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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.

--- a/sys/man/4/plumber

+++ b/sys/man/4/plumber

@@ -6,6 +6,9 @@

 [

 .B -p

 .I plumbing

+] [

+.B -s

+.I srvname

 ]

 .SH DESCRIPTION

 The

@@ -31,11 +34,17 @@

 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

--- a/sys/src/cmd/plumb/fsys.c

+++ b/sys/src/cmd/plumb/fsys.c

@@ -218,9 +218,10 @@

 	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");

--- a/sys/src/cmd/plumb/plumber.c

+++ b/sys/src/cmd/plumb/plumber.c

@@ -15,6 +15,7 @@

 int	printerrors=1;

 jmp_buf	parsejmp;

 char	*lasterror;

+char	*srvname;

 void

 makeports(Ruleset *rules[])

@@ -38,6 +39,13 @@

 }

 void

+usage(void)

+{

+	fprint(2, "usage: %s [-p plumbfile] [-s srvname]\n", argv0);

+	exits("usage");

+}

+

+void

 threadmain(int argc, char *argv[])

 {

 	char buf[512];

@@ -48,7 +56,13 @@

 	ARGBEGIN{

 	case 'p':

-		plumbfile = ARGF();

+		plumbfile = EARGF(usage());

+		break;

+	case 's':

+		srvname = EARGF(usage());

+		break;

+	default:

+		usage();

 		break;

 	}ARGEND

--- a/sys/src/cmd/plumb/plumber.h

+++ b/sys/src/cmd/plumb/plumber.h

@@ -92,3 +92,4 @@

 char		*lasterror;

 char		**ports;

 int		nports;

+char		*srvname;