fix: guard Debug.proc against concurrent CPU-percent reads

server/handlers/api_debug.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"runtime"
 	"runtime/debug"
+	"sync"
 	"time"
 
 	"github.com/shirou/gopsutil/v4/process"
@@ -15,6 +16,11 @@ type Debug struct {
 	version string
 	started time.Time
 	proc    *process.Process
+	// mu guards proc: Percent tracks the previous CPU-time reading on the
+	// *process.Process itself with no internal locking, so concurrent calls
+	// (e.g. multiple browser tabs each polling /api/debug on a timer) race
+	// on that state and can report a corrupted, wildly inflated cpu_percent.
+	mu sync.Mutex
 }
 
 func NewDebug(version string, started time.Time) *Debug {
@@ -35,12 +41,14 @@ func (h *Debug) Get(w http.ResponseWriter, _ *http.Request) {
 	cpuPct := 0.0
 	var rss uint64
 	if h.proc != nil {
+		h.mu.Lock()
 		if v, err := h.proc.Percent(0); err == nil {
 			cpuPct = v
 		}
 		if mi, err := h.proc.MemoryInfo(); err == nil && mi != nil {
 			rss = mi.RSS
 		}
+		h.mu.Unlock()
 	}
 
 	lastGC := ""

server/handlers/api_debug_test.go

@@ -0,0 +1,52 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http/httptest"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestDebugConcurrentAccess guards against a data race in gopsutil's
+// Process.Percent: it tracks the previous CPU-time reading on the *Process
+// itself with no internal locking, so unsynchronized concurrent calls (e.g.
+// several browser tabs each polling /api/debug on its 2s timer) race on that
+// state. Run with `go test -race` to make this test meaningful — without
+// the mutex in Debug, the race detector flags the read/write in
+// gopsutil's PercentWithContext; a plain (non-race) run can't detect it,
+// since a torn read doesn't reliably crash or produce a value outside any
+// fixed bound (rapid back-to-back calls to Percent(0) legitimately produce
+// large, noisy percentages on their own, since the measurement window
+// between consecutive calls shrinks — that's expected behavior of an
+// instantaneous-rate measurement, not corruption).
+func TestDebugConcurrentAccess(t *testing.T) {
+	dbg := NewDebug("test", time.Now())
+
+	var wg sync.WaitGroup
+	for range 20 {
+		wg.Go(func() {
+			for range 20 {
+				w := httptest.NewRecorder()
+				r := httptest.NewRequest("GET", "/api/debug", nil)
+				dbg.Get(w, r)
+
+				var body struct {
+					Data struct {
+						Process struct {
+							CPUPercent float64 `json:"cpu_percent"`
+						} `json:"process"`
+					} `json:"data"`
+				}
+				if err := json.Unmarshal(w.Body.Bytes(), &body); err != nil {
+					t.Errorf("invalid JSON response: %v", err)
+					return
+				}
+				if body.Data.Process.CPUPercent < 0 {
+					t.Errorf("negative cpu_percent: %v", body.Data.Process.CPUPercent)
+				}
+			}
+		})
+	}
+	wg.Wait()
+}