volume: return error instead of panicking when .dat open fails (#11619)

* volume: return error instead of panicking when .dat open fails

When backend.OpenVolumeFile returns an error (e.g. disk below
-minFreeSpace), dataFile is nil.  Calling backend.NewDiskFile(nil)
immediately after caused a nil-pointer panic in f.Stat()/f.Name().

Move the existing error check to run right after OpenVolumeFile, before
NewDiskFile is called, so the error is returned cleanly.

Fixes seaweedfs/seaweedfs#11615

* volume: share .dat load error handling

Extract datFileLoadError helper so open and create paths share one check.

* volume: trim dat-open-fail test comments

* volume(rust): cover unopenable .dat load path

---------

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

seaweed-volume/src/storage/volume.rs

@@ -9687,6 +9687,31 @@ mod tests {
         }
     }
 
+    // A directory occupying the .dat path fails open even as root; load must
+    // return the error. Mirrors Go's TestLoad_DatOpenFail_NoNilPanic.
+    #[test]
+    fn test_load_dat_open_fail_returns_error() {
+        let tmp = TempDir::new().unwrap();
+        let dir = tmp.path().to_str().unwrap();
+
+        {
+            let _v = make_test_volume(dir);
+        }
+
+        let dat_path = format!("{dir}/1.dat");
+        std::fs::remove_file(&dat_path).unwrap();
+        std::fs::create_dir(&dat_path).unwrap();
+
+        let result = Volume::new(
+            dir,
+            dir,
+            VolumeId(1),
+            NeedleMapKind::InMemory,
+            &VolumeSpec::default(),
+        );
+        assert!(result.is_err(), "unopenable .dat must fail load");
+    }
+
     #[test]
     fn test_remote_only_volume_load_reads_from_tier_backend() {
         let tmp = TempDir::new().unwrap();

weed/storage/volume_loading.go

@@ -198,6 +198,9 @@ func (v *Volume) load(alsoLoadIndex bool, createDatIfMissing bool, needleMapKind
 			dataFile, err = backend.OpenVolumeFile(v.FileName(".dat"), os.O_RDONLY)
 			v.noWriteOrDelete = true
 		}
+		if err != nil {
+			return datFileLoadError(v.FileName(".dat"), err)
+		}
 		v.lastModifiedTsSeconds = uint64(modifiedTime.Unix())
 		if fileSize >= super_block.SuperBlockSize {
 			alreadyHasSuperBlock = true
@@ -212,11 +215,7 @@ func (v *Volume) load(alsoLoadIndex bool, createDatIfMissing bool, needleMapKind
 	}
 
 	if err != nil {
-		if !os.IsPermission(err) {
-			return fmt.Errorf("cannot load volume data %s: %v", v.FileName(".dat"), err)
-		} else {
-			return fmt.Errorf("load data file %s: %v", v.FileName(".dat"), err)
-		}
+		return datFileLoadError(v.FileName(".dat"), err)
 	}
 
 	if alreadyHasSuperBlock {
@@ -417,3 +416,10 @@ func (v *Volume) load(alsoLoadIndex bool, createDatIfMissing bool, needleMapKind
 
 	return err
 }
+
+func datFileLoadError(fileName string, err error) error {
+	if os.IsPermission(err) {
+		return fmt.Errorf("load data file %s: %v", fileName, err)
+	}
+	return fmt.Errorf("cannot load volume data %s: %v", fileName, err)
+}

weed/storage/volume_loading_open_fail_test.go

@@ -0,0 +1,35 @@
+package storage
+
+import (
+	"os"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
+)
+
+// A directory occupying the .dat path fails open even as root; load must
+// return the error rather than crash inside backend.NewDiskFile.
+func TestLoad_DatOpenFail_NoNilPanic(t *testing.T) {
+	dir := t.TempDir()
+
+	v, err := NewVolume(dir, dir, "", 1, NeedleMapInMemory, &super_block.ReplicaPlacement{}, &needle.TTL{}, 0, needle.GetCurrentVersion(), 0, 0)
+	if err != nil {
+		t.Fatalf("create volume: %v", err)
+	}
+	v.Close()
+
+	datPath := VolumeFileName(dir, "", 1) + ".dat"
+	if err := os.Remove(datPath); err != nil {
+		t.Fatalf("remove .dat: %v", err)
+	}
+	if err := os.Mkdir(datPath, 0755); err != nil {
+		t.Fatalf("mkdir .dat: %v", err)
+	}
+
+	v2, err := NewVolume(dir, dir, "", 1, NeedleMapInMemory, &super_block.ReplicaPlacement{}, &needle.TTL{}, 0, needle.GetCurrentVersion(), 0, 0)
+	if err == nil {
+		v2.Close()
+		t.Fatal("expected error when .dat cannot be opened, got nil")
+	}
+}