Fix error in etc/default/mailmunge.sample.in; do more sanity-checking on filter program

c/mailmunge-multiplexor.c

@@ -592,6 +592,7 @@ main(int argc, char *argv[], char **env)
     int facility = LOG_MAIL;
     int kidpipe[2] = {-1, -1};
     char kidmsg[256];
+    struct stat sbuf;
 
     time_t now;
 
@@ -987,6 +988,31 @@ main(int argc, char *argv[], char **env)
 	exit(EXIT_FAILURE);
     }
 
+    /* Make sure filter prog exists and is executable */
+    if (stat(Settings.progPath, &sbuf) < 0) {
+        fprintf(stderr, "%s: Unable to stat(%s): %s\n",
+                argv[0], Settings.progPath, strerror(errno));
+        exit(EXIT_FAILURE);
+    }
+    if ((sbuf.st_mode & S_IFMT) != S_IFREG) {
+        fprintf(stderr, "%s: Filter program %s is not a regular file.\n",
+                argv[0], Settings.progPath);
+        exit(EXIT_FAILURE);
+    }
+    if (access(Settings.progPath, R_OK) < 0) {
+        fprintf(stderr, "%s: Filter program %s is not readable.\n",
+                argv[0], Settings.progPath);
+        exit(EXIT_FAILURE);
+    }
+
+    /* If we are not using embedded perl, filter prog must be executable */
+    if (!Settings.useEmbeddedPerl) {
+        if (access(Settings.progPath, X_OK) < 0) {
+            fprintf(stderr, "%s: Filter program %s is not executable.\n",
+                    argv[0], Settings.progPath);
+            exit(EXIT_FAILURE);
+        }
+    }
     /* Fix obvious stupidities */
     if (Settings.maxWorkers < 1) {
 	Settings.maxWorkers = 1;

etc/default/mailmunge.sample.in

@@ -104,7 +104,7 @@
 # MX_EMBED_PERL=no
 
 ### Path to the filter program
-# MX_FILTER=@sysconfdir@@CONFSUBDIR@/mailmunge-filter
+# FILTER=@sysconfdir@@CONFSUBDIR@/mailmunge-filter
 
 ### Time after which an idle worker can be terminated if more than MX_MINIMUM
 # MX_IDLE=300