From: David Levine <levinedl@acm.org>
Date: Thu, 10 Jan 2013 02:50:58 +0000 (-0600)
Subject: When read_more() loads more bytes, save some from the end of the
X-Git-Url: https://diplodocus.org/git/nmh/commitdiff_plain/58aa0b9e7f2147bb1150cb9c438e0c7e5230b2da?hp=--cc

When read_more() loads more bytes, save some from the end of the
current chunk and move them to the front of the buffer.  This
allows backing up of the current position, which is necessary when
looking for the message delimiter in a maildrop file.
---

58aa0b9e7f2147bb1150cb9c438e0c7e5230b2da
diff --git a/sbr/m_getfld.c b/sbr/m_getfld.c
index 577030c1..5b9cfc41 100644
--- a/sbr/m_getfld.c
+++ b/sbr/m_getfld.c
@@ -231,10 +231,14 @@ static int (*eom_action)(int) = NULL;
 
 /* This replaces the old approach, which included direct access to
    stdio internals.  It uses one fread() to load a buffer that we
-   manage. */
+   manage.
+   MSG_INPUT_SIZE is the size of the buffer.
+   MAX_DELIMITER_SIZE is the maximum size of the delimiter used to
+   separate messages in a maildrop, such as mbox "\nFrom ". */
 #define MSG_INPUT_SIZE 8192
+#define MAX_DELIMITER_SIZE 32
 static struct m_getfld_buffer {
-    unsigned char msg_buf[2 * MSG_INPUT_SIZE];
+    unsigned char msg_buf[2 * MSG_INPUT_SIZE + MAX_DELIMITER_SIZE];
     unsigned char *readpos;
     unsigned char *end;  /* One past, like C++, the last character read in. */
 } m;
@@ -255,14 +259,25 @@ setup_buffer (FILE *iob, struct m_getfld_buffer *m) {
 
 static size_t
 read_more (struct m_getfld_buffer *m, FILE *iob) {
+    /* Retain at least edelimlen characters that have already been
+       read so that we can back up to them in m_Eom(). */
+    ssize_t retain = edelimlen;
     size_t num_read;
 
-    /* Move any leftover at the end of buf to the beginning. */
     if (m->end > m->readpos) {
-	memmove (m->msg_buf, m->readpos, m->end - m->readpos);
+	if (retain < m->end - m->readpos) retain = m->end - m->readpos;
+    }
+    if (retain > m->readpos - m->msg_buf) {
+	/* Should not happen:  there have been fewer characters read
+	   than are remaining in the buffer. */
+	retain = m->readpos - m->msg_buf;
     }
 
-    m->readpos = m->msg_buf + (m->end - m->readpos);
+    /* Move any leftover at the end of buf to the beginning. */
+    memmove (m->msg_buf, m->readpos - retain, retain);
+
+    m->readpos = m->msg_buf + retain;
+
     num_read = fread (m->readpos, 1, MSG_INPUT_SIZE, iob);
 
     m->end = m->readpos + num_read;
@@ -593,10 +608,10 @@ void
 m_unknown(FILE *iob)
 {
     register int c;
-    unsigned char *pos;
     char text[10];
     register char *cp;
     register char *delimstr;
+    size_t bytes_read = 0;
 
     setup_buffer (iob, &m);
 
@@ -615,8 +630,8 @@ m_unknown(FILE *iob)
 
     msg_style = MS_UNKNOWN;
 
-    pos = m.readpos; /* ftell */
     for (c = 0, cp = text; c < 5; ++c, ++cp) {
+	++bytes_read;
 	if ((*cp = Getc (iob)) == EOF) {
 	    break;
 	}
@@ -625,11 +640,11 @@ m_unknown(FILE *iob)
     if (c == 5  &&  strncmp (text, "From ", 5) == 0) {
 	msg_style = MS_MBOX;
 	delimstr = "\nFrom ";
-	while ((c = Getc (iob)) != '\n' && c >= 0)
+	while (++bytes_read, (c = Getc (iob)) != '\n' && c >= 0)
 	    ;
     } else {
 	/* not a Unix style maildrop */
-	m.readpos = pos; /* fseek (iob, pos, SEEK_SET) */
+	m.readpos -= bytes_read; /* fseek (iob, pos, SEEK_SET), but relative. */
 	if (mmdlm2 == NULL || *mmdlm2 == 0)
 	    mmdlm2 = "\001\001\001\001\n";
 	delimstr = mmdlm2;
@@ -692,13 +707,13 @@ m_eomsbr (int (*action)(int))
 static int
 m_Eom (int c, FILE *iob)
 {
-    unsigned char *pos;
     register int i;
-    char text[10];
+    size_t bytes_read = 0;
+    char text[MAX_DELIMITER_SIZE];
     char *cp;
 
-    pos = m.readpos; /* ftell */
     for (i = 0, cp = text; i < edelimlen; ++i, ++cp) {
+	++bytes_read;
 	if ((*cp = Getc (iob)) == EOF) {
 	    break;
 	}
@@ -710,8 +725,11 @@ m_Eom (int c, FILE *iob)
 	     * maildrop is part of the delimitter - delete it.
 	     */
 	    return 1;
-	m.readpos = pos - 1;	/* fseek (iob, pos - 1, SEEK_SET) */
-	Getc (iob);		/* should be OK */
+
+	/* Did not find delimiter, so restore the read position.
+	   Note that on input, a character had already been read
+	   with Getc().  It will be unget by m_getfld () on return. */
+	m.readpos -= bytes_read;
 	return 0;
     }