feature: support deleting efi variables.

this is potentially VERY dangerous btw

efi/efivario/context_windows.go

@@ -86,6 +86,22 @@ func (it *varNameIterator) Err() error {
 	return it.err
 }
 
+func convertNameGuid(name string, guid efiguid.GUID) (lpName, lpGuid *uint16, err error) {
+	lpName, err = syscall.UTF16PtrFromString(name)
+	if err != nil {
+		err = fmt.Errorf("utf16(%q): %w", name, err)
+		return
+	}
+
+	lpGuid, err = syscall.UTF16PtrFromString(guid.Braced())
+	if err != nil {
+		err = fmt.Errorf("utf16(%q): %w", guid, err)
+		return
+	}
+
+	return
+}
+
 // WindowsContext provides an implementation of the Context API
 // for the windows platform.
 type WindowsContext struct{}
@@ -131,15 +147,9 @@ func (c WindowsContext) GetSizeHint(name string, guid efiguid.GUID) (int64, erro
 }
 
 func (c WindowsContext) Get(name string, guid efiguid.GUID, out []byte) (a Attributes, n int, err error) {
-	lpName, err := syscall.UTF16PtrFromString(name)
+	lpName, lpGuid, err := convertNameGuid(name, guid)
 	if err != nil {
-		err = fmt.Errorf("efivario/Get: utf16(name): %w", err)
-		return
-	}
-
-	lpGuid, err := syscall.UTF16PtrFromString(guid.Braced())
-	if err != nil {
-		err = fmt.Errorf("efivario/Get: utf16(guid): %w", err)
+		err = fmt.Errorf("efivario/Get: %w", err)
 		return
 	}
 
@@ -159,19 +169,27 @@ func (c WindowsContext) Get(name string, guid efiguid.GUID, out []byte) (a Attri
 }
 
 func (c WindowsContext) Set(name string, guid efiguid.GUID, attributes Attributes, value []byte) error {
-	lpName, err := syscall.UTF16PtrFromString(name)
+	lpName, lpGuid, err := convertNameGuid(name, guid)
 	if err != nil {
-		return fmt.Errorf("efivario/Set: utf16(%q): %w", name, err)
+		return fmt.Errorf("efivario/Set: %w", err)
 	}
 
-	lpGuid, err := syscall.UTF16PtrFromString(guid.Braced())
+	err = efiwindows.SetFirmwareEnvironmentVariableEx(lpName, lpGuid, value, (uint32)(attributes))
 	if err != nil {
-		return fmt.Errorf("efivario/Set: utf16(%q): %w", guid, err)
+		return fmt.Errorf("efivario/Set: %w", err)
 	}
+	return nil
+}
 
-	err = efiwindows.SetFirmwareEnvironmentVariableEx(lpName, lpGuid, value, (uint32)(attributes))
+func (c WindowsContext) Delete(name string, guid efiguid.GUID) error {
+	lpName, lpGuid, err := convertNameGuid(name, guid)
 	if err != nil {
-		return fmt.Errorf("efivario/Set: %w", err)
+		return fmt.Errorf("efivario/Delete: %w", err)
+	}
+
+	err = efiwindows.SetFirmwareEnvironmentVariableEx(lpName, lpGuid, nil, 0)
+	if err != nil {
+		return fmt.Errorf("efivario/Delete: %w", err)
 	}
 	return nil
 }

efi/efivario/contextbase.go

@@ -52,6 +52,10 @@ type Context interface {
 	// Set writes a specific EFI variable.
 	Set(name string, guid efiguid.GUID, attrs Attributes, value []byte) error
 
+	// Delete causes the entire variable to be removed from
+	// existence completely.
+	Delete(name string, guid efiguid.GUID) error
+
 	// VariableNames returns an Iterator which enumerates all
 	// EFI variables that are currently set on the current system.
 	VariableNames() (VariableNameIterator, error)

efi/efivario/fscontext.go

@@ -204,6 +204,30 @@ func (c FsContext) writeEfiVarFileName(name string, value []byte, attrs Attribut
 	return nil
 }
 
+func (c FsContext) deleteEfiFile(name string) error {
+	guard, err := openSafeguard(c.fs, name)
+	if err != nil {
+		return fmt.Errorf("efivario/delete: guard open: %w", err)
+	}
+	if guard != nil {
+		defer multierr.AppendInvoke(&err, multierr.Invoke(func() error {
+			if err := guard.Close(); err != nil {
+				return fmt.Errorf("efivario/set: guard close: %w", err)
+			}
+			return nil
+		}))
+	}
+
+	if _, err := guard.disable(); err != nil {
+		return fmt.Errorf("efivario/delete: guard disable: %w", err)
+	}
+
+	if err := c.fs.Remove(name); err != nil {
+		return fmt.Errorf("efivario/delete: remove: %w", err)
+	}
+	return nil
+}
+
 func (c FsContext) GetSizeHint(name string, guid efiguid.GUID) (int64, error) {
 	fi, err := c.fs.Stat(getFileName(name, guid))
 	if err != nil {
@@ -220,6 +244,10 @@ func (c FsContext) Set(name string, guid efiguid.GUID, attributes Attributes, va
 	return c.writeEfiVarFileName(getFileName(name, guid), value, attributes)
 }
 
+func (c FsContext) Delete(name string, guid efiguid.GUID) error {
+	return c.deleteEfiFile(getFileName(name, guid))
+}
+
 func NewFileSystemContext(fs afero.Fs) *FsContext {
 	return &FsContext{fs: fs}
 }

efi/efivario/fscontext_test.go

@@ -89,6 +89,22 @@ func (s *FsContextTestSuite) TestSetExistingVariable() {
 	require.NoError(s.T(), err)
 }
 
+// TestDelete tests deleting an existing variable.
+func (s *FsContextTestSuite) TestDelete() {
+	// given that ...
+	f, err := s.context.fs.Create("TestVar-3CD99F3F-4B2B-43EB-AC29-F0890A4772B7")
+	require.NoError(s.T(), err)
+	defer func() { require.NoError(s.T(), f.Close()) }()
+
+	// when ...
+	err = s.context.Delete("TestVar", testGuid)
+	require.NoError(s.T(), err)
+
+	// then ...
+	_, err = s.context.fs.Stat("TestVar-3CD99F3F-4B2B-43EB-AC29-F0890A4772B7")
+	require.True(s.T(), errors.Is(err, afero.ErrFileNotFound))
+}
+
 // In order for 'go test' to run this suite, we need to create
 // a normal test function and pass our suite to suite.Run
 func TestFsContextTestSuite(t *testing.T) {