fix: atomic burn-after-read claim (#58)

SoftDelete now reports whether it performed the delete (conditional
UPDATE ... WHERE deleted_at IS NULL checked via RowsAffected).
RegisterRead returns an admitted flag: legacy burn pastes admit exactly
one reader (the atomic soft-delete winner), and burn-after-N pastes
increment reads_used via a conditional UPDATE guarded on
reads_used < reads_limit, so concurrent readers cannot both consume the
final read. API, HTML, and raw read paths return 404 when the reader
loses the burn claim; content is never served twice.

OpenStore pins the SQLite pool to one connection: concurrent writes on
separate pooled connections surfaced SQLITE_BUSY as spurious 500s
instead of serializing.

Adds concurrency regression tests: 24 parallel readers of a burn paste
(exactly one receives content, none of the others leak it) and 30
parallel readers vs a 3-read budget (exactly 3 admitted, then 404).

internal/api/burnrace_test.go

@@ -0,0 +1,103 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// TestBurnAfterReadConcurrentRace is the #58 regression test: N concurrent
+// readers of a burn-after-read paste must receive exactly one success with
+// content; every other reader must get 404 and never any content.
+func TestBurnAfterReadConcurrentRace(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	id := createBurnReads(t, h, 1)
+
+	const readers = 24
+	var wg sync.WaitGroup
+	var mu sync.Mutex
+	wins, losses := 0, 0
+	for i := 0; i < readers; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			req := httptest.NewRequest("GET", "/api/pastes/"+id, nil)
+			req.AddCookie(&http.Cookie{Name: "vwr", Value: fmt.Sprintf("racer-%d", i)})
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, req)
+			mu.Lock()
+			defer mu.Unlock()
+			if rec.Code == 200 {
+				wins++
+				var got struct {
+					Content string `json:"content"`
+				}
+				if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil || got.Content != "limited" {
+					t.Errorf("winning read returned wrong content: %v %q", err, got.Content)
+				}
+			} else if rec.Code == 404 {
+				losses++
+				if strings.Contains(rec.Body.String(), "limited") {
+					t.Errorf("losing read leaked content: %s", rec.Body.String())
+				}
+			} else {
+				t.Errorf("unexpected status %d: %s", rec.Code, rec.Body.String())
+			}
+		}(i)
+	}
+	wg.Wait()
+
+	if wins != 1 {
+		t.Fatalf("expected exactly 1 winning read of burn paste, got %d (losses=%d)", wins, losses)
+	}
+	if losses != readers-1 {
+		t.Fatalf("expected %d losing reads, got %d", readers-1, losses)
+	}
+}
+
+// TestBurnAfterNReadsConcurrentBudget hammers a burn-after-N paste with many
+// more concurrent distinct readers than the budget: total admissions must
+// equal exactly N, and no losing read may see content.
+func TestBurnAfterNReadsConcurrentBudget(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	const budget = 3
+	id := createBurnReads(t, h, budget)
+
+	const readers = 30
+	var wg sync.WaitGroup
+	var mu sync.Mutex
+	wins := 0
+	for i := 0; i < readers; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			req := httptest.NewRequest("GET", "/api/pastes/"+id, nil)
+			req.AddCookie(&http.Cookie{Name: "vwr", Value: fmt.Sprintf("racer-%d", i)})
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, req)
+			mu.Lock()
+			defer mu.Unlock()
+			if rec.Code == 200 {
+				wins++
+			} else if rec.Code != 404 {
+				t.Errorf("unexpected status %d: %s", rec.Code, rec.Body.String())
+			}
+		}(i)
+	}
+	wg.Wait()
+
+	if wins != budget {
+		t.Fatalf("expected exactly %d admitted reads, got %d", budget, wins)
+	}
+
+	// After the race, the paste is burned for everyone.
+	if rec := getWithCookie(t, h, id, "after-the-fact"); rec.Code != 404 {
+		t.Fatalf("paste should be burned after budget exhausted, got %d", rec.Code)
+	}
+}

internal/api/server.go

@@ -29,11 +29,11 @@ type Config struct {
 }
 
 type apiServer struct {
-	store       *store.Store
-	cfg         Config
-	ui          *web.UI
-	settings    *settingsStore
-	adminKey    string
+	store    *store.Store
+	cfg      Config
+	ui       *web.UI
+	settings *settingsStore
+	adminKey string
 }
 
 func NewServer(st *store.Store, cfg Config, ui *web.UI, ss *settingsStore, adminKey string) *apiServer {
@@ -216,7 +216,12 @@ func (a *apiServer) handleGetPaste(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	}
-	rem, _ := a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow()) // #49 (also covers legacy burn)
+	// #58: only the reader that wins the atomic burn claim may see content.
+	rem, admitted := a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow())
+	if !admitted {
+		writeErr(w, 404, "paste not found")
+		return
+	}
 	writeJSON(w, 200, map[string]any{
 		"id": row.ID, "content": row.Content, "content_type": row.ContentType,
 		"language": store.NullStrPtr(row.Language), "title": store.NullStrPtr(row.Title), "created_at": row.CreatedAt,
@@ -240,7 +245,7 @@ func (a *apiServer) handleDeletePaste(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, 403, "not your paste")
 		return
 	}
-	if err := a.store.SoftDelete(row.ID); err != nil {
+	if _, err := a.store.SoftDelete(row.ID); err != nil {
 		writeErr(w, 500, "db error")
 		return
 	}
@@ -319,8 +324,13 @@ func (a *apiServer) handleRaw(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	// #49 decision: raw reads count against the read budget too, with the
-	// same per-viewer dedupe window as page views.
-	a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow())
+	// same per-viewer dedupe window as page views. #58: a reader that loses
+	// the burn claim must not receive the content.
+	_, admitted := a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow())
+	if !admitted {
+		http.Error(w, "not found", 404)
+		return
+	}
 	// #34: content_type is attacker-controlled via the create API. Serving it
 	// verbatim let a paste be stored with text/html (or image/svg+xml) and
 	// render as active content on this origin when fetched from /raw —