From 23654e4b5833feaf83f1cd2c8d453a736c3463db Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Wed, 20 Aug 2025 11:43:05 +1000
Subject: [PATCH] Add a test for seek data right after writing to mmap and not
 syncing

---
 storage/file-io-mmap_test.go | 69 ++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 storage/file-io-mmap_test.go

diff --git a/storage/file-io-mmap_test.go b/storage/file-io-mmap_test.go
new file mode 100644
index 00000000..316dce5c
--- /dev/null
+++ b/storage/file-io-mmap_test.go
@@ -0,0 +1,69 @@
+package storage
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/go-quicktest/qt"
+)
+
+// Note this is AI slop with some modifications. Test if writes to mmap without sync or flush are
+// detected by a subsequent seek data. It would appear they are since the mmap is created from the
+// file descriptor used for the seek, and probably because we're in the same process so MAP_SHARED
+// kicks in.
+func TestFileIoImplementationsSeekDataDetection(t *testing.T) {
+	testData := []byte{0x42}
+	writeOffset := int64(1234567) // Large enough for sparse files to kick in.
+	const fileSize = 12345678
+
+	implementations := []struct {
+		name   string
+		fileIo fileIo
+	}{
+		{"classic", classicFileIo{}},
+		{"mmap", &mmapFileIo{}},
+	}
+
+	for _, impl := range implementations {
+		t.Run(impl.name, func(t *testing.T) {
+			// Create temporary directory and file
+			tempDir := t.TempDir()
+			t.Logf("tempDir: %s", tempDir)
+			testFile := filepath.Join(tempDir, "test.dat")
+
+			// Create empty file
+			f, err := os.Create(testFile)
+			qt.Assert(t, qt.IsNil(err))
+			f.Close()
+
+			// Open for write and write a byte without flushing
+			writer, err := impl.fileIo.openForWrite(testFile, fileSize)
+			qt.Assert(t, qt.IsNil(err))
+
+			_, err = writer.WriteAt(testData, writeOffset)
+			qt.Assert(t, qt.IsNil(err))
+
+			// Close writer (but note: we didn't call flush)
+			err = writer.Close()
+			qt.Assert(t, qt.IsNil(err))
+
+			// Open for read and test seekData detection
+			reader, err := impl.fileIo.openForRead(testFile)
+			qt.Assert(t, qt.IsNil(err))
+			defer reader.Close()
+
+			// Use seekDataOrEof to detect data
+			dataPos, err := reader.seekDataOrEof(writeOffset)
+			qt.Assert(t, qt.IsNil(err))
+			qt.Assert(t, qt.Equals(dataPos, writeOffset))
+
+			var buf bytes.Buffer
+			written, err := reader.writeToN(&buf, 1)
+			qt.Assert(t, qt.IsNil(err))
+			qt.Assert(t, qt.Equals(written, 1))
+			qt.Assert(t, qt.DeepEquals(buf.Bytes(), testData))
+		})
+	}
+}
-- 
2.51.0