plumber: remove $plumbsrv, add optional srvname, usage check

Plumber both posts a service to /srv and sets a $plumbsrv environment
variable.  Our libplumb no longer uses $plumbsrv and nothing else
does.  It's a silly hack;  rc doesn't update /env immediately, and
scripts, which for instance set up subrios, cannot rely on it to
clean up the plumber at the end.

Instead, add the option to specify a srvname, actually check for some
common errors and print a usage string.

Thanks to Ori for input and a preliminary patch.

sys/man/4/plumber

@@ -6,6 +6,9 @@ plumber \- file system for interprocess messaging
 [
 .B -p
 .I plumbing
+] [
+.B -s
+.I srvname
 ]
 .SH DESCRIPTION
 The
@@ -31,11 +34,17 @@ and
 and a set of output
 .I ports
 for dispatching messages to applications.
+.PP
 The service is also published as a
 .IR srv (4)
-file, named in
-.BR $plumbsrv ,
+file,
+.IR srvname ,
 for mounting elsewhere.
+By default, its name is a dot-separated concatenation
+of the program's name and a process id.
+A different one can be specified via the
+.B -s
+option.
 .PP
 Programs use
 .B write

sys/src/cmd/plumb/fsys.c

@@ -218,9 +218,10 @@ startfsys(void)
 	clock = getclock();
 	if(cexecpipe(&mntfd, &srvfd) < 0)
 		error("can't create pipe: %r");
-	sprint(srvfile, "/srv/plumb.%s.%d", user, getpid());
-	if(putenv("plumbsrv", srvfile) < 0)
-		error("can't write $plumbsrv: %r");
+	if(srvname == nil)
+		snprint(srvfile, sizeof(srvfile), "/srv/plumb.%s.%d", user, getpid());
+	else
+		snprint(srvfile, sizeof(srvfile), "/srv/%s", srvname);
 	fd = create(srvfile, OWRITE|OCEXEC|ORCLOSE, 0600);
 	if(fd < 0)
 		error("can't create /srv file: %r");

sys/src/cmd/plumb/plumber.c

@@ -15,6 +15,7 @@ Ruleset **rules;
 int	printerrors=1;
 jmp_buf	parsejmp;
 char	*lasterror;
+char	*srvname;
 
 void
 makeports(Ruleset *rules[])
@@ -37,6 +38,13 @@ mainproc(void *v)
 	sendp(c, nil);
 }
 
+void
+usage(void)
+{
+	fprint(2, "usage: %s [-p plumbfile] [-s srvname]\n", argv0);
+	exits("usage");
+}
+
 void
 threadmain(int argc, char *argv[])
 {
@@ -48,7 +56,13 @@ threadmain(int argc, char *argv[])
 
 	ARGBEGIN{
 	case 'p':
-		plumbfile = ARGF();
+		plumbfile = EARGF(usage());
+		break;
+	case 's':
+		srvname = EARGF(usage());
+		break;
+	default:
+		usage();
 		break;
 	}ARGEND
 

sys/src/cmd/plumb/plumber.h

@@ -92,3 +92,4 @@ jmp_buf	parsejmp;
 char		*lasterror;
 char		**ports;
 int		nports;
+char		*srvname;