input: Allow two consecutive calls to pungetc

The commit ef91d3d6 ([PARSER]
Handle backslash newlines properly after dollar sign) created
cases where we make two consecutive calls to pungetc.  As we
don't explicitly support that there are corner cases where you
end up with garbage input leading to undefined behaviour.

This patch adds explicit support for two consecutive calls to
pungetc.

Reported-by: Jilles Tjoelker <jilles@stack.nl>
Reported-by: Juergen Daubert <jue@jue.li>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/input.c

@@ -102,10 +102,20 @@ RESET {
 int
 pgetc(void)
 {
+	int c;
+
+	if (parsefile->unget)
+		return parsefile->lastc[--parsefile->unget];
+
 	if (--parsefile->nleft >= 0)
-		return (signed char)*parsefile->nextc++;
+		c = (signed char)*parsefile->nextc++;
 	else
-		return preadbuffer();
+		c = preadbuffer();
+
+	parsefile->lastc[1] = parsefile->lastc[0];
+	parsefile->lastc[0] = c;
+
+	return c;
 }
@@ -194,7 +204,7 @@ static int preadbuffer(void)
 #endif
 	char savec;
-	while (unlikely(parsefile->strpush)) {
+	if (unlikely(parsefile->strpush)) {
 		if (
 			parsefile->nleft == -1 &&
 			parsefile->strpush->ap &&
@@ -204,8 +214,7 @@ static int preadbuffer(void)
 			return PEOA;
 		}
 		popstring();
-		if (--parsefile->nleft >= 0)
-			return (signed char)*parsefile->nextc++;
+		return pgetc();
 	}
 	if (unlikely(parsefile->nleft == EOF_NLEFT ||
 		     parsefile->buf == NULL))
@@ -290,15 +299,14 @@ again:
 }
 /*
- * Undo the last call to pgetc.  Only one character may be pushed back.
+ * Undo a call to pgetc.  Only two characters may be pushed back.
  * PEOF may be pushed back.
  */
 void
 pungetc(void)
 {
-	parsefile->nleft++;
-	parsefile->nextc--;
+	parsefile->unget++;
 }
 /*
@@ -322,6 +330,8 @@ pushstring(char *s, void *ap)
 		sp = parsefile->strpush = &(parsefile->basestrpush);
 	sp->prevstring = parsefile->nextc;
 	sp->prevnleft = parsefile->nleft;
+	sp->unget = parsefile->unget;
+	memcpy(sp->lastc, parsefile->lastc, sizeof(sp->lastc));
 	sp->ap = (struct alias *)ap;
 	if (ap) {
 		((struct alias *)ap)->flag |= ALIASINUSE;
@@ -329,6 +339,7 @@ pushstring(char *s, void *ap)
 	}
 	parsefile->nextc = s;
 	parsefile->nleft = len;
+	parsefile->unget = 0;
 	INTON;
 }
@@ -353,6 +364,8 @@ popstring(void)
 	}
 	parsefile->nextc = sp->prevstring;
 	parsefile->nleft = sp->prevnleft;
+	parsefile->unget = sp->unget;
+	memcpy(parsefile->lastc, sp->lastc, sizeof(sp->lastc));
 /*dprintf("*** calling popstring: restoring to '%s'\n", parsenextc);*/
 	parsefile->strpush = sp->prev;
 	if (sp != &(parsefile->basestrpush))
@@ -439,6 +452,7 @@ pushfile(void)
 	pf->fd = -1;
 	pf->strpush = NULL;
 	pf->basestrpush.prev = NULL;
+	pf->unget = 0;
 	parsefile = pf;
 }

src/input.h

@@ -49,6 +49,12 @@ struct strpush {
 	int prevnleft;
 	struct alias *ap;	/* if push was associated with an alias */
 	char *string;		/* remember the string since it may change */
+
+	/* Remember last two characters for pungetc. */
+	int lastc[2];
+
+	/* Number of outstanding calls to pungetc. */
+	int unget;
 };
 /*
@@ -66,6 +72,12 @@ struct parsefile {
 	char *buf;		/* input buffer */
 	struct strpush *strpush; /* for pushing strings at this level */
 	struct strpush basestrpush; /* so pushing one is fast */
+
+	/* Remember last two characters for pungetc. */
+	int lastc[2];
+
+	/* Number of outstanding calls to pungetc. */
+	int unget;
 };
 extern struct parsefile *parsefile;