fix nsh -i/-c appending rules to daemon config files over and over

nsh -i/-c wrote daemon config files in /var/run by opening the file in
append-only mode, adding a single line, closing the file again, and
running chmod on the file. All this over and over, for every line.
This is inefficient and also results in the problem that the full
file content gets re-appended to the file each time nsh -i is run.

Instead, open the file once when a new rules section is encountered,
and close it when a different file needs to be written or when we can
tell that we have left a "rules" section. The last open file might end
up being closed implicitly when nsh -i/-c exits.

While nsh -i should only be run once at boot-time which renders this
issue moot, nsh -c can be run often, and the issue will also trigger
during manual testing of nsh -i which is quite irritating.

testing + OK Tom

ctl.c

@@ -47,7 +47,7 @@ void restart_dhcpd(int, char **, ...);
 /* subroutines */
 int fill_tmpfile(char **, char *, char **);
 int edit_file(char *, mode_t, char *, char **);
-int rule_writeline(char *, mode_t, char *, int);
+int rule_writeline(FILE *, char *, int);
 int acq_lock(char *);
 void rls_lock(int);
 
@@ -626,6 +626,8 @@ ctlhandler(int argc, char **argv, ...)
 	int nargs;
 	va_list ap;
 	char *modhvar;
+	static FILE *rulesfile;
+	static char tmpfile_cur[PATH_MAX];
 
 	va_start(ap, argv);
 	modhvar = va_arg(ap, char *);
@@ -646,9 +648,18 @@ ctlhandler(int argc, char **argv, ...)
 	}
 
 	snprintf(table, sizeof(table), "-T%d", cli_rtable);
-	if (daemons->tmpfile)
+	if (daemons->tmpfile) {
 		snprintf(tmpfile, sizeof(tmpfile), "%s.%d", daemons->tmpfile,
 		    cli_rtable);
+		if (tmpfile_cur[0] != '\0' && rulesfile &&
+		    strcmp(tmpfile_cur, tmpfile) != 0) {
+			fclose(rulesfile);
+			rulesfile = NULL;
+		}
+	} else if (rulesfile) {
+		fclose(rulesfile);
+		rulesfile = NULL;
+	}
 
 	if (modhvar) {
 		/* action specified or indented command specified */
@@ -661,10 +672,24 @@ ctlhandler(int argc, char **argv, ...)
 				printf("%% writeline without tmpfile\n");
 				goto done;
 			}
+			if (rulesfile == NULL) {
+				rulesfile = fopen(tmpfile, "w");
+				if (rulesfile == NULL) {
+					printf("%% error opening %s for "
+					    "writing: %s", tmpfile,
+					    strerror(errno));
+					goto done;
+				}
+				fchmod(fileno(rulesfile), daemons->mode);
+				strlcpy(tmpfile_cur, tmpfile,
+				    sizeof(tmpfile_cur));
+			}
 			/* write indented line to tmp config file */
-			rule_writeline(tmpfile, daemons->mode, saveline,
-			    cli_rtable);
+			rule_writeline(rulesfile, saveline, cli_rtable);
 			goto done;
+		} else if (rulesfile) {
+			fclose(rulesfile);
+			rulesfile = NULL;
 		}
 	}
 	if (argc < 2 || argv[1][0] == '?') {
@@ -1049,24 +1074,16 @@ edit_file(char *tmpfile, mode_t mode, char *propername, char **args)
 }
 
 int
-rule_writeline(char *fname, mode_t mode, char *writeline, int rdomain)
+rule_writeline(FILE *rulesfile, char *writeline, int rdomain)
 {
-	FILE *rulefile;
-
-	rulefile = fopen(fname, "a");
-	if (rulefile == NULL) {
-		printf("%% Rule write failed: %s\n", strerror(errno));
-		return(1);
-	}
 	if (writeline[0] == ' ') {
 		writeline++;
 		/* Lines for rdomain-specific daemons are indented twice. */
 		if (rdomain > 0 && writeline[0] == ' ')
 			writeline++;
 	}
-	fprintf(rulefile, "%s", writeline);
-	fclose(rulefile);
-	chmod(fname, mode);
+	fprintf(rulesfile, "%s", writeline);
+	fflush(rulesfile);
 	return(0);
 }