fix(fs): reject negative offsets when seeking from the start (#3633)

In plugins/fs/src/commands.rs seek, `SeekFrom::Start(offset as u64)`
turned -1 into u64::MAX. The offset is now converted with u64::try_from
and a negative one is an error. E2E spec added.

.changes/fix-fs-seek-negative.md

@@ -0,0 +1,6 @@
+---
+fs: patch
+fs-js: patch
+---
+
+`FileHandle.seek` now rejects a negative offset with `SeekMode.Start` instead of seeking to a huge position.

packages/api-e2e/test/specs/fs.spec.ts

@@ -371,6 +371,19 @@ describePlugin('fs', () => {
     })
   })
 
+  it('FileHandle.seek rejects a negative offset from the start', async () => {
+    const message = await tauriError(async (api, path) => {
+      const baseDir = api.fs.BaseDirectory.AppData
+      const file = await api.fs.create(path, { baseDir })
+      try {
+        await file.seek(-1, api.fs.SeekMode.Start)
+      } finally {
+        await file.close()
+      }
+    }, `${dir}/seek.txt`)
+    expect(message).toMatch(/must not be negative/)
+  })
+
   it('a closed FileHandle cannot be used again', async () => {
     const message = await tauriError(async (api, path) => {
       const baseDir = api.fs.BaseDirectory.AppData

plugins/fs/src/commands.rs

@@ -888,16 +888,17 @@ pub async fn seek<R: Runtime>(
     whence: SeekMode,
 ) -> CommandResult<u64> {
     use std::io::{Seek, SeekFrom};
+    let position = match whence {
+        SeekMode::Start => SeekFrom::Start(u64::try_from(offset).map_err(|_| {
+            format!("invalid seek offset {offset}: it must not be negative with SeekMode.Start")
+        })?),
+        SeekMode::Current => SeekFrom::Current(offset),
+        SeekMode::End => SeekFrom::End(offset),
+    };
     let file: std::sync::Arc<StdFileResource<R>> = webview.resources_table().get(rid)?;
-    StdFileResource::with_lock(&file, |file| {
+    StdFileResource::with_lock(&file, |file| file.seek(position))
-        file.seek(match whence {
+        .map_err(|e| format!("failed to seek file with error: {e}"))
-            SeekMode::Start => SeekFrom::Start(offset as u64),
+        .map_err(Into::into)
-            SeekMode::Current => SeekFrom::Current(offset),
-            SeekMode::End => SeekFrom::End(offset),
-        })
-    })
-    .map_err(|e| format!("failed to seek file with error: {e}"))
-    .map_err(Into::into)
 }
 
 #[cfg(target_os = "android")]