[PATCH] gdb/riscv: delete target descriptions when gdb exits

[Andrew Burgess]

It was pointed out on IRC that the RISC-V target allocates target
descriptions and stores them in a global map, and doesn't delete these
target descriptions when GDB shuts down.

This isn't a particular problem, the total number of target
descriptions we can create is very limited so creating these on demand
and holding them for the entire run on GDB seems reasonable.

However, not deleting these objects on GDB exit means extra warnings
are printed from tools like valgrind, and the address sanitiser,
making it harder to spot real issues.  As it's reasonably easy to have
GDB correctly delete these objects on exit, lets just do that.

I started by noticing that we already have a target_desc_up type, a
wrapper around unique_ptr that calls a function that will correctly
delete target descriptions, so I want to use that, but....

...that type is declared in gdb/target-descriptions.h.  If I try to
include that file in gdb/arch/riscv.c I run into a problem, that file
is compiled into both GDB and GDBServer.

OK, I could guard the include with #ifdef, but surely we can do
better.

So then I decided to move the target_desc_up type into
gdbsupport/tdesc.h, this is the interface file for generic code shared
between GDB and GDBserver (relating to target descriptions).  The
actual implementation for the delete function still lives in
gdb/target-description.c, but now gdb/arch/riscv.c can see the
declaration.  Problem solved....

... but, though RISC-V doesn't use it I've now exposed the
target_desc_up type to gdbserver, so in future someone _might_ start
using it, which is fine, except right now there's no definition of the
delete function - remember the delete I used is only defined in GDB
code.

No problem, I add an implementation of the delete operator into
gdbserver/tdesc.cc, and all is good..... except....

I start getting this error from GCC:

  tdesc.cc:109:10: error: deleting object of polymorphic class type ‘target_desc’ which has non-virtual destructor might cause undefined behavior [-Werror=delete-non-virtual-dtor]

This seems a little strange to me as target_desc is not a polymorphic
class as far as I can tell, so I don't quite understand why GCC is
complaining about this.

Still by making gdbserver's target_desc class final this seems to
resolve this issue.  This should be fine so long as we never try to
inherit from target_desc, but if we do we probably should make the
destructor virtual, so that wouldn't be an issue.

gdb/ChangeLog:

	* arch/riscv.c (riscv_tdesc_cache): Change map type.
	(riscv_lookup_target_description): Return pointer out of
	unique_ptr.
	* target-descriptions.c (allocate_target_description): Add
	comment.
	(target_desc_deleter::operator()): Likewise.
	* target-descriptions.h (struct target_desc_deleter): Moved to
	gdbsupport/tdesc.h.
	(target_desc_up): Likewise.

gdbserver/ChangeLog:

	* tdesc.cc (allocate_target_description): Add header comment.
	(target_desc_deleter::operator()): New function.
	* tdesc.h (struct target_desc): Declare as final.

gdbsupport/ChangeLog:

	* tdesc.h (struct target_desc_deleter): Moved here
	from gdb/target-descriptions.h.
	(target_desc_up): Likewise.
---
 gdb/ChangeLog             | 12 ++++++++++++
 gdb/arch/riscv.c          |  8 +++++---
 gdb/target-descriptions.c |  4 ++++
 gdb/target-descriptions.h | 12 ------------
 gdbserver/ChangeLog       |  6 ++++++
 gdbserver/tdesc.cc        | 10 ++++++++++
 gdbserver/tdesc.h         |  2 +-
 gdbsupport/ChangeLog      |  6 ++++++
 gdbsupport/tdesc.h        | 11 +++++++++++
 9 files changed, 55 insertions(+), 16 deletions(-)

diff --git a/gdb/arch/riscv.c b/gdb/arch/riscv.c
index a02c18b4c91..e43aafc1c2f 100644
--- a/gdb/arch/riscv.c
+++ b/gdb/arch/riscv.c
@@ -91,7 +91,7 @@ struct riscv_gdbarch_features_hasher
 /* Cache of previously seen target descriptions, indexed by the feature set
    that created them.  */
 static std::unordered_map<riscv_gdbarch_features,
-			  const target_desc *,
+			  const target_desc_up,
 			  riscv_gdbarch_features_hasher> riscv_tdesc_cache;
 
 /* See arch/riscv.h.  */
@@ -99,10 +99,12 @@ static std::unordered_map<riscv_gdbarch_features,
 const target_desc *
 riscv_lookup_target_description (const struct riscv_gdbarch_features features)
 {
-  /* Lookup in the cache.  */
+  /* Lookup in the cache.  If we find it then return the pointer out of
+     the target_desc_up (which is a unique_ptr).  This is safe as the
+     riscv_tdesc_cache will exist until GDB exits.  */
   const auto it = riscv_tdesc_cache.find (features);
   if (it != riscv_tdesc_cache.end ())
-    return it->second;
+    return it->second.get ();
 
   target_desc *tdesc = riscv_create_target_description (features);
 
diff --git a/gdb/target-descriptions.c b/gdb/target-descriptions.c
index 2f4b17727b8..20d624c0c65 100644
--- a/gdb/target-descriptions.c
+++ b/gdb/target-descriptions.c
@@ -1206,12 +1206,16 @@ tdesc_create_feature (struct target_desc *tdesc, const char *name)
   return new_feature;
 }
 
+/* See gdbsupport/tdesc.h.  */
+
 struct target_desc *
 allocate_target_description (void)
 {
   return new target_desc ();
 }
 
+/* See gdbsupport/tdesc.h.  */
+
 void
 target_desc_deleter::operator() (struct target_desc *target_desc) const
 {
diff --git a/gdb/target-descriptions.h b/gdb/target-descriptions.h
index 6d842bf07ed..66a2c213dc2 100644
--- a/gdb/target-descriptions.h
+++ b/gdb/target-descriptions.h
@@ -225,18 +225,6 @@ struct type *tdesc_find_type (struct gdbarch *gdbarch, const char *id);
 int tdesc_register_in_reggroup_p (struct gdbarch *gdbarch, int regno,
 				  struct reggroup *reggroup);
 
-
-/* A deleter adapter for a target desc.  */
-
-struct target_desc_deleter
-{
-  void operator() (struct target_desc *desc) const;
-};
-
-/* A unique pointer specialization that holds a target_desc.  */
-
-typedef std::unique_ptr<target_desc, target_desc_deleter> target_desc_up;
-
 /* Methods for constructing a target description.  */
 
 void set_tdesc_architecture (struct target_desc *,
diff --git a/gdbserver/tdesc.cc b/gdbserver/tdesc.cc
index d21688b932b..e639017cc31 100644
--- a/gdbserver/tdesc.cc
+++ b/gdbserver/tdesc.cc
@@ -93,12 +93,22 @@ init_target_desc (struct target_desc *tdesc,
 #endif
 }
 
+/* See gdbsupport/tdesc.h.  */
+
 struct target_desc *
 allocate_target_description (void)
 {
   return new target_desc ();
 }
 
+/* See gdbsupport/tdesc.h.  */
+
+void
+target_desc_deleter::operator() (struct target_desc *target_desc) const
+{
+  delete target_desc;
+}
+
 #ifndef IN_PROCESS_AGENT
 
 static const struct target_desc default_description {};
diff --git a/gdbserver/tdesc.h b/gdbserver/tdesc.h
index f9ca478ff6d..681de64d0aa 100644
--- a/gdbserver/tdesc.h
+++ b/gdbserver/tdesc.h
@@ -27,7 +27,7 @@
 /* A target description.  Inherit from tdesc_feature so that target_desc
    can be used as tdesc_feature.  */
 
-struct target_desc : tdesc_element
+struct target_desc final : tdesc_element
 {
   /* A vector of elements of register definitions that
      describe the inferior's register set.  */
diff --git a/gdbsupport/tdesc.h b/gdbsupport/tdesc.h
index 456e8e070b1..e36137cacd0 100644
--- a/gdbsupport/tdesc.h
+++ b/gdbsupport/tdesc.h
@@ -312,6 +312,17 @@ struct tdesc_feature : tdesc_element
 
 typedef std::unique_ptr<tdesc_feature> tdesc_feature_up;
 
+/* A deleter adapter for a target desc.  */
+
+struct target_desc_deleter
+{
+  void operator() (struct target_desc *desc) const;
+};
+
+/* A unique pointer specialization that holds a target_desc.  */
+
+typedef std::unique_ptr<target_desc, target_desc_deleter> target_desc_up;
+
 /* Allocate a new target_desc.  */
 target_desc *allocate_target_description (void);
 
-- 
2.25.4