libndb: add ndbvalfmt() formatter

Some attributes such as vendor and txt require double-quoting
to be parsable by libndb. Provide the fmtinstall(2) formatter
ndbvalfmt for programs outputting ndb(6) format.

sys/include/ndb.h

@@ -151,3 +151,4 @@ Ndbtuple*	ndbsnext(Ndbs*, char*, char*);
 Ndbtuple*	ndbsubstitute(Ndbtuple*, Ndbtuple*, Ndbtuple*);
 Ndbtuple*	ndbdedup(Ndbtuple*);
 void		ndbsetmalloctag(Ndbtuple*, uintptr);
+int		ndbvalfmt(Fmt*);

sys/man/2/ndb

@@ -1,6 +1,6 @@
 .TH NDB 2
 .SH NAME
-ndbopen, ndbcat, ndbchanged, ndbclose, ndbreopen, ndbsearch, ndbsnext, ndbgetvalue, ndbfree, ipattr, mkptrname, ndbgetipaddr, ndbipinfo, csipinfo, ndbhash, ndbparse, csgetvalue, ndbfindattr, dnsquery, ndbdiscard, ndbconcatenate, ndbreorder, ndbsubstitute, ndbdedup \- network database
+ndbopen, ndbcat, ndbchanged, ndbclose, ndbreopen, ndbsearch, ndbsnext, ndbgetvalue, ndbfree, ipattr, mkptrname, ndbgetipaddr, ndbipinfo, csipinfo, ndbhash, ndbparse, csgetvalue, ndbfindattr, dnsquery, ndbdiscard, ndbconcatenate, ndbreorder, ndbsubstitute, ndbdedup, ndbsetmalloctag, ndbvalfmt \- network database
 .SH SYNOPSIS
 .B #include <u.h>
 .br
@@ -94,6 +94,9 @@ Ndbtuple*	ndbdedup(Ndbtuple *t)
 .PP
 .B
 void	ndbsetmalloctag(Ndbtuple *t, uintptr tag)
+.PP
+.B
+int	ndbvalfmt(Fmt*)
 .SH DESCRIPTION
 These routines are used by network administrative programs to search
 the network database.
@@ -523,6 +526,27 @@ of each tuple in the list
 .I t
 to
 .IR tag .
+.PP
+.I Ndbvalfmt
+formats a
+.B char*
+string
+of a
+.I Ndbtuple
+val,
+adding " quoting if necessary.
+It is typically enabled by calling:
+.IP
+.EX
+fmtinstall('$', ndbvalfmt);
+.EE
+.PP
+And then used like:
+.IP
+.EX
+Ndbtuple *t = ...
+print("%s=%$", t->attr, t->val);
+.EE
 .SH FILES
 .BR /lib/ndb "    directory of network database files
 .SH SOURCE

sys/src/libndb/mkfile

@@ -23,6 +23,7 @@ OFILES=\
 	ndbreorder.$O\
 	ndbsubstitute.$O\
 	ndbdedup.$O\
+	ndbvalfmt.$O\
 
 HFILES=\
 	/sys/include/ndb.h\

sys/src/libndb/ndbvalfmt.c

@@ -0,0 +1,28 @@
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+#include <ndb.h>
+#include "ndbhf.h"
+
+static int
+needquote(char *s)
+{
+	int c;
+
+	while((c = *s++) != '\0'){
+		if(ISWHITE(c) || c == '#')
+			return 1;
+	}
+	return 0;
+}
+
+int
+ndbvalfmt(Fmt *f)
+{
+	char *s = va_arg(f->args, char*);
+	if(s == nil)
+		s = "";
+	if(needquote(s))
+		return fmtprint(f, "\"%s\"", s);
+	return fmtstrcpy(f, s);
+}