Map KstatCtl.kc_chain as a pointer

kstat_ctl_t declares kc_chain as a kstat_t pointer, but KstatCtl mapped it
as an inline Kstat structure, so JNA computed the structure as 200 bytes
against the native 24 (32 on Solaris 11.4). Because a Structure passed by
reference is written before and read after every call, each
kstat_chain_update, kstat_lookup, kstat_read, and kstat_close wrote 176
bytes past the end of the block kstat_open() allocated, reverting
adjacent heap to its contents at the previous call.

Map kc_chain as a Pointer and add KstatCtl.chain() to return the chain
head as a Kstat, using a new Kstat(Pointer) constructor that next() now
shares. Solaris 11.4's trailing kc_private field is private to libkstat
and deliberately left unmapped; the structure only wraps the pointer
kstat_open() returns, so the three-field mapping is a safe prefix there.

Verified on Solaris 11.4 SPARC under libumem with UMEM_DEBUG=default:
size 24, a full walk of the 4675-entry chain, and 1000 update/lookup/read
cycles plus kstat_close with no allocator errors.

Fixes #1740

Author: dbwiddis

CHANGES.md

@@ -10,7 +10,11 @@ Features
 
 Bug Fixes
 ---------
+* [#1740](https://github.com/java-native-access/jna/issues/1740): Fix `c.s.j.p.unix.solaris.LibKstat.KstatCtl` mapping `kc_chain` as an inline `Kstat` instead of a pointer, which made every call taking a `KstatCtl` write past the end of the structure `kstat_open()` allocated - [@dbwiddis](https://github.com/dbwiddis).
 
+Breaking Changes
+----------------
+* `c.s.j.p.unix.solaris.LibKstat.KstatCtl#kc_chain` is now a `Pointer` instead of a `Kstat`. Use `KstatCtl#chain()` to get the head of the kstat chain as a `Kstat`.
 
 Release 5.19.1
 ==============

contrib/platform/src/com/sun/jna/platform/unix/solaris/LibKstat.java

@@ -140,14 +140,20 @@ class Kstat extends Structure {
 
         public Pointer ks_lock; // protects this kstat's data
 
+        public Kstat() {
+            super();
+        }
+
+        public Kstat(Pointer p) {
+            super(p);
+            read();
+        }
+
         public Kstat next() {
             if (ks_next == null) {
                 return null;
             }
-            Kstat n = new Kstat();
-            n.useMemory(ks_next);
-            n.read();
-            return n;
+            return new Kstat(ks_next);
         }
     }
 
@@ -350,16 +356,28 @@ public KstatIO(Pointer p) {
     /**
      * A kstat control structure. Only one thread may actively use a KstatCtl
      * value at any time. Synchronization is left to the application.
+     * <p>
+     * Oracle Solaris 11.4 appends a fourth field, {@code void **kc_private},
+     * which is private to libkstat. It is deliberately not mapped: this
+     * structure only ever wraps the pointer returned by {@code kstat_open()},
+     * so mapping the three public fields reads and writes a prefix of the
+     * native structure on every release, and leaves the private field alone.
      */
     @FieldOrder({"kc_chain_id", "kc_chain", "kc_kd"})
     class KstatCtl extends Structure {
 
         public int kc_chain_id; // current kstat chain ID
 
-        public Kstat kc_chain; // pointer to kstat chain
+        public Pointer kc_chain; // pointer to kstat chain
 
         public int kc_kd; // /dev/kstat descriptor - not public interface
 
+        public Kstat chain() {
+            if (kc_chain == null) {
+                return null;
+            }
+            return new Kstat(kc_chain);
+        }
     }
 
     /**