Regen cost shape: one corpus read, rustfmt once per distinct input (memo + verified per-version cache), rustfmt_spawns on the receipt

dag/gunbc/regen_round_cost.dag

@@ -42,7 +42,6 @@ import std.observation { TimedMeasurement, Measured, MeasuredValue, MeasuredUnav
 type RegenRoundPhase
   = SeedBuild
   | CorpusLoad
-  | CorpusReload
   | Frontend
   | Normalize
   | Reconcile
@@ -105,8 +104,9 @@ type RegenRoundCostReceipt {
   host: String
   tree: String
   tree_dirty: Bool
-  seed_build_compiled_crates: Int
-  rebuild_compiled_crates: Int
+  seed_build_compiled_crates: Nat
+  rebuild_compiled_crates: Nat
+  rustfmt_spawns: Nat
   marks: List<TraceMark>
   changed_paths: List<String>
 }
@@ -115,7 +115,6 @@ fn regen_round_phase_tag(phase: RegenRoundPhase) -> String {
   match phase {
     SeedBuild => "seed_build"
     CorpusLoad => "corpus_load"
-    CorpusReload => "corpus_reload"
     Frontend => "compile.frontend"
     Normalize => "compile.normalize"
     Reconcile => "compile.reconcile"
@@ -141,7 +140,6 @@ fn regen_round_phase_label(phase: RegenRoundPhase) -> String {
   match phase {
     SeedBuild => "round.seed_build"
     CorpusLoad => "regen.corpus_load"
-    CorpusReload => "regen.corpus_reload"
     Frontend => "compile.frontend"
     Normalize => "compile.normalize"
     Reconcile => "compile.reconcile"
@@ -159,7 +157,7 @@ fn regen_round_phase_label(phase: RegenRoundPhase) -> String {
 }
 
 fn regen_round_phases() -> List<RegenRoundPhase> {
-  [SeedBuild, CorpusLoad, CorpusReload, Frontend, Normalize, Reconcile, Analyses, Emit, MirrorWrite, CandidateVerify, Adjudicate, HandVerify, Digest, Install, RebuildFromInstalled, Diff]
+  [SeedBuild, CorpusLoad, Frontend, Normalize, Reconcile, Analyses, Emit, MirrorWrite, CandidateVerify, Adjudicate, HandVerify, Digest, Install, RebuildFromInstalled, Diff]
 }
 
 fn regen_round_phase_of_label(label: String) -> RegenRoundPhaseAttribution {
@@ -217,7 +215,10 @@ fn regen_round_header_line(receipt: RegenRoundCostReceipt) -> String {
       concat(concat(" tree=", receipt.tree), concat(" tree_dirty=", regen_round_bool_text(b: receipt.tree_dirty))),
       concat(
         concat(" seed_build_compiled_crates=", to_string(receipt.seed_build_compiled_crates)),
-        concat(" rebuild_compiled_crates=", to_string(receipt.rebuild_compiled_crates))
+        concat(
+          concat(" rebuild_compiled_crates=", to_string(receipt.rebuild_compiled_crates)),
+          concat(" rustfmt_spawns=", to_string(receipt.rustfmt_spawns))
+        )
       )
     )
   )
@@ -308,6 +309,11 @@ data regen_affected_set_disposition: AffectedSetDisposition = DerivableNow {
 // a round with a real edit; the two cargo builds of the seed (before the round, and
 // rebuild-from-installed) are the majority of such a round even for a one-mirror change,
 // because every mirror compiles into one crate and cargo recompiles that crate whole; the
-// corpus is read twice (corpus_load then corpus_reload, the second inside compile_stage0) and
-// rustfmt normalizes the whole population three times (mirror_write, adjudicate, digest).
+// corpus WAS read twice (corpus_load then a corpus_reload inside compile_stage0) and rustfmt
+// normalized the whole population three times (mirror_write, adjudicate, digest) -- both
+// closed in the change after the measurement: one read threaded to both consumers, and one
+// normalize per distinct input through a memo plus a per-rustfmt-version disk cache under
+// target/stage0-regen-rustfmt-cache whose entries are honoured only when the stored raw input
+// is byte-equal to the request. rustfmt_spawns on the receipt is the count the fixed-point
+// control is checked against.
 data regen_round_cost_shape_note: String = "The instrument, not the number: re-run `claim_executor --regen-round-cost --source-root dag --source-root src/v2` on a tree and read its receipt (target/stage0-regen-round-cost.txt). A change-denominated regen is bounded above by the reverse closure (regen_affected_set_disposition); a change-denominated REBUILD is bounded below by one whole-crate compile until the stage0 partition (gunbc.stage0_crate_partition_generated) carries the mirrors, which is the larger of the two costs on every measured host."

dag/test/claim/regen_round_cost_witness_test.dag → dag/test/claim/self_host_regen_round_cost_witness_test.dag

@@ -1,4 +1,4 @@
-module test.claim.regen_round_cost_witness
+module test.claim.self_host_regen_round_cost_witness
 
 import std.types { Bool, String, List, NonEmptyStr }
 import std.measure { CpuClock, WallClock, Nanosecond, nanosecond, nanosecond_count }
@@ -46,6 +46,7 @@ fn a_receipt(marks: List<TraceMark>) -> RegenRoundCostReceipt {
     tree_dirty: false,
     seed_build_compiled_crates: 0,
     rebuild_compiled_crates: 2,
+    rustfmt_spawns: 7,
     marks: marks,
     changed_paths: ["v1_rt.rs", "v1_compiler_runtime_rust.rs"]
   }
@@ -135,7 +136,7 @@ test fn a_receipt_renders_provenance_marks_totals_and_changed_paths() -> Bool {
     a_mark(label: "compile.emit", wall_ms: 300000, cpu_ms: 299000)
   ]))
   text == concat(
-    "regen-round-cost: producer=claim_executor --regen-round-cost host=srv1 tree=2a11b317d2caf3c37d1d38a4421e8e0c06188925 tree_dirty=false seed_build_compiled_crates=0 rebuild_compiled_crates=2\n",
+    "regen-round-cost: producer=claim_executor --regen-round-cost host=srv1 tree=2a11b317d2caf3c37d1d38a4421e8e0c06188925 tree_dirty=false seed_build_compiled_crates=0 rebuild_compiled_crates=2 rustfmt_spawns=7\n",
     concat(
       "regen-round-cost: phase=seed_build wall_ms=1500 cpu_ms=9000\nregen-round-cost: phase=compile.emit wall_ms=300000 cpu_ms=299000\n",
       "regen-round-cost: total wall_ms=301500 cpu_ms=308000\nregen-round-cost: changed_paths=2 [v1_rt.rs, v1_compiler_runtime_rust.rs]\n"

src/v1/stage0/build.rs

@@ -2,7 +2,18 @@ use std::path::Path;
 use std::process::Command;
 
 fn git_output(args: &[&str]) -> Option<String> {
-    let out = Command::new("git").args(args).output().ok()?;
+    // NO OPTIONAL LOCKS: `git status` refreshes `.git/index` as a side effect, and this script
+    // watches that path (`rerun-if-changed` below). Refreshing it DURING the script's own run
+    // made the NEXT build see a changed input, rerun the script and recompile the whole crate
+    // with nothing changed -- measured 2026-08-30 on srv1 (tree fce29f50, quiescent worktree):
+    // cold 444 s, then a no-change build 382 s, then 0 s only once the index had settled; and
+    // inside every `--regen-round-cost` round, seed_build and rebuild-from-installed each
+    // recompiled the crate at changed_paths=0. `--no-optional-locks` makes the read a read.
+    let out = Command::new("git")
+        .arg("--no-optional-locks")
+        .args(args)
+        .output()
+        .ok()?;
     if out.status.success() {
         Some(String::from_utf8_lossy(&out.stdout).trim().to_string())
     } else {