replace defer with try ... finally (#17753)

lib/core/rlocks.nim

@@ -47,9 +47,9 @@ proc release*(lock: var RLock) =
 
 template withRLock*(lock: var RLock, code: untyped): untyped =
   ## Acquires the given lock and then executes the code.
-  block:
   acquire(lock)
-    defer:
-      release(lock)
   {.locks: [lock].}:
+    try:
       code
+    finally:
+      release(lock)

tests/stdlib/tuserlocks.nim

@@ -1,8 +1,8 @@
 discard """
-  cmd: "nim $target --threads:on $options $file"
+  matrix: "--threads:on"
 """
 
-import rlocks
+import std/rlocks
 
 var r: RLock
 r.initRLock()
@@ -10,4 +10,11 @@ doAssert r.tryAcquire()
 doAssert r.tryAcquire()
 r.release()
 r.release()
+
+block:
+  var x = 12
+  withRLock r:
+    inc x
+  doAssert x == 13
+
 r.deinitRLock()