mmapstorage: add import/export compatibility

doc/news/_preparation_next_release.md

@@ -253,6 +253,11 @@ The following section lists news about the [modules](https://www.libelektra.org/
 - [cache](https://www.libelektra.org/plugins/cache) is a new global caching plugin. It uses [mmapstorage](https://www.libelektra.org/plugins/mmapstorage) as its storage backend and lazily stores keysets from previous ´kdbGet()´ calls. We added initial support for the default resolver and multifile resolver. _(Mihael Pranjić)_
 - Add check of resolved filenames, fixes false cache hits. _(Mihael Pranjić)_
 
+### mmapstorage
+
+- [mmapstorage](https://www.libelektra.org/plugins/mmapstorage) is now able to persist the Global KeySet, which is used by the `cache` plugin. _(Mihael Pranjić)_
+- Fixed support for `kdb import` and `kdb export`. _(Mihael Pranjić)_
+
 ## Libraries
 
 The text below summarizes updates to the [C (and C++)-based libraries](https://www.libelektra.org/libraries/readme) of Elektra.

src/plugins/mmapstorage/README.md

@@ -88,3 +88,8 @@ sudo kdb umount user/tests/mmapstorage
 ## Limitations
 
 Mapped files shall not be altered, otherwise the behavior is undefined.
+
+The `mmap()` system call only supports regular files and so does the mmapstorage
+plugin with one notable exception: The plugin detects when it is called with the
+files `/dev/stdin` and `/dev/stdout` and makes an internal copy. This makes the
+plugin compatible with `kdb import` and `kdb export`.

src/plugins/mmapstorage/internal.h

@@ -37,9 +37,18 @@
 /** Mmap format version */
 #define ELEKTRA_MMAP_FORMAT_VERSION (1)
 
+/** Mmap temp file template */
+#define ELEKTRA_MMAP_TMP_NAME "/tmp/elektraMmapTmpXXXXXX"
+
+/** Magic number used in mmap format */
+#define ELEKTRA_MMAP_BUFSIZE (4096)
+
+#define STDOUT_FILENAME ("/dev/stdout")
+#define STDIN_FILENAME ("/dev/stdin")
+
 /** Suppress warnings in cache mode to debug level */
 #define ELEKTRA_MMAP_LOG_WARNING(...)                                                                                                      \
-	if (mode == MODE_GLOBALCACHE)                                                                                                      \
+	if (test_bit (mode, MODE_GLOBALCACHE))                                                                                             \
 	{                                                                                                                                  \
 		ELEKTRA_LOG_DEBUG (__VA_ARGS__);                                                                                           \
 	}                                                                                                                                  \