tests/lapi: add tests for io functions

The patch adds tests for `io.flush()`, `io.read()`, `io.seek()`,
`io.setvbuf()`, `io.write()`. Other functions are not covered.

Author: ligurio

tests/lapi/io_torture_test.lua

@@ -0,0 +1,99 @@
+--[[
+SPDX-License-Identifier: ISC
+Copyright (c) 2023-2025, Sergey Bronnikov.
+
+5.7 – Input and Output Facilities
+https://www.lua.org/manual/5.1/manual.html#5.7
+https://www.lua.org/pil/21.3.html
+
+Synopsis:
+
+io.close([file])
+io.flush()
+io.open(filename [, mode])
+io.read(...)
+io.tmpfile()
+io.write(...)
+file:seek([whence] [, offset])
+file:setvbuf(mode [, size])
+]]
+
+local luzer = require("luzer")
+-- The maximum file size is 1Mb (1000 * 1000).
+local MAX_N = 1e3
+
+local function io_seek(self)
+    local SEEK_MODE = { "set", "cur", "end" }
+    local mode = self.fdp:oneof(SEEK_MODE)
+    local offset = self.fdp:consume_integer(0, self.MAX_N)
+    self.fh:seek(mode, offset)
+end
+
+local function io_flush(self)
+    self.fh:flush()
+end
+
+local function io_setvbuf(self)
+    local VBUF_MODE = { "no", "full", "line" }
+    local mode = self.fdp:oneof(VBUF_MODE)
+    local size = self.fdp:consume_integer(0, self.MAX_N)
+    self.fh:setvbuf(mode, size)
+end
+
+local function io_write(self)
+    local str = self.fdp:consume_string(self.MAX_N)
+    self.fh:write(str)
+end
+
+local function io_read(self)
+    -- As a special case, `io.read(0)` works as a test for end of
+    -- file: It returns an empty string if there is more to be
+    -- read or `nil` otherwise.
+    local size = self.fdp:consume_integer(0, self.MAX_N)
+    local READ_MODE = { "*number", "*all", "*line", size }
+    local mode = self.fdp:oneof(READ_MODE)
+    local _ = self.fh:read(mode)
+end
+
+local function io_close(self)
+    self.fh:close()
+end
+
+local io_methods = {
+    io_flush,
+    io_read,
+    io_seek,
+    io_setvbuf,
+    io_write,
+}
+
+local function io_random_op(self)
+    local io_method = self.fdp:oneof(io_methods)
+    io_method(self)
+end
+
+local function io_new(fdp)
+    local fh = io.tmpfile()
+    return {
+        close = io_close,
+        fdp = fdp,
+        fh = fh,
+        random_operation = io_random_op,
+        MAX_N = MAX_N,
+    }
+end
+
+local function TestOneInput(buf)
+    local fdp = luzer.FuzzedDataProvider(buf)
+    local nops = fdp:consume_integer(1, MAX_N)
+    local fh = io_new(fdp)
+    for _ = 1, nops do
+        fh:random_operation()
+    end
+    fh:close()
+end
+
+local args = {
+    artifact_prefix = "io_torture_",
+}
+luzer.Fuzz(TestOneInput, nil, args)