[PATCH] Avoid crash in dwarf2/read.c:determine_prefix

[Tom Tromey <tromey@adacore.com>]

I found a gdb crash when using some changes to gnat-llvm to have it
emit unqualified names in the DWARF.  The crash happens because
determine_prefix does this:

	  return dwarf2_full_name (nullptr, parent, cu);

However, dwarf2_full_name can return NULL, causing a crash in the
caller.

The particular DWARF causing this is pretty strange -- it is a
function nested inside another nameless function.  This may be a bug
in gnat-llvm, something I plan to investigate.

Meanwhile, gdb shouldn't crash.  This patch changes determine_prefix
to avoid possible crashes here, by following its contract and not
returning NULL.

I'm not sure if it's worthwhile to write a test case for this.
---
 gdb/dwarf2/read.c | 39 ++++++++++++++++++++++-----------------
 1 file changed, 22 insertions(+), 17 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index bc7b8b46d87..40271cea592 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -16628,23 +16628,28 @@ determine_prefix (struct die_info *die, struct dwarf2_cu *cu)
 	  }
 	return "";
       case DW_TAG_subprogram:
-	/* Nested subroutines in Fortran get a prefix with the name
-	   of the parent's subroutine.  Entry points are prefixed by the
-	   parent's namespace.  */
-	if (cu->lang () == language_fortran)
-	  {
-	    if ((die->tag ==  DW_TAG_subprogram)
-		&& (dwarf2_name (parent, cu) != NULL))
-	      return dwarf2_name (parent, cu);
-	    else if (die->tag == DW_TAG_entry_point)
-	      return determine_prefix (parent, cu);
-	  }
-	else if (cu->lang () == language_ada
-		 && (die->tag == DW_TAG_subprogram
-		     || die->tag == DW_TAG_inlined_subroutine
-		     || die->tag == DW_TAG_lexical_block))
-	  return dwarf2_full_name (nullptr, parent, cu);
-	return "";
+	{
+	  const char *name = nullptr;
+	  /* Nested subroutines in Fortran get a prefix with the name
+	     of the parent's subroutine.  Entry points are prefixed by the
+	     parent's namespace.  */
+	  if (cu->lang () == language_fortran)
+	    {
+	      if ((die->tag ==  DW_TAG_subprogram)
+		  && (dwarf2_name (parent, cu) != NULL))
+		name = dwarf2_name (parent, cu);
+	      else if (die->tag == DW_TAG_entry_point)
+		name = determine_prefix (parent, cu);
+	    }
+	  else if (cu->lang () == language_ada
+		   && (die->tag == DW_TAG_subprogram
+		       || die->tag == DW_TAG_inlined_subroutine
+		       || die->tag == DW_TAG_lexical_block))
+	    name = dwarf2_full_name (nullptr, parent, cu);
+	  if (name == nullptr)
+	    name = "";
+	  return name;
+	}
       case DW_TAG_enumeration_type:
 	parent_type = read_type_die (parent, cu);
 	if (parent_type->is_declared_class ())

base-commit: f797b25fdc7ca4a48c09802082426b71e56898aa
-- 
2.53.0