Introduce deinitIfExists utility and replace deinitIfLive for optional resource cleanup in main.zig.

src/memory/util.zig

@@ -1,23 +1,25 @@
 const std = @import("std");
 
-/// Deinitializes `resource` only if it is still live.
+/// Deinitializes `resource` (allocator owner) only if it is still live.
 ///
-/// Pass a pointer to the original allocator owner,
-/// such as an `ArenaAllocator`.
+/// Pass a pointer to the allocator owner, such as an `ArenaAllocator`.
 ///
-/// If `live` is `true`, calls `allocator.deinit()` and then sets
+/// If `live` is `true`, calls `resource.deinit()` and then sets
 /// `live` to `false`, preventing a later cleanup path from attempting
-/// to deinitialize the same allocator again.
+/// to deinitialize the same resource again.
 ///
-/// The return value of `deinit()` is returned from the function.
+/// The return value of `resource.deinit()` is returned from the function.
 pub fn deinitIfLive(resource: anytype, live: *bool) ?@TypeOf(resource.deinit())
 {
     comptime
     {
-        const T = @TypeOf(resource);
+        const Pointer = @TypeOf(resource);
 
-        if (@typeInfo(T) != .pointer) @compileError("deinitIfLive expects a pointer!");
-        if (T == std.mem.Allocator)   @compileError("deinitIfLive expects a pointer to an allocator owner, not std.mem.Allocator!");
+        if (@typeInfo(Pointer) != .pointer)
+            @compileError("deinitIfLive expects a pointer!");
+
+        if (@typeInfo(Pointer).pointer.size != .one)
+            @compileError("deinitIfLive expects a single-item pointer!");
     }
 
     if (!live.*) return null;
@@ -25,3 +27,78 @@ pub fn deinitIfLive(resource: anytype, live: *bool) ?@TypeOf(resource.deinit())
     live.* = false;
     return resource.deinit();
 }
+
+/// Deinitializes the resources stored in `resource`
+/// (allocator owner) only if still exists.
+///
+/// Pass a pointer to an optional allocator owner, such as
+/// `*?std.heap.ArenaAllocator`.
+///
+/// If `resource.*` is non-null, calls `deinit()` on the payload
+/// and then sets `resource.* = null`, preventing a later cleanup
+/// path from attempting to deinitialize the same resource again.
+///
+/// The return value of `resource.*.?.deinit()` is returned
+/// from the function.
+pub fn deinitIfExists(resource: anytype) ?DeinitReturn(@TypeOf(resource))
+{
+    if (resource.*) |*payload|
+    {
+        const result = payload.deinit();
+        resource.* = null;
+        return result;
+    }
+
+    return null;
+}
+
+//
+
+fn DeinitReturn(comptime Pointer: type) type {
+    return @typeInfo(@TypeOf(Payload(Pointer).deinit)).@"fn".return_type.?;
+}
+
+fn Payload(comptime Pointer: type) type
+{
+    if (@typeInfo(Pointer) != .pointer)
+        @compileError("deinitIfExists expects a pointer!");
+
+    if (@typeInfo(Pointer).pointer.size != .one)
+        @compileError("deinitIfExists expects a single-item pointer!");
+
+    const Child = @typeInfo(Pointer).pointer.child;
+
+    if (@typeInfo(Child) != .optional)
+        @compileError("deinitIfExists expects a pointer to an optional resource (*?T)!");
+
+    return @typeInfo(Child).optional.child;
+}
+
+//
+
+test "deinitIfLive deinitializes an ArenaAllocator once" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    var live  = true;
+
+    const alloc = arena.allocator();
+    _ = try alloc.alloc(u8, 1);
+
+    _ = deinitIfLive(&arena, &live);
+    try std.testing.expect(!live);
+
+    // Must not deinitialize it again.
+    try std.testing.expect(deinitIfLive(&arena, &live) == null);
+}
+
+test "deinitIfExists deinitializes an optional ArenaAllocator once" {
+    var arena: ?std.heap.ArenaAllocator = .init(std.testing.allocator);
+
+    const alloc = arena.?.allocator();
+    _ = try alloc.alloc(u8, 1);
+
+    _ = deinitIfExists(&arena);
+    try std.testing.expect(arena == null);
+
+    // Must not deinitialize it again.
+    try std.testing.expect(deinitIfExists(&arena) == null);
+}