test(txn): add commit and conflict tests

src/txn.zig

@@ -102,3 +102,87 @@ pub const Transaction = struct {
         self.finished = true;
     }
 };
+
+fn openTemp(tmp: *std.testing.TmpDir, allocator: Allocator) !Db {
+    var pathBuf: [std.fs.max_path_bytes]u8 = undefined;
+    const base = try tmp.dir.realpath(".", &pathBuf);
+    var dbPathBuf: [std.fs.max_path_bytes]u8 = undefined;
+    const dbPath = try std.fmt.bufPrint(&dbPathBuf, "{s}/store", .{base});
+    return Db.open(allocator, dbPath, .{});
+}
+
+test "transaction commits buffered writes atomically" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var txn = Transaction.begin(&db);
+    defer txn.deinit();
+    try txn.put("alpha", "1");
+    try txn.put("bravo", "2");
+    try txn.delete("alpha");
+
+    try std.testing.expectEqual(@as(?[]u8, null), try db.get(allocator, "bravo"));
+
+    try txn.commit();
+
+    const bravo = (try db.get(allocator, "bravo")).?;
+    defer allocator.free(bravo);
+    try std.testing.expectEqualStrings("2", bravo);
+    try std.testing.expectEqual(@as(?[]u8, null), try db.get(allocator, "alpha"));
+}
+
+test "transaction reads its own writes" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var txn = Transaction.begin(&db);
+    defer txn.deinit();
+    try txn.put("k", "staged");
+    const staged = (try txn.get(allocator, "k")).?;
+    defer allocator.free(staged);
+    try std.testing.expectEqualStrings("staged", staged);
+
+    try txn.delete("k");
+    try std.testing.expectEqual(@as(?[]u8, null), try txn.get(allocator, "k"));
+}
+
+test "conflicting commit is rejected but clean commit succeeds" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    try db.put("balance", "100");
+
+    var txn = Transaction.begin(&db);
+    defer txn.deinit();
+    const seen = (try txn.get(allocator, "balance")).?;
+    defer allocator.free(seen);
+    try std.testing.expectEqualStrings("100", seen);
+
+    try db.put("balance", "external");
+
+    try txn.put("balance", "150");
+    try std.testing.expectError(error.Conflict, txn.commit());
+
+    var clean = Transaction.begin(&db);
+    defer clean.deinit();
+    const current = (try clean.get(allocator, "balance")).?;
+    defer allocator.free(current);
+    try clean.put("note", "ok");
+    try clean.commit();
+
+    const note = (try db.get(allocator, "note")).?;
+    defer allocator.free(note);
+    try std.testing.expectEqualStrings("ok", note);
+}