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mq: create an automatic root stream group

mq is quite often used in situations where only a single stream group
is needed.  In these cases it is preferable to just create the streams
immediately under the mq mount, avoiding the need for redundant naming
and an extra level in hierarchy.

We allow this by allocating an automatic root stream group at startup.

--- a/man/4/mq

+++ b/man/4/mq

@@ -148,6 +148,7 @@

 being created automatically.

 Similarly, streams are allocated by creating files inside the group

 directory.

+On startup an empty stream group is allocated at the root level.

 .PP

 The file

 .I order

--- a/src/mq.c

+++ b/src/mq.c

@@ -52,12 +52,13 @@

 };

 enum {

-	/* Dirty trick to help clients tell us from most others. */

+	/* Dirty trick to help clients tell our

+	 * root from most others, see pin(1). */

 	Qroot = 0xA,

-		Qmq = 0x1,

-			Qstream,

-			Qorder,

-			Qctl,

+	Qmq = 0x1,

+		Qstream,

+		Qorder,

+		Qctl,

 };

 void

 filesettype(File *f, ushort type)

@@ -66,7 +67,8 @@

 	 * Use four most-significant bits to store the type.

 	 * This depends on the 9pfile(2) library generating

 	 * simple incremental qid paths.

-	*/

+	 */

+	f->qid.path &= ~(uvlong)0xF<<60;

 	f->qid.path |= (uvlong)(type&0xF)<<60;

 }

@@ -91,7 +93,13 @@

 	mq->replay = Replayoff;

 	ctl = order = nil;

-	if((d = createfile(parent, name, uid, perm, mq)) == nil)

+	if(strcmp(name, "/") == 0){

+		d = parent;

+		d->aux = mq;

+	}

+	else

+		d = createfile(parent, name, uid, perm, mq);

+	if(d == nil)

 		goto err;

 	filesettype(d, Qmq);

@@ -428,11 +436,6 @@

 	switch(filetype(parent)){

 	case Qroot:

-		if(!(perm&DMDIR)){

-			respond(r, "forbidden");

-			return;

-		}

-		/* fallthrough */

 	case Qmq:

 		if(perm&DMDIR)

 			f = mqcreate(parent, name, uid, perm);

@@ -441,7 +444,7 @@

 		break;

 	}

 	if(f == nil)

-		responderror(r);

+		respond(r, "internal failure");

 	else

 		respond(r, nil);

 }

@@ -652,6 +655,7 @@

 	}ARGEND;

 	fs.tree = alloctree(nil, nil, DMDIR|0777, xdestroyfile);

+	mqcreate(fs.tree->root, "/", nil, 0);

 	filesettype(fs.tree->root, Qroot);

 	if(name || mtpt){