server: Warn instead of error if a -D (debug) flag is not used.

This was a hard error before, but that's not very useful.  It's best
to ignore -D options that we don't understand (but we still warn about
them).

server/debug-flags.c

@@ -59,17 +59,17 @@ apply_debug_flags (void *dl, const char *name)
 
       /* Find the symbol. */
       sym = dlsym (dl, var);
-      if (sym == NULL) {
+      if (sym) {
+        /* Set the flag. */
+        *sym = flag->value;
+      }
+      else {
         fprintf (stderr,
-                 "%s: -D %s.%s: %s does not contain a "
+                 "%s: warning: -D %s.%s: %s does not contain a "
                  "global variable called %s\n",
                  program_name, name, flag->flag, name, var);
-        exit (EXIT_FAILURE);
       }
 
-      /* Set the flag. */
-      *sym = flag->value;
-
       /* Mark this flag as used. */
       flag->used = true;
     }
@@ -82,11 +82,9 @@ free_debug_flags (void)
   while (debug_flags != NULL) {
     struct debug_flag *next = debug_flags->next;
 
-    if (!debug_flags->used) {
+    if (!debug_flags->used)
       fprintf (stderr, "%s: warning: debug flag -D %s.%s was not used\n",
                program_name, debug_flags->name, debug_flags->flag);
-      exit (EXIT_FAILURE);
-    }
     free (debug_flags->name);
     free (debug_flags->flag);
     free (debug_flags);

tests/test-debug-flags.sh

@@ -56,19 +56,31 @@ check_error ()
     fi
 }
 
+check_warning ()
+{
+    cat debug-flags.out
+    if ! grep -sq "warning.*$1" debug-flags.out; then
+        echo "$0: expected warning message not present in above output: '$1'"
+        exit 1
+    fi
+}
+
 # This is expected to fail because we didn't set the file= parameter,
 # but it should not fail because of the debug flag.
 nbdkit -f -D example2.extra=1 example2 2>debug-flags.out && expected_failure
 check_error "you must supply the file="
 
-# This should fail because the -D flag refers to an unknown global in
-# a known plugin.
+# This should fail because we didn't set the file= parameter, but it
+# should also print a warning about the unknown -D flag.
 nbdkit -f -D example2.unknown=1 example2 2>debug-flags.out && expected_failure
-check_error "does not contain a global variable called example2_debug_unknown"
+check_error "you must supply the file="
+check_warning "does not contain a global variable called example2_debug_unknown"
 
-# This should fail because the -D flag is unused.
+# This should fail because we didn't set the file= parameter, but it
+# should also print a warning because the -D flag is unused.
 nbdkit -f -D example1.foo=1 example2 2>debug-flags.out && expected_failure
-check_error "was not used"
+check_error "you must supply the file="
+check_warning "was not used"
 
 # These should fail because the -D flag has a bad format.
 nbdkit -f -D = example2 2>debug-flags.out && expected_failure