Modules callbacks for lazy free effort, and unlink (#7912)

Add two optional callbacks to the RedisModuleTypeMethods structure, which is `free_effort`
and `unlink`. the `free_effort` callback indicates the effort required to free a module memory.
Currently, if the effort exceeds LAZYFREE_THRESHOLD, the module memory may be released
asynchronously. the `unlink` callback indicates the key has been removed from the DB by redis, and
may soon be freed by a background thread.

Add `lazyfreed_objects` info field, which represents the number of objects that have been
lazyfreed since redis was started.

Add `RM_GetTypeMethodVersion` API, which return the current redis-server runtime value of
`REDISMODULE_TYPE_METHOD_VERSION`. You can use that when calling `RM_CreateDataType` to know
which fields of RedisModuleTypeMethods are gonna be supported and which will be ignored.

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/unit/moduleapi/test_lazyfree.tcl b/tests/unit/moduleapi/test_lazyfree.tcl
new file mode 100644
index 000000000..9f482769c
--- /dev/null
+++ b/tests/unit/moduleapi/test_lazyfree.tcl
@@ -0,0 +1,32 @@
+set testmodule [file normalize tests/modules/test_lazyfree.so]
+
+start_server {tags {"modules"}} {
+    r module load $testmodule
+
+    test "modules allocated memory can be reclaimed in the background" {
+        set orig_mem [s used_memory]
+        set rd [redis_deferring_client]
+
+        # LAZYFREE_THRESHOLD is 64
+        for {set i 0} {$i < 10000} {incr i} {
+            $rd lazyfreelink.insert lazykey $i
+        }
+
+        for {set j 0} {$j < 10000} {incr j} {
+            $rd read 
+        }
+
+        assert {[r lazyfreelink.len lazykey] == 10000}
+
+        set peak_mem [s used_memory]
+        assert {[r unlink lazykey] == 1}
+        assert {$peak_mem > $orig_mem+10000}
+        wait_for_condition 50 100 {
+            [s used_memory] < $peak_mem &&
+            [s used_memory] < $orig_mem*2 &&
+            [string match {*lazyfreed_objects:1*} [r info Memory]]
+        } else {
+            fail "Module memory is not reclaimed by UNLINK"
+        }
+    }
+}
\ No newline at end of file