Reserve toolsrv process slots atomically

cmd/toolsrv/internal/server/proc.go

@@ -38,10 +38,11 @@ type State struct {
 	pathLocks pathLockTable
 
 	// Process management
-	procMu    sync.Mutex
-	procs     map[int]*Proc
-	nextID    int
-	procLimit int
+	procMu       sync.Mutex
+	procs        map[int]*Proc
+	nextID       int
+	procLimit    int
+	procReserved int
 
 	// Callbacks
 	OnToolsChanged func()
@@ -223,13 +224,23 @@ func (st *State) NewProc(ctx context.Context, payload string, background bool) (
 		return "", 0, fmt.Errorf("missing 'tool' in payload")
 	}
 
-	// Check proc limit
+	// Reserve a process slot before setup so concurrent callers cannot all pass
+	// the limit check while their processes are still being initialized.
 	st.procMu.Lock()
-	if len(st.procs) >= st.procLimit {
+	if len(st.procs)+st.procReserved >= st.procLimit {
 		st.procMu.Unlock()
 		return "", 0, fmt.Errorf("process limit reached (%d)", st.procLimit)
 	}
+	st.procReserved++
 	st.procMu.Unlock()
+	reserved := true
+	defer func() {
+		if reserved {
+			st.procMu.Lock()
+			st.procReserved--
+			st.procMu.Unlock()
+		}
+	}()
 
 	// Look up tool
 	st.mu.RLock()
@@ -289,7 +300,9 @@ func (st *State) NewProc(ctx context.Context, payload string, background bool) (
 
 	st.procMu.Lock()
 	st.procs[proc.ID] = proc
+	st.procReserved--
 	st.procMu.Unlock()
+	reserved = false
 
 	// For background procs, stream output directly into the proc buffer.
 	// For foreground, use nil (local buffer in exec path).