From 7a0698d87b88d6aeb13e6bfd89792223a0c487de Mon Sep 17 00:00:00 2001
From: Rob Zolkos <rob@37signals.com>
Date: Mon, 12 Oct 2026 13:26:50 -0400
Subject: [PATCH] Shrink two linear-time tests under the race detector, and
 shuffle the race run

TestFindBareLinksPastRejectedPrefixesIsLinear and
TestFindBareLinksInALongUnspacedParagraphIsLinear scan one large string on
one goroutine. Under -race they took 44s and 13s, against 2.7s and 0.8s
without it, and made internal/markdown the floor of Race Detection while
giving the detector nothing to find. A race-tagged linearInputScale now
divides their inputs by ten under -race; every build without -race runs
them at full size, where the linear-time bound is held.

Race Detection also runs with -shuffle=on, so a test that only passes in
source order fails there with the seed to reproduce it.
---
 .github/workflows/test.yml           | 2 +-
 internal/markdown/bare_links_test.go | 9 +++++----
 internal/markdown/bound_race_test.go | 6 ++++++
 internal/markdown/bound_test.go      | 4 ++++
 4 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index ecd4015b2..d81cd685d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -165,7 +165,7 @@ jobs:
       - name: Run tests with race detector
         env:
           GOCACHE: ${{ runner.temp }}/go-build-race
-        run: go test -race -count=1 ./...
+        run: go test -race -shuffle=on -count=1 ./...
 
   e2e:
     name: Bats e2e
diff --git a/internal/markdown/bare_links_test.go b/internal/markdown/bare_links_test.go
index ccd22d5e2..fb04d0108 100644
--- a/internal/markdown/bare_links_test.go
+++ b/internal/markdown/bare_links_test.go
@@ -537,7 +537,7 @@ func TestRenderWrapsANonASCIILinkWhole(t *testing.T) {
 // text with a link in every sentence; finding them must not scan to the paragraph's end
 // once for each.
 func TestFindBareLinksInALongUnspacedParagraphIsLinear(t *testing.T) {
-	const links = 20_000
+	const links = 20_000 / linearInputScale
 	for _, sentence := range []string{
 		"「mailto:jane@example.org」と",
 		"「mailto:jane@example.org?subject=資料」と",
@@ -619,8 +619,9 @@ func TestLinkifyURLsPastManyHyperlinksIsLinear(t *testing.T) {
 // A run of text full of prefixes that never become a link is read once, not once per
 // prefix, and a real link after them is still found.
 func TestFindBareLinksPastRejectedPrefixesIsLinear(t *testing.T) {
+	const repeats = 50_000 / linearInputScale
 	for _, prefix := range []string{"https://", "www.", "mailto:", "https://a.b_", "mailto:a@b.co_"} {
-		run := strings.Repeat(prefix, 50_000) + " https://docs.example.com/room_list"
+		run := strings.Repeat(prefix, repeats) + " https://docs.example.com/room_list"
 		found := make(chan []bareLink, 1)
 		go func() {
 			links, _ := findBareLinks(run)
@@ -629,10 +630,10 @@ func TestFindBareLinksPastRejectedPrefixesIsLinear(t *testing.T) {
 		select {
 		case links := <-found:
 			if len(links) == 0 || links[len(links)-1].target != "https://docs.example.com/room_list" {
-				t.Errorf("%q×50000: links end %v, want the room list last", prefix, links[max(0, len(links)-1):])
+				t.Errorf("%q×%d: links end %v, want the room list last", prefix, repeats, links[max(0, len(links)-1):])
 			}
 		case <-time.After(linearBound):
-			t.Fatalf("%q×50000: findBareLinks did not finish within %v", prefix, linearBound)
+			t.Fatalf("%q×%d: findBareLinks did not finish within %v", prefix, repeats, linearBound)
 		}
 	}
 }
diff --git a/internal/markdown/bound_race_test.go b/internal/markdown/bound_race_test.go
index 3321a9d65..ef5814911 100644
--- a/internal/markdown/bound_race_test.go
+++ b/internal/markdown/bound_race_test.go
@@ -7,3 +7,9 @@ import "time"
 // linearBound is longer under the race detector, which runs this code several times
 // slower; a quadratic run is slower by the same factor and still far past it.
 const linearBound = 60 * time.Second
+
+// linearInputScale shrinks the largest linear-time inputs under the race detector. Those
+// tests scan one string on one goroutine, so the detector has nothing to find in them and
+// only multiplies their cost (44s and 13s, against 2.7s and 0.8s without it). The
+// full-size inputs run in every build without -race, where the bound is held.
+const linearInputScale = 10
diff --git a/internal/markdown/bound_test.go b/internal/markdown/bound_test.go
index 45a769ccc..b34ae5e97 100644
--- a/internal/markdown/bound_test.go
+++ b/internal/markdown/bound_test.go
@@ -8,3 +8,7 @@ import "time"
 // quadratic run it guards against: in one pass each takes well under a second, and the
 // quadratic runs took tens of seconds.
 const linearBound = 5 * time.Second
+
+// linearInputScale divides the largest linear-time inputs. Without the race detector
+// they run at full size: this build is where the linear-time guarantee is held.
+const linearInputScale = 1