gdb: add convenience variables around linker namespace debugging

This commit adds 2 simple built-in convenience variables to help users
debug an inferior with multiple linker namespaces. The first is
$_active_linker_namespaces, which just counts how many namespaces have SOs
loaded onto them. The second is $_current_linker_namespace, and it tracks
which namespace the current location in the inferior belongs to.

This commit also introduces a test ensuring that we track namespaces
correctly, and that a user can use the $_current_linker_namespace
variable to set a conditional breakpoint, while linespec changes aren't
finalized to make it more convenient.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-by: Kevin Buettner <kevinb@redhat.com>

gdb/NEWS

@@ -40,6 +40,12 @@
   namespace into which the library was loaded, if more than one namespace
   is active.
 
+* New built-in convenience variables $_active_linker_namespaces and
+  $_current_linker_namespace.  These show the number of active linkage
+  namespaces, and the namespace to which the current location belongs to.
+  In systems that don't support linkage namespaces, these always return 1
+  and [[0]] respectively.
+
  * Add record full support for rv64gc architectures
 
 * New commands

gdb/doc/gdb.texinfo

@@ -13046,6 +13046,18 @@ variable which may be @samp{truecolor} or @samp{24bit}. Other color spaces are
 determined by the "Co" termcap which in turn depends on the @env{TERM}
 environment variable.
 
+@vindex $_active_linker_namespaces@r{, convenience variable}
+@item $_active_linker_namespaces
+Number of active linkage namespaces in the inferior.  In systems with no
+support for linkage namespaces, this variable will always be set to @samp{1}.
+
+@vindex $_current_linker_namespace@r{, convenience variable}
+@item $_current_linker_namespace
+The namespace which contains the current location in the inferior.  This
+returns GDB's internal identifier for namespaces, which is @samp{[[@var{n}]]}
+where @var{n} is a zero-based namespace number.  In systems with no support
+for linkage namespaces, this variable will always be set to @samp{[[0]]}.
+
 @end table
 
 @node Convenience Funs

gdb/solib-svr4.c

@@ -451,6 +451,12 @@ svr4_maybe_add_namespace (svr4_info *info, CORE_ADDR lmid)
     info->namespace_id.push_back (lmid);
 
   info->active_namespaces.insert (i);
+
+  /* Create or update the convenience variable "active_namespaces".
+     It only needs to be updated here, as this only changes when a
+     dlmopen or dlclose call happens.  */
+  set_internalvar_integer (lookup_internalvar ("_active_linker_namespaces"),
+			   info->active_namespaces.size ());
 }
 
 /* Return whether DEBUG_BASE is the default namespace of INFO.  */

gdb/solib.c

@@ -1715,6 +1715,44 @@ default_find_solib_addr (solib &so)
   return {};
 }
 
+/* Implementation of the current_linker_namespace convenience variable.
+   This returns the GDB internal identifier of the linker namespace,
+   for the current frame, in the form '[[<number>]]'.  If the inferior
+   doesn't support linker namespaces, this always returns [[0]].  */
+
+static value *
+current_linker_namespace_make_value (gdbarch *gdbarch, internalvar *var,
+				     void *ignore)
+{
+  const solib_ops *ops = gdbarch_so_ops (gdbarch);
+  const language_defn *lang = language_def (get_frame_language
+					      (get_current_frame ()));
+  std::string nsid = "[[0]]";
+  if (ops->find_solib_ns != nullptr)
+    {
+      CORE_ADDR curr_pc = get_frame_pc (get_current_frame ());
+      for (const solib &so : current_program_space->solibs ())
+	if (solib_contains_address_p (so, curr_pc))
+	  {
+	    nsid = string_printf ("[[%d]]", ops->find_solib_ns (so));
+	    break;
+	  }
+    }
+
+
+  /* If the PC is not in an SO, or the solib_ops doesn't support
+     linker namespaces, the inferior is in the default namespace.  */
+  return lang->value_string (gdbarch, nsid.c_str (), nsid.length ());
+}
+
+/* Implementation of `$_current_linker_namespace' variable.  */
+
+static const struct internalvar_funcs current_linker_namespace_funcs =
+{
+  current_linker_namespace_make_value,
+  nullptr,
+};
+
 void _initialize_solib ();
 
 void
@@ -1727,6 +1765,13 @@ _initialize_solib ()
   },
     "solib");
 
+  /* Convenience variables for debugging linker namespaces.  These are
+     set here, even if the solib_ops doesn't support them,
+     for consistency.  */
+  create_internalvar_type_lazy ("_current_linker_namespace",
+				&current_linker_namespace_funcs, nullptr);
+  set_internalvar_integer (lookup_internalvar ("_active_linker_namespaces"), 1);
+
   add_com (
     "sharedlibrary", class_files, sharedlibrary_command,
     _ ("Load shared object library symbols for files matching REGEXP."));

gdb/testsuite/gdb.base/default.exp

@@ -699,6 +699,8 @@ set show_conv_list \
 	{$_gdb_minor = 1} \
 	{$_shell_exitsignal = void} \
 	{$_shell_exitcode = 0} \
+	{$_active_linker_namespaces = 1} \
+	{$_current_linker_namespace = <error: No registers.>}\
     }
 if [allow_python_tests] {
     append show_conv_list \

gdb/testsuite/gdb.base/dlmopen-ns-ids-main.c

@@ -41,6 +41,12 @@ main (void)
   handle[2] = dlmopen (LM_ID_NEWLM, DSO_NAME, RTLD_LAZY | RTLD_LOCAL);
   assert (handle[2] != NULL);
 
+  for (dl = 2; dl >= 0; dl--)
+    {
+      fun = dlsym (handle[dl], "inc");
+      fun (dl);
+    }
+
   dlclose (handle[0]); /* TAG: first dlclose */
   dlclose (handle[1]); /* TAG: second dlclose */
   dlclose (handle[2]); /* TAG: third dlclose */

gdb/testsuite/gdb.base/dlmopen-ns-ids.exp

@@ -105,4 +105,59 @@ proc test_info_shared {} {
 	"after unloading everything"
 }
 
+# Run all tests related to the linkage namespaces convenience
+# variables, _active_namespaces and _current_namespaces.
+proc_with_prefix test_conv_vars {} {
+    clean_restart $::binfile
+
+    gdb_test "print \$_active_linker_namespaces" "1" \
+	"1 namespace before starting inferior"
+    gdb_test "print \$_current_linker_namespace" "No registers." \
+	"No current namespace before starting inferior"
+
+    if { ![runto_main] } {
+	return
+    }
+
+    gdb_test "print \$_active_linker_namespaces" "1" \
+	"Before activating namespaces"
+    gdb_test "print \$_current_linker_namespace" ".*\"\\\[\\\[0\\\]\\\]\"" \
+	"Still in the default namespace"
+
+    gdb_breakpoint "inc" allow-pending
+    gdb_breakpoint [gdb_get_line_number "TAG: first dlclose"]
+
+    foreach_with_prefix dl {3 2 1} {
+	gdb_continue_to_breakpoint "inc"
+
+	gdb_test "print \$_current_linker_namespace" ".*\"\\\[\\\[$dl\\\]\\\]\"" \
+	    "Verify we're in namespace $dl"
+    }
+
+    gdb_continue_to_breakpoint "first dlclose"
+    gdb_test "print \$_active_linker_namespaces" "4" "all SOs loaded"
+
+    gdb_test "next" ".*second dlclose.*" "close one SO"
+    gdb_test "print \$_active_linker_namespaces" "3" "one SOs unloaded"
+    gdb_test "next" ".*third dlclose.*" "close another SO"
+    gdb_test "print \$_active_linker_namespaces" "2" "two SOs unloaded"
+
+    # Restarting GDB so that we can test setting a breakpoint
+    # using the convenience variable, while a proper bp syntax
+    # isn't implemented for namespaces
+    clean_restart $::binfile
+    if {![runto_main]} {
+	return
+    }
+
+    # We need to load one SO because you can't have confitional
+    # breakpoints and pending breakpoints at the same time with
+    # gdb_breakpoint.
+    gdb_test "next" ".*assert.*" "load the first SO"
+    gdb_breakpoint "inc if \$_streq(\$_current_linker_namespace, \"\[\[2\]\]\")"
+    gdb_continue_to_breakpoint "inc"
+    gdb_continue_to_end "" continue 1
+}
+
 test_info_shared
+test_conv_vars