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exportfs: fix endless loop for eof/error on second read in localread9pmsg()

the ini buffer was not reset when we got eof in the message
body read causing a endless loop. instead of defining our
own read9pmsg() function, just handle the first read specially
when we consumed the first 4 bytes for the "impo" protocol
escape check.

--- a/sys/src/cmd/exportfs/exportfs.c

+++ b/sys/src/cmd/exportfs/exportfs.c

@@ -51,7 +51,6 @@

 int	readonly;

 static void mksecret(char *, uchar *);

-static int localread9pmsg(int, void *, uint, void *);

 static char *anstring  = "tcp!*!0";

 char *netdir = "", *local = "", *remote = "";

@@ -391,60 +390,37 @@

 	if(ai != nil)

 		auth_freeAI(ai);

+	if(ini != nil){

+		r = getsbuf();

+		memmove(r->buf, ini, BIT32SZ);

+		n = GBIT32(r->buf);

+		if(n <= BIT32SZ || n > messagesize)

+			fatal("bad length in 9P2000 message header");

+		n -= BIT32SZ;

+		if(readn(0, r->buf+BIT32SZ, n) != n)

+			fatal(nil);

+		n += BIT32SZ;

+		goto Message;

+	}

+

 	/*

 	 * Start serving file requests from the network

 	 */

 	for(;;) {

 		r = getsbuf();

-		while((n = localread9pmsg(0, r->buf, messagesize, ini)) == 0)

+		while((n = read9pmsg(0, r->buf, messagesize)) == 0)

 			;

-		if(n < 0)

+		if(n <= 0)

 			fatal(nil);

+	Message:

 		if(convM2S(r->buf, n, &r->work) == 0)

 			fatal("convM2S format error");

 		DEBUG(DFD, "%F\n", &r->work);

 		(fcalls[r->work.type])(r);

-		ini = nil;

 	}

 }

-/*

- * WARNING: Replace this with the original version as soon as all 

- * _old_ imports have been replaced with negotiating imports.  Also

- * cpu relies on this (which needs to be fixed!) -- pb.

- */

-static int

-localread9pmsg(int fd, void *abuf, uint n, void *ini)

-{

-	int m, len;

-	uchar *buf;

-

-	buf = abuf;

-

-	/* read count */

-	if(ini != nil)

-		memcpy(buf, ini, BIT32SZ);

-	else {

-		m = readn(fd, buf, BIT32SZ);

-		if(m != BIT32SZ){

-			if(m < 0)

-				return -1;

-			return 0;

-		}

-	}

-

-	len = GBIT32(buf);

-	if(len <= BIT32SZ || len > n){

-		werrstr("bad length in 9P2000 message header");

-		return -1;

-	}

-	len -= BIT32SZ;

-	m = readn(fd, buf+BIT32SZ, len);

-	if(m < len)

-		return 0;

-	return BIT32SZ+m;

-}

 void

 reply(Fcall *r, Fcall *t, char *err)

 {