Fix: prevent panic when swap file creation fails (#7957)

* Fix: prevent panic when swap file creation fails

* weed mount: fix race condition in swap file initialization

Ensure thread-safe access to sf.file and other state in NewSwapFileChunk
and FreeResource by using sf.chunkTrackingLock consistently. Also
set sf.file to nil after closing to prevent reuse.

* weed mount: improve swap directory creation logic

- Check error for os.MkdirAll and log it if it fails.
- Use 0700 permissions for the swap directory for better security.
- Improve error logging context.

* weed mount: add unit tests for swap file creation

Add tests to verify:
- Concurrent initialization of the swap file.
- Correct directory permissions (0700).
- Automatic directory recreation if deleted.

* weed mount: fix thread-safety in swap file unit tests

Use atomic.Uint32 to track failures within goroutines in
TestSwapFile_NewSwapFileChunk_Concurrent to avoid unsafe calls to
t.Errorf from multiple goroutines.

* weed mount: simplify swap file creation logic

Refactor the directory check and retry logic for better readability and
to avoid re-using the main error variable for directory creation errors.
Remove redundant error logging.

* weed mount: improve error checking in swap file tests

Explicitly check if NewSwapFileChunk returns nil to provide more
informative failures.

* weed mount: update DirtyPages interface to return error

Propagate errors from SaveDataAt when swap file creation fails. This
prevents potential panics in the write path.

* weed mount: handle AddPage errors in write paths

Update ChunkedDirtyPages and PageWriter to propagate errors and update
WFS.Write and WFS.CopyFileRange to return fuse.EIO on failure.

* weed mount: update swap directory creation error message

Change "recreate" to "create/recreate" to better reflect that this path
is also taken during the initial creation of the swap directory.

---------

Co-authored-by: lixiang58 <lixiang58@lenovo.com>
Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/mount/page_writer/page_chunk_swapfile.go

@@ -1,12 +1,13 @@
 package page_writer
 
 import (
-	"github.com/seaweedfs/seaweedfs/weed/glog"
-	"github.com/seaweedfs/seaweedfs/weed/util"
-	"github.com/seaweedfs/seaweedfs/weed/util/mem"
 	"io"
 	"os"
 	"sync"
+
+	"github.com/seaweedfs/seaweedfs/weed/glog"
+	"github.com/seaweedfs/seaweedfs/weed/util"
+	"github.com/seaweedfs/seaweedfs/weed/util/mem"
 )
 
 var (
@@ -42,23 +43,34 @@ func NewSwapFile(dir string, chunkSize int64) *SwapFile {
 	}
 }
 func (sf *SwapFile) FreeResource() {
+	sf.chunkTrackingLock.Lock()
+	defer sf.chunkTrackingLock.Unlock()
 	if sf.file != nil {
 		sf.file.Close()
 		os.Remove(sf.file.Name())
+		sf.file = nil
 	}
 }
 
 func (sf *SwapFile) NewSwapFileChunk(logicChunkIndex LogicChunkIndex) (tc *SwapFileChunk) {
+	sf.chunkTrackingLock.Lock()
+	defer sf.chunkTrackingLock.Unlock()
+
 	if sf.file == nil {
 		var err error
 		sf.file, err = os.CreateTemp(sf.dir, "")
+		if os.IsNotExist(err) {
+			if mkdirErr := os.MkdirAll(sf.dir, 0700); mkdirErr != nil {
+				glog.Errorf("create/recreate swap directory %s: %v", sf.dir, mkdirErr)
+				return nil
+			}
+			sf.file, err = os.CreateTemp(sf.dir, "")
+		}
 		if err != nil {
-			glog.Errorf("create swap file: %v", err)
+			glog.Errorf("create swap file in %s: %v", sf.dir, err)
 			return nil
 		}
 	}
-	sf.chunkTrackingLock.Lock()
-	defer sf.chunkTrackingLock.Unlock()
 
 	sf.activeChunkCount++