Pad/internal/io/pool/real/filesystem_test.go

package real

// Data-integrity tests for the persistence layer's atomicity contract. The
// editor's corruption protection against crashes rests on WriteFileAtomic's
// write-sibling-temp-then-rename scheme; these tests pin the observable
// consequences of that contract:
//
//   - a written file is byte-exact (no truncation/padding at any size),
//   - a concurrent reader NEVER observes a torn file: every read returns
//     exactly one of the known complete payloads (the "crash/kill mid-write
//     leaves a complete snapshot" property, without needing a real crash),
//   - a FAILED write leaves the original file byte-identical,
//   - a stale leftover temp file is consumed by the next successful write.

import (
	"bytes"
	"math/rand"
	"os"
	"path/filepath"
	"strconv"
	"sync"
	"sync/atomic"
	"testing"
)

func randomPayload(t *testing.T, seed int64, size int) []byte {
	t.Helper()
	rng := rand.New(rand.NewSource(seed))
	b := make([]byte, size)
	rng.Read(b)
	return b
}

func TestWriteFileAtomic_RoundTripExact(t *testing.T) {
	for _, size := range []int{0, 1, 2, 65537, 5 * 1024 * 1024} {
		d := t.TempDir()
		fs := NewRealFileSystem(d)
		data := randomPayload(t, int64(size), size)
		if err := fs.WriteFileAtomic("/rt.txt", data); err != nil {
			t.Fatalf("size %d: write: %v", size, err)
		}
		got, err := fs.ReadFile("/rt.txt")
		if err != nil {
			t.Fatalf("size %d: read: %v", size, err)
		}
		if !bytes.Equal(got, data) {
			t.Fatalf("size %d: round trip mismatch (got %d bytes)", size, len(got))
		}
		// Overwrite with different content of the same size.
		data2 := randomPayload(t, int64(size)+1, size)
		if err := fs.WriteFileAtomic("/rt.txt", data2); err != nil {
			t.Fatalf("size %d: overwrite: %v", size, err)
		}
		got, err = fs.ReadFile("/rt.txt")
		if err != nil {
			t.Fatalf("size %d: read after overwrite: %v", size, err)
		}
		if !bytes.Equal(got, data2) {
			t.Fatalf("size %d: overwrite mismatch", size)
		}
	}
}

// TestWriteFileAtomic_ConcurrentReader_NeverTorn hammers the file with two
// alternating 2 MB payloads while a reader spins; every single read must
// return exactly one of the two payloads in full. Any torn read (a mix, a
// truncation, a padding) is a data-corruption failure.
func TestWriteFileAtomic_ConcurrentReader_NeverTorn(t *testing.T) {
	d := t.TempDir()
	fs := NewRealFileSystem(d)
	const size = 2 * 1024 * 1024
	payloadA := randomPayload(t, 1, size)
	payloadB := randomPayload(t, 2, size)

	if err := fs.WriteFileAtomic("/torn.txt", payloadA); err != nil {
		t.Fatal(err)
	}

	stop := make(chan struct{})
	var (
		wg         sync.WaitGroup
		reads      atomic.Int64
		torn       atomic.Int64
		tornDetail string // written only by the reader, read after wg.Wait()
	)
	wg.Add(1)
	go func() {
		defer wg.Done()
		for {
			select {
			case <-stop:
				return
			default:
			}
			data, err := fs.ReadFile("/torn.txt")
			if err != nil {
				continue // transient (rename boundary); next read decides
			}
			reads.Add(1)
			if !bytes.Equal(data, payloadA) && !bytes.Equal(data, payloadB) {
				torn.Store(1)
				at := 0
				ref := payloadA
				if len(data) < len(ref) {
					ref = data
				}
				for at < len(data) && at < len(ref) && data[at] == ref[at] {
					at++
				}
				tornDetail = "first diff at " + itoa(at) + " (read " + itoa(len(data)) + " bytes, want " + itoa(size) + ")"
				return
			}
		}
	}()

	const iters = 150
	for i := 0; i < iters; i++ {
		payload := payloadA
		if i%2 == 1 {
			payload = payloadB
		}
		if err := fs.WriteFileAtomic("/torn.txt", payload); err != nil {
			t.Fatal(err)
		}
	}
	close(stop)
	wg.Wait()

	if torn.Load() == 1 {
		t.Fatalf("torn read observed: %s", tornDetail) // safe: reader exited (wg.Wait)
	}
	if n := reads.Load(); n < 20 {
		t.Fatalf("only %d concurrent reads happened; test not meaningful", n)
	}
}

// TestWriteFileAtomic_FailedWriteLeavesOriginalIntact verifies the crash
// property directly: when the write cannot complete (read-only directory),
// the original file is untouched, byte for byte.
func TestWriteFileAtomic_FailedWriteLeavesOriginalIntact(t *testing.T) {
	if os.Geteuid() == 0 {
		t.Skip("running as root: permission bits are bypassed")
	}
	d := t.TempDir()
	fs := NewRealFileSystem(d)
	original := []byte("original content that must survive\n")
	if err := fs.WriteFileAtomic("/f.txt", original); err != nil {
		t.Fatal(err)
	}
	if err := os.Chmod(d, 0o555); err != nil {
		t.Fatal(err)
	}
	defer func() { _ = os.Chmod(d, 0o755) }()

	if err := fs.WriteFileAtomic("/f.txt", []byte("new content")); err == nil {
		t.Fatal("expected write to fail in read-only directory")
	}
	got, err := fs.ReadFile("/f.txt")
	if err != nil {
		t.Fatal(err)
	}
	if !bytes.Equal(got, original) {
		t.Fatalf("failed write modified the original file: got %q", got)
	}
}

// TestWriteFileAtomic_StaleTempFileIsConsumed verifies a crash-leftover temp
// file (garbage from an interrupted write) does not poison the next
// successful write and does not survive it.
func TestWriteFileAtomic_StaleTempFileIsConsumed(t *testing.T) {
	d := t.TempDir()
	fs := NewRealFileSystem(d)
	if err := os.WriteFile(filepath.Join(d, ".f.txt.tmp"), []byte("stale garbage from a crashed write"), 0o644); err != nil {
		t.Fatal(err)
	}
	fresh := []byte("fresh content")
	if err := fs.WriteFileAtomic("/f.txt", fresh); err != nil {
		t.Fatal(err)
	}
	got, err := fs.ReadFile("/f.txt")
	if err != nil {
		t.Fatal(err)
	}
	if !bytes.Equal(got, fresh) {
		t.Fatalf("target file != fresh content: %q", got)
	}
	entries, err := os.ReadDir(d)
	if err != nil {
		t.Fatal(err)
	}
	for _, e := range entries {
		if e.Name() == ".f.txt.tmp" {
			t.Fatal("stale temp file was not consumed by the successful write")
		}
	}
}

func itoa(n int) string { return strconv.Itoa(n) }