Do not auto-dereference null pointers in Ada MI varobj

The Ada varobj code automatically dereferences access types.  This is
often handy, but it also does so for null pointers -- showing children
with empty values.

These children are weird, but even weirder when a variant type is
involved, because only the non-varying parts of the type are
displayed.  This behavior conflicts a bit with my ongoing quest to
move the Ada code to use DWARF rather than gnat encodings, in that
reproducing this behavior with the DWARF code seems rather hacky.

So, this patch instead changes the Ada varobj code so that it does not
automatically dereference null pointers.

As this patch only affects Ada, and it was already reviewed internally
by Joel, I am checking it in.

2020-09-02  Tom Tromey  <tromey@adacore.com>

	* ada-varobj.c (ada_varobj_get_ptr_number_of_children): Return 0
	for null pointers.
	(ada_varobj_adjust_for_child_access): Special-case null pointers.

gdb/testsuite/ChangeLog
2020-09-02  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mi_var_access.exp: Test children of access variable.
	* gdb.ada/mi_var_access/mi_access.adb: Add new stop markers.
	* gdb.ada/mi_var_array.exp: Update.

6 files changed, 43 insertions, 11 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ab003af..8ace83b 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2020-09-02  Tom Tromey  <tromey@adacore.com>
+
+	* ada-varobj.c (ada_varobj_get_ptr_number_of_children): Return 0
+	for null pointers.
+	(ada_varobj_adjust_for_child_access): Special-case null pointers.
+
 2020-09-01  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* bcache.h (struct bcache) <insert>: Change type of `added` to
diff --git a/gdb/ada-varobj.c b/gdb/ada-varobj.c
index a72653a..f2a291a 100644
--- a/gdb/ada-varobj.c
+++ b/gdb/ada-varobj.c
@@ -209,6 +209,8 @@ ada_varobj_adjust_for_child_access (struct value **value,
   if ((*type)->code () == TYPE_CODE_PTR
       && (TYPE_TARGET_TYPE (*type)->code () == TYPE_CODE_STRUCT
           || TYPE_TARGET_TYPE (*type)->code () == TYPE_CODE_UNION)
+      && *value != nullptr
+      && value_as_address (*value) != 0
       && !ada_is_array_descriptor_type (TYPE_TARGET_TYPE (*type))
       && !ada_is_constrained_packed_array_type (TYPE_TARGET_TYPE (*type)))
     ada_varobj_ind (*value, *type, value, type);
@@ -333,6 +335,10 @@ ada_varobj_get_ptr_number_of_children (struct value *parent_value,
       || child_type->code () == TYPE_CODE_VOID)
     return 0;
 
+  /* Only show children for non-null pointers.  */
+  if (parent_value == nullptr || value_as_address (parent_value) == 0)
+    return 0;
+
   /* All other types have 1 child.  */
   return 1;
 }
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 7e60a11..63dcad7 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2020-09-02  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.ada/mi_var_access.exp: Test children of access variable.
+	* gdb.ada/mi_var_access/mi_access.adb: Add new stop markers.
+	* gdb.ada/mi_var_array.exp: Update.
+
 2020-08-31  Kevin Buettner  <kevinb@redhat.com>
 
 	* gdb.base/corefile.exp (warning-free): XFAIL test when running
diff --git a/gdb/testsuite/gdb.ada/mi_var_access.exp b/gdb/testsuite/gdb.ada/mi_var_access.exp
index 5ffc0ae..14e8992 100644
--- a/gdb/testsuite/gdb.ada/mi_var_access.exp
+++ b/gdb/testsuite/gdb.ada/mi_var_access.exp
@@ -50,3 +50,25 @@ mi_continue_to_line \
 mi_gdb_test "-var-create A_String_Access * A_String_Access" \
     "\\^done,name=\"A_String_Access\",numchild=\"1\",.*" \
     "Create varobj"
+
+set bp_location [gdb_get_line_number "STOP2" ${testdir}/mi_access.adb]
+mi_continue_to_line \
+    "mi_access.adb:$bp_location" \
+    "stop at stop 2"
+
+mi_gdb_test "-var-update A_String_Access" \
+    [string_to_regexp {^done,changelist=[{name="A_String_Access",in_scope="true",type_changed="false",has_more="0"}]}] \
+    "update at stop 2"
+
+mi_gdb_test "-var-list-children A_String_Access" \
+    [string_to_regexp {^done,numchild="1",children=[child={name="A_String_Access.A_String_Access.all",exp="A_String_Access.all",numchild="3",type="array (3 .. 5) of character",thread-id="1"}],has_more="0"}] \
+    "list children at stop 2"
+
+set bp_location [gdb_get_line_number "STOP3" ${testdir}/mi_access.adb]
+mi_continue_to_line \
+    "mi_access.adb:$bp_location" \
+    "stop at stop 3"
+
+mi_gdb_test "-var-update A_String_Access" \
+    [string_to_regexp {^done,changelist=[{name="A_String_Access",in_scope="true",type_changed="true",new_type="pck.string_access",new_num_children="0",has_more="0"}]}] \
+    "update at stop 3"
diff --git a/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb b/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb
index d5f5f88..b1dc7be 100644
--- a/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb
+++ b/gdb/testsuite/gdb.ada/mi_var_access/mi_access.adb
@@ -24,7 +24,7 @@ begin
    A_String (4) := '6';
    A_String_Access := Copy (A_String);
    A_Pointer.P := A_String_Access;
-   Do_Nothing (A_String_Access'Address);
+   Do_Nothing (A_String_Access'Address); -- STOP2
    A_String_Access (4) := 'a';
    Do_Nothing (A_Pointer'Address);
    A_String_Access := Copy("Hi");
@@ -32,6 +32,6 @@ begin
    Do_Nothing (A_String_Access'Address);
    A_String_Access := null;
    A_Pointer.P := null;
-   Do_Nothing (A_Pointer'Address);
+   Do_Nothing (A_Pointer'Address); -- STOP3
    Do_Nothing (A_String'Address);
 end Mi_Access;
diff --git a/gdb/testsuite/gdb.ada/mi_var_array.exp b/gdb/testsuite/gdb.ada/mi_var_array.exp
index fdc2249..4ef213b 100644
--- a/gdb/testsuite/gdb.ada/mi_var_array.exp
+++ b/gdb/testsuite/gdb.ada/mi_var_array.exp
@@ -52,14 +52,6 @@ foreach_with_prefix scenario {none all minimal} {
 	"stop at start of main Ada procedure"
 
     mi_gdb_test "-var-create vta * vta" \
-	"\\^done,name=\"vta\",numchild=\"2\",.*" \
+	"\\^done,name=\"vta\",numchild=\"0\",.*" \
 	"create bt varobj"
-
-    # In the "minimal" mode, we don't currently have the ability to
-    # print the subrange type properly.  So, we just allow anything
-    # for the array range here.  The correct result would be to fix
-    # this to read "(1 .. n)".
-    mi_gdb_test "-var-list-children vta" \
-	"\\^done,numchild=\"2\",children=\\\[child={name=\"vta.n\",exp=\"n\",numchild=\"0\",type=\"bar\\.int\",thread-id=\"$decimal\"},child={name=\"vta.f\",exp=\"f\",numchild=\"0\",type=\"array .* of character\",thread-id=\"$decimal\"}\\\],.*" \
-	"list vta's children"
 }