The upcoming follow-fork-mode child support needs to perform certain
actions when GDB switches between the stopped parent and the stopped
child. Introduce a user-specific hook for this.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 gdbstub/gdbstub.c   | 11 +++++++++--
 gdbstub/internals.h |  1 +
 gdbstub/user.c      |  5 +++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/gdbstub/gdbstub.c b/gdbstub/gdbstub.c
index 43d79dfdd59..adcd977cd57 100644
--- a/gdbstub/gdbstub.c
+++ b/gdbstub/gdbstub.c
@@ -1066,6 +1066,7 @@ static void handle_cont_with_sig(GArray *params, void *user_ctx)
 
 static void handle_set_thread(GArray *params, void *user_ctx)
 {
+    uint32_t pid, tid;
     CPUState *cpu;
 
     if (params->len != 2) {
@@ -1083,8 +1084,14 @@ static void handle_set_thread(GArray *params, void *user_ctx)
         return;
     }
 
-    cpu = gdb_get_cpu(get_param(params, 1)->thread_id.pid,
-                      get_param(params, 1)->thread_id.tid);
+    pid = get_param(params, 1)->thread_id.pid;
+    tid = get_param(params, 1)->thread_id.tid;
+#ifdef CONFIG_USER_ONLY
+    if (gdb_handle_set_thread_user(pid, tid)) {
+        return;
+    }
+#endif
+    cpu = gdb_get_cpu(pid, tid);
     if (!cpu) {
         gdb_put_packet("E22");
         return;
diff --git a/gdbstub/internals.h b/gdbstub/internals.h
index e6063835b1f..b4905c7181a 100644
--- a/gdbstub/internals.h
+++ b/gdbstub/internals.h
@@ -197,6 +197,7 @@ void gdb_handle_v_file_readlink(GArray *params, void *user_ctx); /* user */
 void gdb_handle_query_xfer_exec_file(GArray *params, void *user_ctx); /* user */
 void gdb_handle_set_catch_syscalls(GArray *params, void *user_ctx); /* user */
 void gdb_handle_query_supported_user(const char *gdb_supported); /* user */
+bool gdb_handle_set_thread_user(uint32_t pid, uint32_t tid); /* user */
 
 void gdb_handle_query_attached(GArray *params, void *user_ctx); /* both */
 
diff --git a/gdbstub/user.c b/gdbstub/user.c
index 2b8c67972c0..ee6b47b9b9c 100644
--- a/gdbstub/user.c
+++ b/gdbstub/user.c
@@ -386,6 +386,11 @@ void gdb_handle_query_supported_user(const char *gdb_supported)
 {
 }
 
+bool gdb_handle_set_thread_user(uint32_t pid, uint32_t tid)
+{
+    return false;
+}
+
 /*
  * Execution state helpers
  */
-- 
2.43.0