complete_builds, 1,
+        "a build that walked the whole catalog is the complete one",
+    );
 }
 
 /// Tiny dummy ScoredCandidate (the vtree is never inspected by select_peak_band).

src/tests/bundle/components/candidates.rs

@@ -171,18 +171,18 @@ fn what_the_run_config_allows_reaches_the_portfolio() {
         "the run config's candidate width must reach the portfolio",
     );
 
-    // The same config with an already-spent deadline: the construction that
-    // succeeded above can now only fail by obeying it. Matched by hand rather
-    // than `expect_err`: the `Ok` side does not derive `Debug`.
+    // The same config with an already-spent deadline. The build still returns a
+    // vtree — the portfolio gives its first candidate one short attempt rather
+    // than skipping the whole catalog — but it obeys the deadline by leaving the
+    // rest of the catalog unstarted, which a complete build never reports.
     let spent = RunConfig {
         deadline: Some(std::time::Instant::now()),
         ..candidates_config(1)
     };
-    match build_vtree(&formula, &spent, &SelectionCtx::plain()) {
-        Ok(_) => panic!("a spent deadline on the run config must bound the construction"),
-        Err(err) => assert!(
-            matches!(err, crate::error::VitriError::Construction { .. }),
-            "expected a construction error, got {err:?}",
-        ),
-    }
+    let bounded = build_vtree(&formula, &spent, &SelectionCtx::plain())
+        .expect("a spent deadline must still hand back a vtree");
+    assert!(
+        !bounded.limits.skipped.is_empty(),
+        "a spent deadline on the run config must bound the construction",
+    );
 }

src/tests/bundle/run.rs

@@ -286,10 +286,11 @@ fn a_refuted_run_writes_the_bundle_and_names_no_vtree() {
 /// construction budget that cannot build anything never reaches the run.
 ///
 /// One work unit buys no construction at all: the budget is spent at the
-/// instant construction would start, every portfolio entry is skipped, and
-/// nothing is built — which the irreducible instance below shows is a hard
-/// error. The refuted instance takes the same configuration and succeeds,
-/// because construction is never asked for a vtree over it.
+/// instant construction would start. The irreducible instance below shows what
+/// that costs — the portfolio gives its first candidate one short attempt and
+/// leaves the rest of the catalog unstarted. The refuted instance takes the
+/// same configuration and reports no construction at all, because construction
+/// is never asked for a vtree over it.
 #[test]
 fn a_refuted_run_answers_before_construction_can_be_asked_for_a_vtree() {
     let config = RunConfig {
@@ -298,11 +299,14 @@ fn a_refuted_run_answers_before_construction_can_be_asked_for_a_vtree() {
     };
 
     let (formula, meta) = parse(IRREDUCIBLE_5);
-    let err = run(&formula, &meta, &config, &SelectionCtx::plain())
-        .expect_err("one work unit must leave the portfolio nothing to build with");
+    let produced = run(&formula, &meta, &config, &SelectionCtx::plain())
+        .expect("one work unit still buys the first candidate its one attempt");
+    let RunVtree::Built(built) = &produced.vtree else {
+        panic!("the irreducible instance reaches construction and is built over");
+    };
     assert!(
-        matches!(err, VitriError::Construction { .. }),
-        "the exhausted construction budget must be what fails, got: {err:?}",
+        !built.limits.skipped.is_empty(),
+        "a construction budget spent at entry leaves the rest of the catalog unstarted",
     );
 
     let (formula, meta) = parse(REFUTED);