This is the v1 version, had the v2 before.

backend/backendutil/body.go

@@ -0,0 +1,121 @@
+package backendutil
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"mime"
+	nettextproto "net/textproto"
+	"strings"
+
+	"github.com/emersion/go-imap"
+	"github.com/emersion/go-message/textproto"
+)
+
+var errNoSuchPart = errors.New("backendutil: no such message body part")
+
+func multipartReader(header textproto.Header, body io.Reader) *textproto.MultipartReader {
+	contentType := header.Get("Content-Type")
+	if !strings.HasPrefix(strings.ToLower(contentType), "multipart/") {
+		return nil
+	}
+
+	_, params, err := mime.ParseMediaType(contentType)
+	if err != nil {
+		return nil
+	}
+
+	return textproto.NewMultipartReader(body, params["boundary"])
+}
+
+// FetchBodySection extracts a body section from a message.
+func FetchBodySection(header textproto.Header, body io.Reader, section *imap.BodySectionName) (imap.Literal, error) {
+	// First, find the requested part using the provided path
+	for i := 0; i < len(section.Path); i++ {
+		n := section.Path[i]
+
+		mr := multipartReader(header, body)
+		if mr == nil {
+			// First part of non-multipart message refers to the message itself.
+			// See RFC 3501, Page 55.
+			if len(section.Path) == 1 && section.Path[0] == 1 {
+				break
+			}
+			return nil, errNoSuchPart
+		}
+
+		for j := 1; j <= n; j++ {
+			p, err := mr.NextPart()
+			if err == io.EOF {
+				return nil, errNoSuchPart
+			} else if err != nil {
+				return nil, err
+			}
+
+			if j == n {
+				body = p
+				header = p.Header
+
+				break
+			}
+		}
+	}
+
+	// Then, write the requested data to a buffer
+	b := new(bytes.Buffer)
+
+	resHeader := header
+	if section.Fields != nil {
+		// Copy header so we will not change value passed to us.
+		resHeader = header.Copy()
+
+		if section.NotFields {
+			for _, fieldName := range section.Fields {
+				resHeader.Del(fieldName)
+			}
+		} else {
+			fieldsMap := make(map[string]struct{}, len(section.Fields))
+			for _, field := range section.Fields {
+				fieldsMap[nettextproto.CanonicalMIMEHeaderKey(field)] = struct{}{}
+			}
+
+			for field := resHeader.Fields(); field.Next(); {
+				if _, ok := fieldsMap[field.Key()]; !ok {
+					field.Del()
+				}
+			}
+		}
+	}
+
+	// Write the header
+	err := textproto.WriteHeader(b, resHeader)
+	if err != nil {
+		return nil, err
+	}
+
+	switch section.Specifier {
+	case imap.TextSpecifier:
+		// The header hasn't been requested. Discard it.
+		b.Reset()
+	case imap.EntireSpecifier:
+		if len(section.Path) > 0 {
+			// When selecting a specific part by index, IMAP servers
+			// return only the text, not the associated MIME header.
+			b.Reset()
+		}
+	}
+
+	// Write the body, if requested
+	switch section.Specifier {
+	case imap.EntireSpecifier, imap.TextSpecifier:
+		if _, err := io.Copy(b, body); err != nil {
+			return nil, err
+		}
+	}
+
+	var l imap.Literal = b
+	if section.Partial != nil {
+		l = bytes.NewReader(section.ExtractPartial(b.Bytes()))
+	}
+	return l, nil
+}