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

package nfsfs

import (
	"fmt"
	"os"
	"path/filepath"
	"testing"
)

// openFDs counts the descriptors the process holds through /proc. The
// count is a lower bound of live descriptors and good enough to prove a
// cache does not leak: a thousand operations over one file must not add a
// thousand descriptors.
func openFDs(t *testing.T) int {
	t.Helper()
	entries, err := os.ReadDir("/proc/self/fd")
	if err != nil {
		t.Skipf("/proc/self/fd is unavailable: %v", err)
	}
	return len(entries)
}

// TestCacheNoDescriptorLeak drives more operations over one file than the
// cache can hold and requires the process descriptor count to stay flat.
func TestCacheNoDescriptorLeak(t *testing.T) {
	l, h, _ := cacheTestRoot(t, "leak")
	before := openFDs(t)
	for range 1000 {
		if _, err := l.Read(h, 0, 4); err != nil {
			t.Fatalf("Read: %v", err)
		}
	}
	after := openFDs(t)
	if after-before > 8 {
		t.Fatalf("descriptors grew from %d to %d over 1000 reads", before, after)
	}
}

// TestCacheBound holds when many distinct files flow through: after
// touching twice the bound, at most the bound of cache entries may remain.
func TestCacheBound(t *testing.T) {
	root := t.TempDir()
	l, err := NewLocal(root)
	if err != nil {
		t.Fatalf("NewLocal: %v", err)
	}
	rh, err := l.Root()
	if err != nil {
		t.Fatalf("Root: %v", err)
	}
	for i := range 2 * fdCacheLimit {
		name := fmt.Sprintf("f%d", i)
		if err := os.WriteFile(filepath.Join(root, name), []byte("x"), 0o644); err != nil {
			t.Fatalf("WriteFile: %v", err)
		}
		h, _, err := l.Lookup(rh, name)
		if err != nil {
			t.Fatalf("Lookup: %v", err)
		}
		if _, err := l.Read(h, 0, 1); err != nil {
			t.Fatalf("Read %s: %v", name, err)
		}
	}
	l.fdMu.Lock()
	n := len(l.fds)
	l.fdMu.Unlock()
	if n > fdCacheLimit {
		t.Fatalf("cache holds %d entries, bound is %d", n, fdCacheLimit)
	}
}