Add AttachFromIOFS and EmbedFromIOFS functions

Introduce new methods AttachFromIOFS and EmbedFromIOFS to handle attachments and embeds from a general file system (fs.FS). Updated tests to cover these new functionalities and modified error messages for consistency. Updated README to reflect support for fs.FS.

README.md

@@ -52,7 +52,7 @@ Here are some highlights of go-mail's featureset:
 * [X] RFC5322 compliant mail address validation
 * [X] Support for common mail header field generation (Message-ID, Date, Bulk-Precedence, Priority, etc.)
 * [X] Concurrency-safe reusing the same SMTP connection to send multiple mails
-* [X] Support for attachments and inline embeds (from file system, `io.Reader` or `embed.FS`)
+* [X] Support for attachments and inline embeds (from file system, `io.Reader`, `embed.FS` or `fs.FS`)
 * [X] Support for different encodings
 * [X] Middleware support for 3rd-party libraries to alter mail messages
 * [X] Support sending mails via a local sendmail command

msg.go

@@ -1963,9 +1963,28 @@ func (m *Msg) AttachTextTemplate(
 //   - https://datatracker.ietf.org/doc/html/rfc2183
 func (m *Msg) AttachFromEmbedFS(name string, fs *embed.FS, opts ...FileOption) error {
 	if fs == nil {
-		return fmt.Errorf("embed.FS must not be nil")
+		return errors.New("embed.FS must not be nil")
 	}
-	file, err := fileFromIOFS(name, fs)
+	return m.AttachFromIOFS(name, *fs, opts...)
+}
+
+// AttachFromIOFS attaches a file from a generic file system to the message.
+//
+// This function retrieves a file by name from an fs.FS instance, processes it, and appends it to the
+// message's attachment collection. Additional file options can be provided for further customization.
+//
+// Parameters:
+//   - name: The name of the file to retrieve from the file system.
+//   - iofs: The file system (must not be nil).
+//   - opts: Optional file options to customize the attachment process.
+//
+// Returns:
+//   - An error if the file cannot be retrieved, the fs.FS is nil, or any other issue occurs.
+func (m *Msg) AttachFromIOFS(name string, iofs fs.FS, opts ...FileOption) error {
+	if iofs == nil {
+		return errors.New("fs.FS must not be nil")
+	}
+	file, err := fileFromIOFS(name, iofs)
 	if err != nil {
 		return err
 	}
@@ -2109,9 +2128,28 @@ func (m *Msg) EmbedTextTemplate(
 //   - https://datatracker.ietf.org/doc/html/rfc2183
 func (m *Msg) EmbedFromEmbedFS(name string, fs *embed.FS, opts ...FileOption) error {
 	if fs == nil {
-		return fmt.Errorf("embed.FS must not be nil")
+		return errors.New("embed.FS must not be nil")
 	}
-	file, err := fileFromIOFS(name, fs)
+	return m.EmbedFromIOFS(name, *fs, opts...)
+}
+
+// EmbedFromIOFS embeds a file from a generic file system into the message.
+//
+// This function retrieves a file by name from an fs.FS instance, processes it, and appends it to the
+// message's embed collection. Additional file options can be provided for further customization.
+//
+// Parameters:
+//   - name: The name of the file to retrieve from the file system.
+//   - iofs: The file system (must not be nil).
+//   - opts: Optional file options to customize the embedding process.
+//
+// Returns:
+//   - An error if the file cannot be retrieved, the fs.FS is nil, or any other issue occurs.
+func (m *Msg) EmbedFromIOFS(name string, iofs fs.FS, opts ...FileOption) error {
+	if iofs == nil {
+		return errors.New("fs.FS must not be nil")
+	}
+	file, err := fileFromIOFS(name, iofs)
 	if err != nil {
 		return err
 	}
@@ -2684,22 +2722,26 @@ func (m *Msg) addDefaultHeader() {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc2183
 func fileFromIOFS(name string, iofs fs.FS) (*File, error) {
+	if iofs == nil {
+		return nil, errors.New("fs.FS is nil")
+	}
+
 	_, err := iofs.Open(name)
 	if err != nil {
-		return nil, fmt.Errorf("failed to open file from embed.FS: %w", err)
+		return nil, fmt.Errorf("failed to open file from fs.FS: %w", err)
 	}
 	return &File{
 		Name:   filepath.Base(name),
 		Header: make(map[string][]string),
 		Writer: func(writer io.Writer) (int64, error) {
-			file, err := iofs.Open(name)
-			if err != nil {
-				return 0, err
+			file, ferr := iofs.Open(name)
+			if ferr != nil {
+				return 0, fmt.Errorf("failed to open file from fs.FS: %w", ferr)
 			}
-			numBytes, err := io.Copy(writer, file)
-			if err != nil {
+			numBytes, ferr := io.Copy(writer, file)
+			if ferr != nil {
 				_ = file.Close()
-				return numBytes, fmt.Errorf("failed to copy file to io.Writer: %w", err)
+				return numBytes, fmt.Errorf("failed to copy file from fs.FS to io.Writer: %w", ferr)
 			}
 			return numBytes, file.Close()
 		},

msg_test.go

@@ -4970,6 +4970,75 @@ func TestMsg_AttachFromEmbedFS(t *testing.T) {
 	})
 }
 
+func TestMsg_AttachFromIOFS(t *testing.T) {
+	t.Run("AttachFromIOFS successful", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.AttachFromIOFS("testdata/attachment.txt", efs,
+			WithFileName("attachment.txt")); err != nil {
+			t.Fatalf("failed to attach from embed FS: %s", err)
+		}
+		attachments := message.GetAttachments()
+		if len(attachments) != 1 {
+			t.Fatalf("failed to retrieve attachments list")
+		}
+		if attachments[0] == nil {
+			t.Fatal("expected attachment to be not nil")
+		}
+		if attachments[0].Name != "attachment.txt" {
+			t.Errorf("expected attachment name to be %s, got: %s", "attachment.txt", attachments[0].Name)
+		}
+		messageBuf := bytes.NewBuffer(nil)
+		_, err := attachments[0].Writer(messageBuf)
+		if err != nil {
+			t.Errorf("writer func failed: %s", err)
+		}
+		got := strings.TrimSpace(messageBuf.String())
+		if !strings.EqualFold(got, "This is a test attachment") {
+			t.Errorf("expected message body to be %s, got: %s", "This is a test attachment", got)
+		}
+	})
+	t.Run("AttachFromIOFS with invalid path", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		err := message.AttachFromIOFS("testdata/invalid.txt", efs, WithFileName("attachment.txt"))
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+	})
+	t.Run("AttachFromIOFS with nil embed FS", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		err := message.AttachFromIOFS("testdata/invalid.txt", nil, WithFileName("attachment.txt"))
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+	})
+	t.Run("AttachFromIOFS with fs.FS fails on copy", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.AttachFromIOFS("testdata/attachment.txt", efs); err != nil {
+			t.Fatalf("failed to attach file from fs.FS: %s", err)
+		}
+		attachments := message.GetAttachments()
+		if len(attachments) != 1 {
+			t.Fatalf("failed to get attachments, expected 1, got: %d", len(attachments))
+		}
+		_, err := attachments[0].Writer(failReadWriteSeekCloser{})
+		if err == nil {
+			t.Error("writer func expected to fail, but didn't")
+		}
+	})
+}
+
 func TestMsg_EmbedFile(t *testing.T) {
 	t.Run("EmbedFile with file", func(t *testing.T) {
 		message := NewMsg()
@@ -5435,6 +5504,58 @@ func TestMsg_EmbedFromEmbedFS(t *testing.T) {
 	})
 }
 
+func TestMsg_EmbedFromIOFS(t *testing.T) {
+	t.Run("EmbedFromIOFS successful", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		if err := message.EmbedFromIOFS("testdata/embed.txt", efs,
+			WithFileName("embed.txt")); err != nil {
+			t.Fatalf("failed to embed from embed FS: %s", err)
+		}
+		embeds := message.GetEmbeds()
+		if len(embeds) != 1 {
+			t.Fatalf("failed to retrieve embeds list")
+		}
+		if embeds[0] == nil {
+			t.Fatal("expected embed to be not nil")
+		}
+		if embeds[0].Name != "embed.txt" {
+			t.Errorf("expected embed name to be %s, got: %s", "embed.txt", embeds[0].Name)
+		}
+		messageBuf := bytes.NewBuffer(nil)
+		_, err := embeds[0].Writer(messageBuf)
+		if err != nil {
+			t.Errorf("writer func failed: %s", err)
+		}
+		got := strings.TrimSpace(messageBuf.String())
+		if !strings.EqualFold(got, "This is a test embed") {
+			t.Errorf("expected message body to be %s, got: %s", "This is a test embed", got)
+		}
+	})
+	t.Run("EmbedFromIOFS with invalid path", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		err := message.EmbedFromIOFS("testdata/invalid.txt", efs, WithFileName("embed.txt"))
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+	})
+	t.Run("EmbedFromIOFS with nil embed FS", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		err := message.EmbedFromIOFS("testdata/invalid.txt", nil, WithFileName("embed.txt"))
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+	})
+}
+
 func TestMsg_Reset(t *testing.T) {
 	message := NewMsg()
 	if message == nil {
@@ -6440,6 +6561,15 @@ func TestMsg_addDefaultHeader(t *testing.T) {
 	})
 }
 
+func TestMsg_fileFromIOFS(t *testing.T) {
+	t.Run("file from fs.FS where fs is nil ", func(t *testing.T) {
+		_, err := fileFromIOFS("testfile.txt", nil)
+		if err == nil {
+			t.Fatal("expected error for fs.FS that is nil")
+		}
+	})
+}
+
 // uppercaseMiddleware is a middleware type that transforms the subject to uppercase.
 type uppercaseMiddleware struct{}