[PATCH v2 1/5] GDB: Ignore `max-value-size' setting with value history accesses

["Maciej W. Rozycki" <macro@embecosm.com>]

We have an inconsistency in value history accesses where array element 
accesses cause an error for entries exceeding the currently selected 
`max-value-size' setting even where such accesses successfully complete 
for elements located in the inferior, e.g.:

  (gdb) p/d one
  $1 = 0
  (gdb) p/d one_hundred
  $2 = {0 <repeats 100 times>}
  (gdb) p/d one_hundred[99]
  $3 = 0
  (gdb) set max-value-size 25
  (gdb) p/d one_hundred
  value requires 100 bytes, which is more than max-value-size
  (gdb) p/d one_hundred[99]
  $7 = 0
  (gdb) p/d $2
  value requires 100 bytes, which is more than max-value-size
  (gdb) p/d $2[99]
  value requires 100 bytes, which is more than max-value-size
  (gdb) 

According to our documentation the `max-value-size' setting is a safety 
guard against allocating an overly large amount of memory.  Moreover a 
statement in documentation says, concerning this setting, that: "Setting 
this variable does not effect values that have already been allocated 
within GDB, only future allocations."  While in the implementer-speak 
the sentence may be unambiguous I think the outside user may well infer 
that the setting only applies to values that need to be retrieved from 
the debuggee.

Therefore rather than just fixing this inconsistency it seems reasonable 
to lift the setting for value history accesses, under an implication 
that by having been retrieved from the debuggee they have already passed 
the safety check.  Do it then, making the last two commands succeed:

  (gdb) p/d $2
  $8 = {0 <repeats 100 times>}
  (gdb) p/d $2 [99]
  $9 = 0
  (gdb) 

Expand the testsuite accordingly.
---
 gdb/testsuite/gdb.base/max-value-size.exp |    3 +
 gdb/value.c                               |   56 ++++++++++++++++++++----------
 2 files changed, 42 insertions(+), 17 deletions(-)

gdb-value-history-size.diff
Index: src/gdb/testsuite/gdb.base/max-value-size.exp
===================================================================
--- src.orig/gdb/testsuite/gdb.base/max-value-size.exp
+++ src/gdb/testsuite/gdb.base/max-value-size.exp
@@ -53,6 +53,9 @@ proc do_value_printing { max_value_size
 	    gdb_test "p/d one_hundred" " = \\{0 <repeats 100 times>\\}"
 	}
 	gdb_test "p/d one_hundred \[99\]" " = 0"
+	# Verify that accessing value history is undisturbed.
+	gdb_test "p/d \$2" " = \\{0 <repeats 100 times>\\}"
+	gdb_test "p/d \$2 \[99\]" " = 0"
     }
 }
 
Index: src/gdb/value.c
===================================================================
--- src.orig/gdb/value.c
+++ src/gdb/value.c
@@ -1034,31 +1034,42 @@ check_type_length_before_alloc (const st
     }
 }
 
-/* Allocate the contents of VAL if it has not been allocated yet.  */
+/* Allocate the contents of VAL if it has not been allocated yet.
+   If CHECK_SIZE is true, then apply the usual max-value-size checks.  */
 
 static void
-allocate_value_contents (struct value *val)
+allocate_value_contents (struct value *val, bool check_size)
 {
   if (!val->contents)
     {
-      check_type_length_before_alloc (val->enclosing_type);
+      if (check_size)
+	check_type_length_before_alloc (val->enclosing_type);
       val->contents.reset
 	((gdb_byte *) xzalloc (val->enclosing_type->length ()));
     }
 }
 
-/* Allocate a  value  and its contents for type TYPE.  */
+/* Allocate a value and its contents for type TYPE.  If CHECK_SIZE is true,
+   then apply the usual max-value-size checks.  */
 
-struct value *
-allocate_value (struct type *type)
+static struct value *
+allocate_value (struct type *type, bool check_size)
 {
   struct value *val = allocate_value_lazy (type);
 
-  allocate_value_contents (val);
+  allocate_value_contents (val, check_size);
   val->lazy = 0;
   return val;
 }
 
+/* Allocate a value and its contents for type TYPE.  */
+
+struct value *
+allocate_value (struct type *type)
+{
+  return allocate_value (type, true);
+}
+
 /* Allocate a  value  that has the correct length
    for COUNT repetitions of type TYPE.  */
 
@@ -1169,7 +1180,7 @@ value_contents_raw (struct value *value)
   struct gdbarch *arch = get_value_arch (value);
   int unit_size = gdbarch_addressable_memory_unit_size (arch);
 
-  allocate_value_contents (value);
+  allocate_value_contents (value, true);
 
   ULONGEST length = value_type (value)->length ();
   return gdb::make_array_view
@@ -1179,7 +1190,7 @@ value_contents_raw (struct value *value)
 gdb::array_view<gdb_byte>
 value_contents_all_raw (struct value *value)
 {
-  allocate_value_contents (value);
+  allocate_value_contents (value, true);
 
   ULONGEST length = value_enclosing_type (value)->length ();
   return gdb::make_array_view (value->contents.get (), length);
@@ -1752,12 +1763,14 @@ value_release_to_mark (const struct valu
   return result;
 }
 
-/* Return a copy of the value ARG.
-   It contains the same contents, for same memory address,
-   but it's a different block of storage.  */
+/* Return a copy of the value ARG.  It contains the same contents,
+   for the same memory address, but it's a different block of storage.
+   If CHECK_SIZE is true, then throw an exception whenever the size
+   of memory allocated for the contents of the value would exceed
+   max-value-size.  */
 
-struct value *
-value_copy (const value *arg)
+static struct value *
+value_copy (const value *arg, bool check_size)
 {
   struct type *encl_type = value_enclosing_type (arg);
   struct value *val;
@@ -1765,7 +1778,7 @@ value_copy (const value *arg)
   if (value_lazy (arg))
     val = allocate_value_lazy (encl_type);
   else
-    val = allocate_value (encl_type);
+    val = allocate_value (encl_type, check_size);
   val->type = arg->type;
   VALUE_LVAL (val) = arg->lval;
   val->location = arg->location;
@@ -1802,6 +1815,15 @@ value_copy (const value *arg)
   return val;
 }
 
+/* Return a copy of the value ARG.  It contains the same contents,
+   for the same memory address, but it's a different block of storage.  */
+
+struct value *
+value_copy (const value *arg)
+{
+  return value_copy (arg, true);
+}
+
 /* Return a "const" and/or "volatile" qualified version of the value V.
    If CNST is true, then the returned value will be qualified with
    "const".
@@ -1965,7 +1987,7 @@ access_value_history (int num)
 
   absnum--;
 
-  return value_copy (value_history[absnum].get ());
+  return value_copy (value_history[absnum].get (), false);
 }
 
 /* See value.h.  */
@@ -4162,7 +4184,7 @@ void
 value_fetch_lazy (struct value *val)
 {
   gdb_assert (value_lazy (val));
-  allocate_value_contents (val);
+  allocate_value_contents (val, true);
   /* A value is either lazy, or fully fetched.  The
      availability/validity is only established as we try to fetch a
      value.  */