Improve RM_ModuleTypeReplaceValue() API.

With the previous API, a NULL return value was ambiguous and could
represent either an old value of NULL or an error condition. The new API
returns a status code and allows the old value to be returned
by-reference.

This commit also includes test coverage based on
tests/modules/datatype.c which did not exist at the time of the original
commit.

src/module.c

@@ -7308,22 +7308,30 @@ int RM_GetLFU(RedisModuleKey *key, long long *lfu_freq) {
  * Unlike RM_ModuleTypeSetValue() which will free the old value, this function
  * simply swaps the old value with the new value.
  *
- * The function returns the old value, or NULL if any of the above conditions is
- * not met.
+ * The function returns REDISMODULE_OK on success, REDISMODULE_ERR on errors
+ * such as:
+ *
+ * 1. Key is not opened for writing.
+ * 2. Key is not a module data type key.
+ * 3. Key is a module datatype other than 'mt'.
+ *
+ * If old_value is non-NULL, the old value is returned by reference.
  */
-void *RM_ModuleTypeReplaceValue(RedisModuleKey *key, moduleType *mt, void *new_value) {
+int RM_ModuleTypeReplaceValue(RedisModuleKey *key, moduleType *mt, void *new_value, void **old_value) {
     if (!(key->mode & REDISMODULE_WRITE) || key->iter)
-        return NULL;
+        return REDISMODULE_ERR;
     if (!key->value || key->value->type != OBJ_MODULE)
-        return NULL;
+        return REDISMODULE_ERR;
 
     moduleValue *mv = key->value->ptr;
     if (mv->type != mt)
-        return NULL;
+        return REDISMODULE_ERR;
 
-    void *old_val = mv->value;
+    if (old_value)
+        *old_value = mv->value;
     mv->value = new_value;
-    return old_val;
+
+    return REDISMODULE_OK;
 }
 
 /* Register all the APIs we export. Keep this function at the end of the

src/redismodule.h

@@ -525,7 +525,7 @@ int REDISMODULE_API_FUNC(RedisModule_GetContextFlags)(RedisModuleCtx *ctx);
 void *REDISMODULE_API_FUNC(RedisModule_PoolAlloc)(RedisModuleCtx *ctx, size_t bytes);
 RedisModuleType *REDISMODULE_API_FUNC(RedisModule_CreateDataType)(RedisModuleCtx *ctx, const char *name, int encver, RedisModuleTypeMethods *typemethods);
 int REDISMODULE_API_FUNC(RedisModule_ModuleTypeSetValue)(RedisModuleKey *key, RedisModuleType *mt, void *value);
-void *REDISMODULE_API_FUNC(RedisModule_ModuleTypeReplaceValue)(RedisModuleKey *key, RedisModuleType *mt, void *new_value);
+int REDISMODULE_API_FUNC(RedisModule_ModuleTypeReplaceValue)(RedisModuleKey *key, RedisModuleType *mt, void *new_value, void **old_value);
 RedisModuleType *REDISMODULE_API_FUNC(RedisModule_ModuleTypeGetType)(RedisModuleKey *key);
 void *REDISMODULE_API_FUNC(RedisModule_ModuleTypeGetValue)(RedisModuleKey *key);
 int REDISMODULE_API_FUNC(RedisModule_IsIOError)(RedisModuleIO *io);

tests/modules/datatype.c

@@ -124,6 +124,28 @@ static int datatype_dump(RedisModuleCtx *ctx, RedisModuleString **argv, int argc
     return REDISMODULE_OK;
 }
 
+static int datatype_swap(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
+    if (argc != 3) {
+        RedisModule_WrongArity(ctx);
+        return REDISMODULE_OK;
+    }
+
+    RedisModuleKey *a = RedisModule_OpenKey(ctx, argv[1], REDISMODULE_WRITE);
+    RedisModuleKey *b = RedisModule_OpenKey(ctx, argv[2], REDISMODULE_WRITE);
+    void *val = RedisModule_ModuleTypeGetValue(a);
+
+    int error = (RedisModule_ModuleTypeReplaceValue(b, datatype, val, &val) == REDISMODULE_ERR ||
+                 RedisModule_ModuleTypeReplaceValue(a, datatype, val, NULL) == REDISMODULE_ERR);
+    if (!error)
+        RedisModule_ReplyWithSimpleString(ctx, "OK");
+    else
+        RedisModule_ReplyWithError(ctx, "ERR failed");
+
+    RedisModule_CloseKey(a);
+    RedisModule_CloseKey(b);
+
+    return REDISMODULE_OK;
+}
 
 int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
     REDISMODULE_NOT_USED(argv);
@@ -157,5 +179,8 @@ int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
     if (RedisModule_CreateCommand(ctx,"datatype.dump", datatype_dump,"",1,1,1) == REDISMODULE_ERR)
         return REDISMODULE_ERR;
 
+    if (RedisModule_CreateCommand(ctx,"datatype.swap", datatype_swap,"",1,1,1) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
     return REDISMODULE_OK;
 }

tests/unit/moduleapi/datatype.tcl

@@ -24,4 +24,21 @@ start_server {tags {"modules"}} {
         catch {r datatype.restore dtkeycopy $truncated} e
         set e
     } {*Invalid*}
+
+    test {DataType: ModuleTypeReplaceValue() happy path works} {
+        r datatype.set key-a 1 AAA
+        r datatype.set key-b 2 BBB
+
+        assert {[r datatype.swap key-a key-b] eq {OK}}
+        assert {[r datatype.get key-a] eq {2 BBB}}
+        assert {[r datatype.get key-b] eq {1 AAA}}
+    }
+
+    test {DataType: ModuleTypeReplaceValue() fails on non-module keys} {
+        r datatype.set key-a 1 AAA
+        r set key-b RedisString
+
+        catch {r datatype.swap key-a key-b} e
+        set e
+    } {*ERR*}
 }