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pipe: bring back wstat, set size (in bytes) of the pipe buffer

--- a/sys/man/2/pipe

+++ b/sys/man/2/pipe

@@ -52,6 +52,12 @@

 on a pipe (see

 .IR stat (2)).

 .PP

+The limit of data bytes in the pipe's buffer can be set by

+.I fwstat

+or

+.I dirfwstat

+on either of the pipe's descriptors.

+.PP

 When all the data has been read from a pipe and the writer has closed the pipe or exited,

 .IR read (2)

 will return 0 bytes.  Writes to a pipe with no reader will generate a note

--- a/sys/src/9/port/devpipe.c

+++ b/sys/src/9/port/devpipe.c

@@ -365,6 +365,42 @@

 	return n;

 }

+static int

+pipewstat(Chan *c, uchar *dp, int n)

+{

+	Dir d;

+	Pipe *p;

+

+	if(waserror()){

+		/* avoid notes when pipe is a mounted queue */

+		if((c->flag & CMSG) == 0)

+			postnote(up, 1, "sys: wstat on closed pipe", NUser);

+		nexterror();

+	}

+

+	n = convM2D(dp, n, &d, nil);

+	if(n == 0)

+		error(Eshortstat);

+	if(d.length < 1 || d.length > conf.pipeqsize)

+		error(Ebadarg);

+

+	p = c->aux;

+	switch(NETTYPE(c->qid.path)){

+	case Qdir:

+		error(Eperm);

+	case Qdata0:

+	case Qdata1:

+		qsetlimit(p->q[0], d.length);

+		qsetlimit(p->q[1], d.length);

+		break;

+	default:

+		panic("pipewstat");

+	}

+

+	poperror();

+	return n;

+}

+

 Dev pipedevtab = {

 	'|',

 	"pipe",

@@ -383,5 +419,5 @@

 	pipewrite,

 	pipebwrite,

 	devremove,

-	devwstat,

+	pipewstat,

 };