mailinfo: handle charset conversion errors in the caller

Instead of dying in convert_to_utf8(), just report an error and let
the callers handle it.  Between the two callers:

 - decode_header() silently punts when it cannot parse a broken
   RFC2047 encoded text (e.g. when it sees anything other than B or
   Q after it sees "=?<charset>") by jumping to release_return,
   returning the string it successfully parsed out so far, to the
   caller.  A piece of string that convert_to_utf8() cannot handle
   can be treated the same way.

 - handle_commit_msg() doesn't cope with a malformed line well, so
   die there for now.  We'll lift this even higher in later changes
   in this series.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

mailinfo.c

@@ -344,21 +344,22 @@ static struct strbuf *decode_b_segment(const struct strbuf *b_seg)
 	return out;
 }
 
-static void convert_to_utf8(struct mailinfo *mi,
-			    struct strbuf *line, const char *charset)
+static int convert_to_utf8(struct mailinfo *mi,
+			   struct strbuf *line, const char *charset)
 {
 	char *out;
 
 	if (!mi->metainfo_charset || !charset || !*charset)
-		return;
+		return 0;
 
 	if (same_encoding(mi->metainfo_charset, charset))
-		return;
+		return 0;
 	out = reencode_string(line->buf, mi->metainfo_charset, charset);
 	if (!out)
-		die("cannot convert from %s to %s",
-		    charset, mi->metainfo_charset);
+		return error("cannot convert from %s to %s",
+			     charset, mi->metainfo_charset);
 	strbuf_attach(line, out, strlen(out), strlen(out));
+	return 0;
 }
 
 static void decode_header(struct mailinfo *mi, struct strbuf *it)
@@ -424,7 +425,8 @@ static void decode_header(struct mailinfo *mi, struct strbuf *it)
 			dec = decode_q_segment(&piecebuf, 1);
 			break;
 		}
-		convert_to_utf8(mi, dec, charset_q.buf);
+		if (convert_to_utf8(mi, dec, charset_q.buf))
+			goto release_return;
 
 		strbuf_addbuf(&outbuf, dec);
 		strbuf_release(dec);
@@ -637,7 +639,8 @@ static int handle_commit_msg(struct mailinfo *mi, struct strbuf *line)
 		mi->header_stage = 0;
 
 	/* normalize the log message to UTF-8. */
-	convert_to_utf8(mi, line, mi->charset.buf);
+	if (convert_to_utf8(mi, line, mi->charset.buf))
+		exit(128);
 
 	if (mi->use_scissors && is_scissors_line(line)) {
 		int i;