Added custom formatter for the Newsgroups: header.

Fixes issue #162

gmime/gmime-header.c

@@ -75,6 +75,7 @@ static struct {
  { "Content-Disposition", g_mime_header_format_content_disposition },
  { "Content-Id", g_mime_header_format_message_id },
  { "Disposition-Notification-To", g_mime_header_format_addrlist },
+ { "Newsgroups", g_mime_header_format_newsgroups },
 };
 
 
@@ -905,6 +906,50 @@ g_mime_header_format_received (GMimeHeader *header, GMimeFormatOptions *options,
 }
 
 
+/**
+ * g_mime_header_format_newsgroups:
+ * @header: a #GMimeHeader
+ * @options: (nullable): a #GMimeFormatOptions or %NULL
+ * @value: a Newsgroups header value
+ * @charset: a charset (note: unused)
+ *
+ * Parses the @value and then re-formats it to conform to the formatting options,
+ * folding the value if necessary.
+ *
+ * Returns: a newly allocated string containing the reformatted value.
+ **/
+char *
+g_mime_header_format_newsgroups (GMimeHeader *header, GMimeFormatOptions *options, const char *value, const char *charset)
+{
+ /* https://www.rfc-editor.org/rfc/rfc5536#section-3.1.4
+ *
+ * The Newsgroups header field specifies the newsgroup(s) to which the
+ * article is posted.
+ *
+ * newsgroups = "Newsgroups:" SP newsgroup-list CRLF
+ *
+ * newsgroup-list = *WSP newsgroup-name
+ * *( [FWS] "," [FWS] newsgroup-name ) *WSP
+ *
+ * newsgroup-name = component *( "." component )
+ *
+ * component = 1*component-char
+ *
+ * component-char = ALPHA / DIGIT / "+" / "-" / "_"
+ *
+ * Not all servers support optional FWS in the list of newsgroups. In
+ * particular, folding the Newsgroups header field over several lines
+ * has been shown to harm propagation significantly. Optional FWS in
+ * the <newsgroup-list> SHOULD NOT be generated, but MUST be accepted.
+ *
+ * Based on this, it seems better to avoid reformatting the value at all.
+ */
+ const char *newline = g_mime_format_options_get_newline (options);
+
+ return g_strdup_printf (" %s%s", value, newline);
+}
+
+
 /**
  * g_mime_header_format_default:
  * @header: a #GMimeHeader

gmime/gmime-header.h

@@ -70,6 +70,7 @@ char *g_mime_header_format_message_id (GMimeHeader *header, GMimeFormatOptions *
 char *g_mime_header_format_references (GMimeHeader *header, GMimeFormatOptions *options, const char *value, const char *charset);
 char *g_mime_header_format_addrlist (GMimeHeader *header, GMimeFormatOptions *options, const char *value, const char *charset);
 char *g_mime_header_format_received (GMimeHeader *header, GMimeFormatOptions *options, const char *value, const char *charset);
+char *g_mime_header_format_newsgroups (GMimeHeader *header, GMimeFormatOptions *options, const char *value, const char *charset);
 char *g_mime_header_format_default (GMimeHeader *header, GMimeFormatOptions *options, const char *value, const char *charset);