restore back pool logic

ffi/ffi_context.nim

@@ -371,6 +371,31 @@ proc initContextResources*[T](ctx: ptr FFIContext[T]): Result[void, string] =
   success = true
   return ok()
 
+const MaxFFIContexts* = 32
+  ## Maximum number of concurrently live FFI contexts when using FFIContextPool.
+  ## Fds and threads are only consumed for slots that are actually acquired,
+  ## so this value only affects the upfront memory of the pool array.
+
+type FFIContextPool*[T] = object
+  ## Fixed-size pool of FFI contexts. Avoids dynamic heap allocation per context
+  ## and bounds the total number of file descriptors consumed by ThreadSignalPtrs
+  ## to at most MaxFFIContexts * 2.
+  slots: array[MaxFFIContexts, FFIContext[T]]
+  inUse: array[MaxFFIContexts, Atomic[bool]]
+
+proc acquireSlot[T](pool: var FFIContextPool[T]): Result[ptr FFIContext[T], string] =
+  for i in 0 ..< MaxFFIContexts:
+    var expected = false
+    if pool.inUse[i].compareExchange(expected, true):
+      return ok(pool.slots[i].addr)
+  return err("FFI context pool exhausted (max " & $MaxFFIContexts & " contexts)")
+
+proc releaseSlot[T](pool: var FFIContextPool[T], ctx: ptr FFIContext[T]) =
+  for i in 0 ..< MaxFFIContexts:
+    if pool.slots[i].addr == ctx:
+      pool.inUse[i].store(false)
+      return
+
 # ── Public API ────────────────────────────────────────────────────────────────
 
 proc createFFIContext*[T](): Result[ptr FFIContext[T], string] =
@@ -382,6 +407,18 @@ proc createFFIContext*[T](): Result[ptr FFIContext[T], string] =
     return err(error)
   return ok(ctx)
 
+proc createFFIContext*[T](
+    pool: var FFIContextPool[T]
+): Result[ptr FFIContext[T], string] =
+  ## Acquires a slot from the fixed pool and initialises it as an FFI context.
+  ## Bounded fd usage: at most MaxFFIContexts * 2 ThreadSignalPtr fds are ever open.
+  let ctx = pool.acquireSlot().valueOr:
+    return err(error)
+  initContextResources(ctx).isOkOr:
+    pool.releaseSlot(ctx)
+    return err(error)
+  return ok(ctx)
+
 proc signalStop*[T](ctx: ptr FFIContext[T]): Result[void, string] =
   ctx.running.store(false)
   let ffiSignaled = ctx.reqSignal.fireSync().valueOr: