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u9fs: fix misuse of Fcall.afid in p9any authentication module. (from sources patch/u9fs-afid)

U9fs(4) misuses Fcall.afid in its p9any authentication module.

The afid field of Fcall structure is only valid with Tauth or Tattach.
Tread, Twrite, Tclunk should use rx->fid instead.  It's been lucky so
far to get the job done because rx->afid survives from previous
Tauth/Tattach.  The issue pops up when several authentications happen
concurrently.

Test case:
	u9fs = 'host with u9fs on tcp!*!564'
	9fs $u9fs; for(i in 1 2 3 4 5 6 7 8){ 9fs $u9fs & }; wait
Should:
	no errors
Should not:
	mount failed: authentication failed

--- a/sys/src/cmd/unix/u9fs/authp9any.c

+++ b/sys/src/cmd/unix/u9fs/authp9any.c

@@ -367,9 +367,11 @@

 		return ep;

 	if (chatty9p)

 		fprint(2, "p9anyattach: afid %d state %d\n", rx->afid, sp->state);

-	if (sp->state == Established && strcmp(rx->uname, sp->uname) == 0

-		&& strcmp(rx->aname, sp->aname) == 0)

+	if(sp->state == Established && strcmp(rx->uname, sp->uname) == 0

+		&& strcmp(rx->aname, sp->aname) == 0){

+		rx->uname = sp->t.suid;

 		return nil;

+	}

 	return "authentication failed";

 }

@@ -392,7 +394,7 @@

 	char *ep;

 	Fid *f;

-	f = oldauthfid(rx->afid, (void **)&sp, &ep);

+	f = oldauthfid(rx->fid, (void **)&sp, &ep);

 	if (f == nil)

 		return ep;

 	if (chatty9p)

@@ -437,7 +439,7 @@

 	Fid *f;

-	f = oldauthfid(rx->afid, (void **)&sp, &ep);

+	f = oldauthfid(rx->fid, (void **)&sp, &ep);

 	if (f == nil)

 		return ep;

 	if (chatty9p)

@@ -515,7 +517,7 @@

 	AuthSession *sp;

 	char *ep;

-	f = oldauthfid(rx->afid, (void **)&sp, &ep);

+	f = oldauthfid(rx->fid, (void **)&sp, &ep);

 	if (f == nil)

 		return ep;

 	if (chatty9p)