The Annoyance Filter.pdf - Fourmilab

122 MAIL FOLDER ANNOYANCE-FILTER §139
139. Each message in a folder begins with a line containing the text “From␣” starting in the first
column. Well, more or less. . . . In the beginning there were BSD mail folders, in which messages were
simply concatenated together with the start of each message indicated by a line beginning with the
“From␣” sentinel. In this scheme, any line in a message body which matches this pattern must be
quoted, usually by inserting a “>” character in column 1, but this is not universal. This was kind of
ugly, and could cause problems when messages began to contain content other than human-readable
text, so then there were Sun message folders, where each message header indicated the number of bytes
in the message with a “Content−Length” header item. You can imagine how disastrous this was in
the typical UNIX environment where people pass mail folders and messages through all kinds of text
filters—’nuff said; better forgotten. These days the most common form of text file mail folder is a
compromise in which the basic BSD scheme is used, but the “From␣” sentinel only designates the start
of a message if it appears following a blank line. This avoids quoting many cases in body copy, while
remaining robust against editing and ease of parsing by simple programs.
If BSDfolder is set, we follow the original BSD semantics and recognise any “From␣” as beginning a
new message. Otherwise, we only treat the sentinel as denoting the start of message if it follows a blank
line or appears at the start of the folder.
Upon finding the start of a message, we increment the number of messages in the folder, mark the
start of a new message, and set the inHeader flag to indicate we’re parsing the header section of the
message.
One complication is that some mail systems which store messages as files in a directory do not include
the “From␣” sentinel at the start of message files. We use the expectingNewMessage flag to cope with
this. This flag gets set at the start of every new file we begin to read (whether a concatenated mail
folder or a file within a directory). When this flag is set, the first nonblank line in the file is considered
the start of message, even if it isn’t the “From␣” sentinel.
#define messageSentinel "From␣" /∗ First line of each message in folder ∗/
〈 Check for start of new message in folder 139 〉 ≡
#ifdef BSD_DIAG
if (s.substr (0, (sizeof messageSentinel ) − 1) ≡ messageSentinel ) {
if (¬BSDfolder ∧ ¬lastLineBlank ) {
cerr ≪ "***␣NonBSD␣From␣line␣ditched:␣" ≪ s ≪ endl ;
}
}
#endif
if (((s.substr (0, (sizeof messageSentinel ) − 1) ≡ messageSentinel ) ∧ (BSDfolder ∨ lastLineBlank )) ∨
(expectingNewMessage ∧ (s.length ( ) > 0) ∧ (¬isISOspace (s[0])))) {
nMessages ++;
newMessage = true ;
expectingNewMessage = false ;
inHeader = true ;
multiPart = false ;
inPartHeader = false ;
partHeaderLines = 0;
bodyContentType = bodyContentTypeCharset = bodyContentTypeName =
bodyContentTransferEncoding = "";
fromLine = s; /∗ Save last “From␣” line for diagnostics ∗/
lastFromLine = lastMessageID = messageID = "";
while (¬partBoundaryStack .empty ( )) {
ostringstream os ;
os ≪ "Orphaned␣part␣boundary␣on␣stack:␣\"" ≪ partBoundaryStack .top( ) ≪ "\"";
reportParserDiagnostic(os );
partBoundaryStack .pop( );