// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package nfsfs

import (
	"errors"
	"os"
	"path/filepath"
	"sync"
	"testing"
)

// cacheTestRoot builds a backend over a fresh directory holding one file
// and returns the backend, the file handle and the path.
func cacheTestRoot(t *testing.T, content string) (*Local, Handle, string) {
	t.Helper()
	root := t.TempDir()
	l, err := NewLocal(root)
	if err != nil {
		t.Fatalf("NewLocal: %v", err)
	}
	p := filepath.Join(root, "file")
	if err := os.WriteFile(p, []byte(content), 0o644); err != nil {
		t.Fatalf("WriteFile: %v", err)
	}
	rh, err := l.Root()
	if err != nil {
		t.Fatalf("Root: %v", err)
	}
	h, _, err := l.Lookup(rh, "file")
	if err != nil {
		t.Fatalf("Lookup: %v", err)
	}
	return l, h, p
}

// TestCacheStaleAfterRemove covers the identity contract on a cache
// hit: a file removed while a descriptor of it is cached answers stale,
// exactly as it does without the cache.
func TestCacheStaleAfterRemove(t *testing.T) {
	l, h, p := cacheTestRoot(t, "hello")
	if _, err := l.Read(h, 0, 5); err != nil {
		t.Fatalf("warm read: %v", err)
	}
	if err := os.Remove(p); err != nil {
		t.Fatalf("Remove: %v", err)
	}
	if _, err := l.Read(h, 0, 5); !errors.Is(err, ErrStale) {
		t.Fatalf("read after remove: %v, want ErrStale", err)
	}
}

// TestCacheStaleAfterReplace covers a name swapped for another inode: the
// cached descriptor of the old inode must never serve through it.
func TestCacheStaleAfterReplace(t *testing.T) {
	l, h, p := cacheTestRoot(t, "old")
	if _, err := l.Read(h, 0, 3); err != nil {
		t.Fatalf("warm read: %v", err)
	}
	if err := os.Remove(p); err != nil {
		t.Fatalf("Remove: %v", err)
	}
	if err := os.WriteFile(p, []byte("new"), 0o644); err != nil {
		t.Fatalf("WriteFile: %v", err)
	}
	if _, err := l.Read(h, 0, 3); !errors.Is(err, ErrStale) {
		t.Fatalf("read after replace: %v, want ErrStale", err)
	}
}

// TestCacheInodeReuse covers the recycled inode number at the same path:
// a cached descriptor of the unlinked old inode must never serve the
// identity the recycled file now carries. The map entry is forged onto
// the new file, which is exactly the state a reuse produces.
func TestCacheInodeReuse(t *testing.T) {
	l, h, p := cacheTestRoot(t, "stale data")
	if _, err := l.Read(h, 0, 4); err != nil {
		t.Fatalf("warm read: %v", err)
	}
	// Remove the file behind the backend's back and recreate a fresh one
	// at the same path, then point the old handle's identity at it the
	// way a recycled inode number would.
	fi, err := os.Lstat(p)
	if err != nil {
		t.Fatalf("Lstat: %v", err)
	}
	oldID := fileID{dev: stat(fi).Dev, ino: stat(fi).Ino}
	if err := os.Remove(p); err != nil {
		t.Fatalf("Remove: %v", err)
	}
	if err := os.WriteFile(p, []byte("fresh"), 0o644); err != nil {
		t.Fatalf("WriteFile: %v", err)
	}
	fi2, err := os.Lstat(p)
	if err != nil {
		t.Fatalf("Lstat: %v", err)
	}
	newID := fileID{dev: stat(fi2).Dev, ino: stat(fi2).Ino}
	l.mu.Lock()
	l.paths[newID] = p
	if newID == oldID {
		// The filesystem handed back the very same inode; the scenario
		// holds without forging anything.
		l.paths[oldID] = p
	} else {
		delete(l.paths, oldID)
	}
	l.mu.Unlock()
	// Whatever the inode numbers did, a fresh read must serve the fresh
	// content, never the unlinked inode's bytes.
	got, err := l.Read(h, 0, 5)
	if err != nil {
		// A stale answer is also safe: the identity broke and the cache
		// refused. Serving the old bytes is the only failure.
		if !errors.Is(err, ErrStale) {
			t.Fatalf("read after reuse: %v", err)
		}
		return
	}
	if string(got) != "fresh" {
		t.Fatalf("read after reuse: %q, want the fresh content", got)
	}
}

// TestCacheWriteThrough covers that writes land and that a follow up read
// of the same cached file sees them.
func TestCacheWriteThrough(t *testing.T) {
	l, h, p := cacheTestRoot(t, "0123456789")
	if _, err := l.Read(h, 0, 10); err != nil {
		t.Fatalf("warm read: %v", err)
	}
	if n, err := l.Write(h, 2, []byte("AB")); err != nil || n != 2 {
		t.Fatalf("Write: %d, %v", n, err)
	}
	got, err := l.Read(h, 0, 10)
	if err != nil {
		t.Fatalf("Read: %v", err)
	}
	if string(got) != "01AB456789" {
		t.Fatalf("Read: %q", got)
	}
	raw, err := os.ReadFile(p)
	if err != nil {
		t.Fatalf("ReadFile: %v", err)
	}
	if string(raw) != "01AB456789" {
		t.Fatalf("file on disk: %q", raw)
	}
}

// TestCacheConcurrent drives reads and writes of one file from many
// goroutines; the race detector is the judge.
func TestCacheConcurrent(t *testing.T) {
	l, h, _ := cacheTestRoot(t, "concurrent")
	var wg sync.WaitGroup
	for i := range 8 {
		wg.Go(func() {
			for range 50 {
				if _, err := l.Read(h, 0, 4); err != nil {
					t.Errorf("Read: %v", err)
					return
				}
				if i%2 == 0 {
					if _, err := l.Write(h, 0, []byte("writ")); err != nil {
						t.Errorf("Write: %v", err)
						return
					}
				}
			}
		})
	}
	wg.Wait()
}