From f4840cdba548979969cb2ad25b4c6acbc3e63f8c Mon Sep 17 00:00:00 2001
From: Jacob Moody <moody@posixcafe.org>
Date: Tue, 7 Jun 2022 05:25:44 +0000
Subject: [PATCH] kernel: add devskel for pc and pc64

---
 sys/man/3/skel           |  35 ++++++
 sys/src/9/pc/pc          |   1 +
 sys/src/9/pc64/pc64      |   1 +
 sys/src/9/port/devskel.c | 237 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 274 insertions(+)
 create mode 100644 sys/man/3/skel
 create mode 100644 sys/src/9/port/devskel.c

diff --git a/sys/man/3/skel b/sys/man/3/skel
new file mode 100644
index 000000000..cc8dfbc17
--- /dev/null
+++ b/sys/man/3/skel
@@ -0,0 +1,35 @@
+.TH SKEL 3 
+.SH NAME
+skel \- skeleton builder
+.SH SYNOPSIS
+.B bind #z/\fIskel\fR
+.I dir
+.PP
+.B bind #zd/\fIskel\fR
+.I dir
+.PP
+.B bind #ze/
+.I dir
+.SH DESCRIPTION
+.PP
+This device serves a single child directory with a single empty child
+file. The name of these files is determined by the first walk away
+from the root. After which, the hierarchy for that attached session
+becomes stable. The
+.B d
+attach option causes the child file to be an empty directory. The 
+.B e
+attach option presents a completly empty root directory.
+.SH EXAMPLES
+Skeleton files can be used for constructing arbitrary bind targets.
+.EX
+.IP
+bind -b '#zd/newroot' /
+bind '#zd/bin' /newroot
+bind '#z/walk' /newroot/bin
+bind /bin/walk /newroot/bin/walk
+bind /newroot /
+walk /
+.EE
+.SH SOURCE
+.B /sys/src/9/port/devskel.c
diff --git a/sys/src/9/pc/pc b/sys/src/9/pc/pc
index 2cacee652..3581fe207 100644
--- a/sys/src/9/pc/pc
+++ b/sys/src/9/pc/pc
@@ -18,6 +18,7 @@ dev
 	kprof
 	fs
 	dtracy
+	skel
 
 	ether		netif
 	bridge		netif log
diff --git a/sys/src/9/pc64/pc64 b/sys/src/9/pc64/pc64
index bafda3d72..e78a2d47f 100644
--- a/sys/src/9/pc64/pc64
+++ b/sys/src/9/pc64/pc64
@@ -41,6 +41,7 @@ dev
 	segment
 	vmx
 	dtracy
+	skel
 
 link
 #	devpccard	pci
diff --git a/sys/src/9/port/devskel.c b/sys/src/9/port/devskel.c
new file mode 100644
index 000000000..95e5b6038
--- /dev/null
+++ b/sys/src/9/port/devskel.c
@@ -0,0 +1,237 @@
+#include	"u.h"
+#include	"../port/lib.h"
+#include	"mem.h"
+#include	"dat.h"
+#include	"fns.h"
+#include	"../port/error.h"
+
+#include	"netif.h"
+
+typedef struct Skel Skel;
+struct Skel {
+	int ref;
+	QLock lk;
+	char name[KNAMELEN];
+	char mode;
+};
+
+struct
+{
+	QLock lk;
+	ulong path;
+} skelalloc;
+
+enum{
+	Qroot,
+	Qdir,
+	Qskel,
+};
+
+static Chan*
+skelattach(char *spec)
+{
+	Chan *c;
+	Skel *f;
+	uvlong path;
+
+	c = devattach('z', spec);
+
+	f = smalloc(sizeof *f);
+	if(spec != nil && spec[0] != '\0' && strchr("de", spec[0]) != nil)
+		f->mode = spec[0];
+	else
+		f->mode = 'f';
+
+	f->ref = 1;
+
+	qlock(&skelalloc.lk);
+	path = skelalloc.path++;
+	qunlock(&skelalloc.lk);
+
+	mkqid(&c->qid, NETQID(path, Qroot), 0, QTDIR);
+	c->aux = f;
+	return c;
+}
+
+static int
+step(Chan *c, Dir *dp, int direction)
+{
+	Skel *f;
+	Qid qid;
+	ulong perm;
+	uvlong path;
+	char *name;
+
+	perm = 0555|DMDIR;
+	path = NETTYPE(c->qid.path);
+	f = c->aux;
+	name = f->name;
+
+	path += direction;
+	if(!f->name[0] && path != Qroot)
+		return -1;
+
+	switch(path){
+	case Qroot:
+		mkqid(&qid, Qroot, 0, QTDIR);
+		name = "#z";
+		break;
+	case Qdir:
+		mkqid(&qid, Qdir, 0, QTDIR);
+		break;
+	case Qskel:
+		switch(f->mode){
+		case 'd':
+			mkqid(&qid, Qskel, 0, QTDIR);
+			break;
+		case 'f':
+		default:
+			mkqid(&qid, Qskel, 0, QTFILE);
+			perm = 0666;
+			break;
+		}
+		break;
+	default:
+		return -1;
+	}
+
+	qid.path = NETQID(NETID(c->qid.path), qid.path);
+	devdir(c, qid, name, 0, eve, perm, dp);
+	return 1;
+}
+
+
+static int
+skelgen(Chan *c, char *name, Dirtab *, int, int s, Dir *dp)
+{
+	Skel *f;
+
+	f = c->aux;
+	//First walk away from root
+	if(name && !f->name[0] && f->mode != 'e' && NETTYPE(c->qid.path) == Qroot)
+		utfecpy(f->name, &f->name[sizeof f->name-1], name);
+
+	if(s != DEVDOTDOT)
+		s++;
+
+	return step(c, dp, s);
+}
+
+static Walkqid*
+skelwalk(Chan *c, Chan *nc, char **name, int nname)
+{
+	Walkqid *wq;
+	Skel *f;
+
+	f = c->aux;
+	qlock(&f->lk);
+	if(waserror()){
+		qunlock(&f->lk);
+		nexterror();
+	}
+
+	wq = devwalk(c, nc, name, nname, nil, 0, skelgen);
+	if(wq != nil && wq->clone != nil && wq->clone != c){
+		if(f->ref <= 0)
+			panic("devskel ref");
+		f->ref++;
+	}
+	qunlock(&f->lk);
+	poperror();
+	return wq;
+}
+
+static Chan*
+skelopen(Chan *c, int omode)
+{
+	if(!(c->qid.type & QTDIR))
+		error(Eperm);
+	if(omode != OREAD)
+		error(Ebadarg);
+
+	c->mode = omode;
+	c->flag |= COPEN;
+	c->offset = 0;
+	return c;
+}
+
+static void
+skelclose(Chan *c)
+{
+	Skel *f;
+
+	f = c->aux;
+	qlock(&f->lk);
+	f->ref--;
+	if(f->ref == 0){
+		qunlock(&f->lk);
+		free(f);
+	} else
+		qunlock(&f->lk);
+}
+
+static long
+skelread(Chan *c, void *va, long n, vlong)
+{
+	Skel *f;
+	long nout;
+
+	if(!(c->qid.type & QTDIR))
+		error(Eperm);
+
+	f = c->aux;
+	qlock(&f->lk);
+	if(waserror()){
+		qunlock(&f->lk);
+		nexterror();
+	}
+	nout = devdirread(c, va, n, nil, 0, skelgen);
+	qunlock(&f->lk);
+	poperror();
+	return nout;
+}
+
+static long
+skelwrite(Chan *, void *, long, vlong)
+{
+	error(Eperm);
+	return 0;
+}
+
+static int
+skelstat(Chan *c, uchar *db, int n)
+{
+	Skel *f;
+	Dir dir;
+
+	f = c->aux;
+	qlock(&f->lk);
+	step(c, &dir, 0);
+	qunlock(&f->lk);
+
+	n = convD2M(&dir, db, n);
+	if(n < BIT16SZ)
+		error(Eshortstat);
+	return n;
+}
+
+Dev skeldevtab = {
+	'z',
+	"skel",
+
+	devreset,
+	devinit,
+	devshutdown,
+	skelattach,
+	skelwalk,
+	skelstat,
+	skelopen,
+	devcreate,
+	skelclose,
+	skelread,
+	devbread,
+	skelwrite,
+	devbwrite,
+	devremove,
+	devwstat,
+};