From cdfe4378310c567f7562bbe0aa395d7a8fdf81d8 Mon Sep 17 00:00:00 2001
From: kidfromjupiter <limahaliyana@gmail.com>
Date: Fri, 6 Feb 2026 12:28:58 +0000
Subject: [PATCH] l2cap socket psm is not generated once per BleV2Medium
 initialisation. Accept() waits till all other l2cap connections are closed to
 create a new connection

---
 .../linux/ble_l2cap_server_socket.cc          | 23 +++++++++++--------
 .../linux/ble_l2cap_server_socket.h           |  3 +--
 .../implementation/linux/ble_v2_medium.cc     |  9 +++++---
 .../implementation/linux/ble_v2_medium.h      |  2 ++
 4 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/internal/platform/implementation/linux/ble_l2cap_server_socket.cc b/internal/platform/implementation/linux/ble_l2cap_server_socket.cc
index 3c03ff3d..3df1e631 100644
--- a/internal/platform/implementation/linux/ble_l2cap_server_socket.cc
+++ b/internal/platform/implementation/linux/ble_l2cap_server_socket.cc
@@ -72,12 +72,13 @@ void BleL2capServerSocket::AcceptPoll(int& client_fd, sockaddr_l2& client_addr,
     }
 
     // Stop requested
-if (fds[1].revents & POLLIN) {
-  DrainFd(stop_pipe_[0]);
-  client_fd = -1;
-  errno = EINTR;              // optional: helps caller treat as "interrupted"
-  return;                     // <-- THIS is the key
-}
+    if (fds[1].revents & POLLIN) {
+      absl::MutexLock l(&mutex_);
+      DrainFd(stop_pipe_[0]);
+      client_fd = -1;
+      errno = EINTR;              // optional: helps caller treat as "interrupted"
+      return;                     // <-- THIS is the key
+    }
 
     // Listen socket error/hangup
     if (fds[0].revents & (POLLERR | POLLHUP | POLLNVAL)) {
@@ -106,10 +107,13 @@ if (fds[1].revents & POLLIN) {
   }
 }
 std::unique_ptr<api::ble::BleL2capSocket> BleL2capServerSocket::Accept() {
-    Prng prng;
-    psm_ = 0x80 + (prng.NextUint32() % 0x80);
+  // blocking accept until the previous server_fd_ is closed.
+  // We need to keep accepting connections on the same psm. That's why we do this
+  absl::MutexLock lock(&mutex_);
+  auto idle = [this] {return server_fd_ == -1;};
+  mutex_.Await(absl::Condition(&idle));
 
-    server_fd_ = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);
+  server_fd_ = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);
     if (server_fd_ < 0) {
       LOG(ERROR) << "Failed to create L2CAP server socket: "
                  << std::strerror(errno);
@@ -222,5 +226,6 @@ Exception BleL2capServerSocket::Close() {
 }
 
 
+
 }  // namespace linux
 }  // namespace nearby
diff --git a/internal/platform/implementation/linux/ble_l2cap_server_socket.h b/internal/platform/implementation/linux/ble_l2cap_server_socket.h
index e77203c5..bee205ce 100644
--- a/internal/platform/implementation/linux/ble_l2cap_server_socket.h
+++ b/internal/platform/implementation/linux/ble_l2cap_server_socket.h
@@ -48,8 +48,7 @@ class BleL2capServerSocket final : public api::ble::BleL2capServerSocket {
   void SetCloseNotifier(absl::AnyInvocable<void()> notifier);
 
  private:
-  mutable absl::Mutex mutex_;
-  absl::CondVar cond_;
+  absl::Mutex mutex_;
   int psm_ = 0;
   int server_fd_ = -1;
   int stop_pipe_[2] = {-1, -1};   // read end [0], write end [1]
diff --git a/internal/platform/implementation/linux/ble_v2_medium.cc b/internal/platform/implementation/linux/ble_v2_medium.cc
index 7d4ff3cf..e03c1740 100644
--- a/internal/platform/implementation/linux/ble_v2_medium.cc
+++ b/internal/platform/implementation/linux/ble_v2_medium.cc
@@ -65,6 +65,11 @@ BleV2Medium::BleV2Medium(BluetoothAdapter &adapter)
     adv_manager_(std::make_unique<bluez::LEAdvertisementManager>(*system_bus_,
                                                                  adapter)),
     cur_adv_(nullptr) {
+
+  // generating psm value for l2cap socket
+  Prng prng;
+  psm_ = 0x80 + (prng.NextUint32() % 0x80);
+
   if (adv_monitor_manager_) {
     LOG(INFO)
         << __func__
@@ -464,9 +469,7 @@ BleV2Medium::OpenL2capServerSocket(const std::string &service_id) {
   LOG(INFO) << __func__ << ": Opening L2CAP server socket for service "
             << service_id;
 
-  Prng prng;
-  auto psm = 0x80 + (prng.NextUint32() % 0x80);
-  auto server_socket = std::make_unique<linux::BleL2capServerSocket>(psm);
+  auto server_socket = std::make_unique<linux::BleL2capServerSocket>(psm_);
 
   return server_socket;
 }
diff --git a/internal/platform/implementation/linux/ble_v2_medium.h b/internal/platform/implementation/linux/ble_v2_medium.h
index b3b997e3..e9c3baea 100644
--- a/internal/platform/implementation/linux/ble_v2_medium.h
+++ b/internal/platform/implementation/linux/ble_v2_medium.h
@@ -158,6 +158,8 @@ class BleV2Medium final : public api::ble::BleMedium {
 
   std::unique_ptr<bluez::LEAdvertisementManager> adv_manager_;
 
+    int psm_;
+
   absl::Mutex cur_adv_mutex_;
   std::unique_ptr<bluez::LEAdvertisement> cur_adv_
       ABSL_GUARDED_BY(cur_adv_mutex_);