Make Wave home config writes atomic and serialized to avoid watcher partial reads (#2945)

`WriteWaveHomeConfigFile()` previously used direct `os.WriteFile`, which
can expose truncation/partial-write states to the JSON file watcher.
This change switches config persistence to temp-file + rename semantics
and serializes writes through a single process-wide lock for config file
writes.

- **Atomic file write helper**
  - Added `AtomicWriteFile()` in `pkg/util/fileutil/fileutil.go`.
- Writes to `<filename>.tmp` in the same directory, then renames to the
target path.
  - Performs temp-file cleanup on error paths.
- Introduced a shared suffix constant (`TempFileSuffix`) used by
implementation/tests.

- **Config write path update**
- Updated `WriteWaveHomeConfigFile()` in `pkg/wconfig/settingsconfig.go`
to:
- Use a package-level mutex (`configWriteLock`) so only one config write
runs at a time (across all config files).
- Call `fileutil.AtomicWriteFile(...)` instead of direct
`os.WriteFile(...)`.

- **Focused coverage for atomic behavior**
  - Added `pkg/util/fileutil/fileutil_test.go` with tests for:
- Successful atomic write (target file contains expected payload and no
leftover `.tmp` file).
    - Rename-failure path cleanup (temp file is removed).

```go
func WriteWaveHomeConfigFile(fileName string, m waveobj.MetaMapType) error {
    configWriteLock.Lock()
    defer configWriteLock.Unlock()

    fullFileName := filepath.Join(wavebase.GetWaveConfigDir(), fileName)
    barr, err := jsonMarshalConfigInOrder(m)
    if err != nil {
        return err
    }
    return fileutil.AtomicWriteFile(fullFileName, barr, 0644)
}
```

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Author: Copilot

pkg/util/fileutil/fileutil.go

@@ -14,10 +14,8 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
-	"time"
 
 	"github.com/wavetermdev/waveterm/pkg/wavebase"
-	"github.com/wavetermdev/waveterm/pkg/wshrpc"
 )
 
 func FixPath(path string) (string, error) {
@@ -145,13 +143,21 @@ func DetectMimeTypeWithDirEnt(path string, dirEnt fs.DirEntry) string {
 	return ""
 }
 
-func AddMimeTypeToFileInfo(path string, fileInfo *wshrpc.FileInfo) {
-	if fileInfo == nil {
-		return
+func AtomicWriteFile(fileName string, data []byte, perm os.FileMode) error {
+	tmpFileName := fileName + TempFileSuffix
+	if err := os.WriteFile(tmpFileName, data, perm); err != nil {
+		if removeErr := os.Remove(tmpFileName); removeErr != nil && !os.IsNotExist(removeErr) {
+			return fmt.Errorf("failed to write temp file %q: %w (also failed to remove temp file: %v)", tmpFileName, err, removeErr)
+		}
+		return err
 	}
-	if fileInfo.MimeType == "" {
-		fileInfo.MimeType = DetectMimeType(path, ToFsFileInfo(fileInfo), false)
+	if err := os.Rename(tmpFileName, fileName); err != nil {
+		if removeErr := os.Remove(tmpFileName); removeErr != nil && !os.IsNotExist(removeErr) {
+			return fmt.Errorf("failed to rename temp file %q to %q: %w (also failed to remove temp file: %v)", tmpFileName, fileName, err, removeErr)
+		}
+		return err
 	}
+	return nil
 }
 
 var (
@@ -203,56 +209,8 @@ func IsInitScriptPath(input string) bool {
 	return true
 }
 
-type FsFileInfo struct {
-	NameInternal    string
-	ModeInternal    os.FileMode
-	SizeInternal    int64
-	ModTimeInternal int64
-	IsDirInternal   bool
-}
-
-func (f FsFileInfo) Name() string {
-	return f.NameInternal
-}
-
-func (f FsFileInfo) Size() int64 {
-	return f.SizeInternal
-}
-
-func (f FsFileInfo) Mode() os.FileMode {
-	return f.ModeInternal
-}
-
-func (f FsFileInfo) ModTime() time.Time {
-	return time.Unix(0, f.ModTimeInternal)
-}
-
-func (f FsFileInfo) IsDir() bool {
-	return f.IsDirInternal
-}
-
-func (f FsFileInfo) Sys() interface{} {
-	return nil
-}
-
-var _ fs.FileInfo = FsFileInfo{}
-
-// ToFsFileInfo converts wshrpc.FileInfo to FsFileInfo.
-// It panics if fi is nil.
-func ToFsFileInfo(fi *wshrpc.FileInfo) FsFileInfo {
-	if fi == nil {
-		panic("ToFsFileInfo: nil FileInfo")
-	}
-	return FsFileInfo{
-		NameInternal:    fi.Name,
-		ModeInternal:    fi.Mode,
-		SizeInternal:    fi.Size,
-		ModTimeInternal: fi.ModTime,
-		IsDirInternal:   fi.IsDir,
-	}
-}
-
 const (
+	TempFileSuffix  = ".tmp"
 	MaxEditFileSize = 5 * 1024 * 1024 // 5MB
 )
 

pkg/util/fileutil/fileutil_test.go

@@ -0,0 +1,45 @@
+package fileutil
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestAtomicWriteFile(t *testing.T) {
+	tmpDir := t.TempDir()
+	fileName := filepath.Join(tmpDir, "settings.json")
+
+	err := AtomicWriteFile(fileName, []byte(`{"key":"value"}`), 0644)
+	if err != nil {
+		t.Fatalf("AtomicWriteFile failed: %v", err)
+	}
+
+	data, err := os.ReadFile(fileName)
+	if err != nil {
+		t.Fatalf("ReadFile failed: %v", err)
+	}
+	if string(data) != `{"key":"value"}` {
+		t.Fatalf("unexpected file contents: %q", string(data))
+	}
+	if _, err := os.Stat(fileName + TempFileSuffix); !os.IsNotExist(err) {
+		t.Fatalf("temporary file should not exist, stat err: %v", err)
+	}
+}
+
+func TestAtomicWriteFileRenameErrorCleansTempFile(t *testing.T) {
+	tmpDir := t.TempDir()
+	fileName := filepath.Join(tmpDir, "settings.json")
+
+	if err := os.Mkdir(fileName, 0755); err != nil {
+		t.Fatalf("Mkdir failed: %v", err)
+	}
+
+	err := AtomicWriteFile(fileName, []byte(`{"key":"value"}`), 0644)
+	if err == nil {
+		t.Fatalf("AtomicWriteFile expected error")
+	}
+	if _, statErr := os.Stat(fileName + TempFileSuffix); !os.IsNotExist(statErr) {
+		t.Fatalf("temporary file should be removed on rename error, stat err: %v", statErr)
+	}
+}

pkg/wconfig/settingsconfig.go

@@ -14,7 +14,9 @@ import (
 	"reflect"
 	"sort"
 	"strings"
+	"sync"
 
+	"github.com/wavetermdev/waveterm/pkg/util/fileutil"
 	"github.com/wavetermdev/waveterm/pkg/util/utilfn"
 	"github.com/wavetermdev/waveterm/pkg/wavebase"
 	"github.com/wavetermdev/waveterm/pkg/waveobj"
@@ -25,6 +27,8 @@ const SettingsFile = "settings.json"
 const ConnectionsFile = "connections.json"
 const ProfilesFile = "profiles.json"
 
+var configWriteLock sync.Mutex
+
 const AnySchema = `
 {
   "type": "object",
@@ -502,13 +506,16 @@ func ReadWaveHomeConfigFile(fileName string) (waveobj.MetaMapType, []ConfigError
 }
 
 func WriteWaveHomeConfigFile(fileName string, m waveobj.MetaMapType) error {
+	configWriteLock.Lock()
+	defer configWriteLock.Unlock()
+
 	configDirAbsPath := wavebase.GetWaveConfigDir()
 	fullFileName := filepath.Join(configDirAbsPath, fileName)
 	barr, err := jsonMarshalConfigInOrder(m)
 	if err != nil {
 		return err
 	}
-	return os.WriteFile(fullFileName, barr, 0644)
+	return fileutil.AtomicWriteFile(fullFileName, barr, 0644)
 }
 
 // simple merge that overwrites