Add simple mocking mechanism.

tests/utils/mock.nim

@@ -0,0 +1,48 @@
+# Sourced from https://forum.nim-lang.org/t/9255#60617
+
+import posix
+
+type Instr {.union.} = object
+  bytes: array[8, byte]
+  value: uint64
+
+proc mockImpl*(target, replacement: pointer) =
+  # YOLO who needs alignment
+  #doAssert (cast[ByteAddress](target) and ByteAddress(0x07)) == 0
+  var page = cast[pointer](cast[ByteAddress](target) and (not 0xfff))
+  doAssert mprotect(page, 4096, PROT_WRITE or PROT_EXEC) == 0
+  let rel = cast[ByteAddress](replacement) - cast[ByteAddress](target) - 5
+  var instr = Instr(
+    bytes: [
+      0xe9.byte,
+      (rel shr 0).byte,
+      (rel shr 8).byte,
+      (rel shr 16).byte,
+      (rel shr 24).byte,
+      0,
+      0,
+      0,
+    ]
+  )
+  cast[ptr uint64](target)[] = instr.value
+  doAssert mprotect(page, 4096, PROT_EXEC) == 0
+
+# Note: Requires manual cleanup
+# Usage Example:
+# proc helloWorld(): string = 
+#   "Hello, World!"
+#
+# echo helloWorld() # "Hello, World!"
+#
+# let backup = helloWorld
+# mock(helloWorld):
+#   proc mockedHellWorld(): string =
+#     "Mocked Hello, World!"
+#   mockedHellWorld
+#
+# echo helloWorld() # "Mocked Hello, World!"
+# 
+# mock(helloWorld):
+#   backup
+template mock*(target, replacement: untyped): untyped =
+  mockImpl(cast[pointer](target), cast[pointer](replacement))