From: Robert Luberda <robert@debian.org>
Date: Thu, 13 Apr 2017 23:39:39 +0200
Subject: Add MIME headers unless set by user

Generate the three following headers by default:
  MIME-Version: 1.0
  Content-Type: text/plain; charset="<current charset from $LC_CTYPE>"
  Content-Transfer-Encoding: 8bit

However allow a user to override each of them with the -a flag.
Example:
   bsd-mailx -a "Content-Type: text/html; charset=UTF-8"
uses the above user-provided Content-Type, but still adds
the default MIME-Version and Content-Transfer-Encoding headers.

Bugs-Debian: https://bugs.debian.org/859935
---
 mail.1 | 28 +++++++++++++++++++++++++++-
 main.c |  4 ++++
 send.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/mail.1 b/mail.1
index 5683e56..f429557 100644
--- a/mail.1
+++ b/mail.1
@@ -67,7 +67,13 @@ The options are as follows:
 .Bl -tag -width Ds
 .It Fl a
 Specify additional header fields on the command line such as "X-Loop:
-foo@bar" etc.  You have to use quotes if the string contains spaces.
+foo@bar" etc.
+It can be also used to override MIME headers
+.Nm mail
+adds by default to each outgoing mail, see
+.Sx Character sets and MIME
+below.
+You have to use quotes if the string contains spaces.
 This argument may be specified more than once, the headers will then
 be concatenated.
 .It Fl b Ar bcc-addr
@@ -366,6 +372,26 @@ If the
 .Ic expandaddr
 option is not set (the default), no expansion is performed and
 the recipient is treated as a local or network mail address.
+.Ss Character sets and MIME
+Generally
+.Nm mail
+does not handle neither different character sets nor any other MIME
+feature.  Especially it does not perform any any conversions between
+character sets while displaying or sending mails.
+.Pp
+Starting from April 2017, however, as a Debian extension this version of
+.Nm mail
+adds a few MIME headers to every outgoing mail in order to indicate
+that the mail is sent as 8-bit plain text data that uses character
+set encoding detected from the current
+.Xr locale 7
+settings.
+The
+.Fl a
+command-line option can be used to override those headers, for example:
+.Dl $  mail -a 'Content-Type: text/plain; charset="ISO-8859-1"'
+sets header indicating legacy character encoding.
+.Pp
 .Sh SUMMARY
 (Adapted from the
 .Dq Mail Reference Manual . )
diff --git a/main.c b/main.c
index 07ad856..4cba833 100644
--- a/main.c
+++ b/main.c
@@ -34,6 +34,7 @@
 #include <fcntl.h>
 #include <sys/ioctl.h>
 #include "extern.h"
+#include <locale.h>
 
 static void	usage(void);
 	int	main(int, char **);
@@ -70,6 +71,9 @@ main(int argc, char **argv)
           err(1, "setuid");
 	}
 
+	/* Set the current user locale's charset. */
+	setlocale(LC_CTYPE, "");
+
 	/*
 	 * Set up a reasonable environment.
 	 * Figure out whether we are being run interactively,
diff --git a/send.c b/send.c
index cc75f49..771ecc9 100644
--- a/send.c
+++ b/send.c
@@ -30,6 +30,7 @@
  * SUCH DAMAGE.
  */
 
+#include <langinfo.h>
 #include <time.h>
 #include "rcv.h"
 #include "extern.h"
@@ -558,6 +559,36 @@ infix(struct header *hp, FILE *fi)
 	rewind(nfi);
 	return(nfi);
 }
+/*
+ * Check if headers (a list separated by new line characters) contains a header
+ * which name is given in the headername argument of length given in headernamelen.
+ */
+static int hasheader(const char* headers, const char* const headername, const size_t headernamelen)
+{
+	while (headers) {
+		/*  Skip leading white spaces, including '\n' from
+		 *  the previous iteration of the loop
+		 */
+		while (isspace(*headers))
+			++headers;
+
+		if (strncasecmp(headers, headername, headernamelen) == 0) {
+			/* Found a match, check if it is followed by ':' */
+			const char* end = headers + headernamelen;
+			/* Permit optional white spaces before ':' */
+			while (isspace(*end) && *end != '\n')
+				++end;
+
+			if (*end == ':')
+				return 1;
+
+			headers = end; /* Small optimization for the following strchr() call */
+		}
+
+		headers = strchr(headers, '\n'); /* Find start of the next header */
+	}
+	return 0;
+}
 
 /*
  * Dump the to, subject, cc header on the
@@ -583,6 +614,18 @@ puthead(struct header *hp, FILE *fo, int w)
 		fmt("Bcc:", hp->h_bcc, fo, w&GCOMMA), gotcha++;
 	if (hp->h_header != NULL && w)
 		fprintf(fo, "%s\n", hp->h_header), gotcha++;
+	if (w)
+	{
+#define ADDHEADER(name, value, ...) if (!hasheader(hp->h_header, name, sizeof(name) - 1)) \
+				     fprintf(fo, name ": " value "\n", ##__VA_ARGS__), gotcha++
+
+		const char* const cs = nl_langinfo(CODESET);
+		ADDHEADER("MIME-Version", "1.0");
+		ADDHEADER("Content-Type", "text/plain; charset=\"%s\"",
+				cs && *cs ? cs : "ANSI_X3.4-1968");
+		ADDHEADER("Content-Transfer-Encoding", "8bit");
+#undef ADDHEADER
+	}
         if (hp->h_replyto != NULL && w & GREPLYTO)
                 fprintf(fo, "Reply-To: %s\n", hp->h_replyto), gotcha++;
         if (hp->h_inreplyto != NULL && w & GINREPLYTO)