examples/configdata: exercise unregister/register lifetime

Add a regression test for apache/nuttx#20166 to the configdata
example, which runs automatically in every sim:configdata build.
After the normal configdata test loops, close the long-lived
descriptor and run a full lifetime cycle on /dev/config:

- mtdconfig_unregister() must succeed,
- the unregistered device must no longer be openable (ENOENT),
- the same MTD device must be registerable again and usable,
- a final unregister must succeed, leaving the device unregistered.

Before the driver fix this crashes deterministically: the old
mtdconfig_unregister_by_path() freed the private device structure
before closing its temporary file, so the subsequent open() faults
in mm_malloc on the corrupted heap (SIGSEGV observed on sim right
after unregister returns).  With the fix the whole example,
934706/934706 checks included, completes cleanly.

The test uses only generic configdata APIs, so it also passes
unchanged under CONFIG_MTD_CONFIG_NVS, whose open/close callbacks
are no-ops.

Assisted-by: Claude:claude-opus
Signed-off-by: Arnav Sharma <2006arnavsharma@gmail.com>

Author: arnavsharma990

examples/configdata/configdata_main.c

@@ -609,6 +609,80 @@ static void configdata_cleardeleted(void)
   g_ndeleted = 0;
 }
 
+/****************************************************************************
+ * Name: configdata_testunregister
+ *
+ * Description:
+ *   Exercise the /dev/config register/unregister lifetime.  Unregister
+ *   the device, verify that it can no longer be opened, then register
+ *   it again and verify that it is usable.  This is a regression test:
+ *   freeing the private device structure before the temporary file used
+ *   during unregister is closed corrupts the heap and crashes the open
+ *   below.
+ *
+ * Input Parameters:
+ *   mtd - Pointer to the MTD device bound to the /dev/config device
+ *
+ ****************************************************************************/
+
+static void configdata_testunregister(FAR struct mtd_dev_s *mtd)
+{
+  int fd;
+  int ret;
+
+  close(g_fd);
+  g_fd = -1;
+
+  ret = mtdconfig_unregister();
+  if (ret < 0)
+    {
+      printf("ERROR: /dev/config unregistration failed: %d\n", ret);
+      fflush(stdout);
+      exit(3);
+    }
+
+  fd = open("/dev/config", O_RDONLY);
+  if (fd >= 0 || errno != ENOENT)
+    {
+      printf("ERROR: /dev/config still accessible after unregister: "
+             "fd=%d errno=%d\n", fd, errno);
+      fflush(stdout);
+
+      if (fd >= 0)
+        {
+          close(fd);
+        }
+
+      exit(3);
+    }
+
+  ret = mtdconfig_register(mtd);
+  if (ret < 0)
+    {
+      printf("ERROR: /dev/config re-registration failed: %d\n", ret);
+      fflush(stdout);
+      exit(3);
+    }
+
+  fd = open("/dev/config", O_RDONLY);
+  if (fd < 0)
+    {
+      printf("ERROR: Failed to re-open /dev/config %d\n", errno);
+      fflush(stdout);
+      exit(3);
+    }
+
+  close(fd);
+
+  ret = mtdconfig_unregister();
+  if (ret < 0)
+    {
+      printf("ERROR: /dev/config final unregistration failed: %d\n", ret);
+      fflush(stdout);
+      exit(3);
+    }
+}
+
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
@@ -786,6 +860,8 @@ int main(int argc, FAR char *argv[])
   configdata_delallfiles();
 #endif
 
+  configdata_testunregister(mtd);
+
   configdata_endmemusage();
   fflush(stdout);
   return 0;