xregcomp: don't consider every regex compilation failure an internal error.

This pushes the error handling up one step, but makes the function
more flexible. Running out of memory still triggers an internal error,
in the spirit of other xutils functions.

notmuch-restore.c

@@ -88,9 +88,10 @@ notmuch_restore_command (unused (void *ctx), int argc, char *argv[])
      * non-space characters for the message-id, then one or more
      * spaces, then a list of space-separated tags as a sequence of
      * characters within literal '(' and ')'. */
-    xregcomp (&regex,
+    if ( xregcomp (&regex,
-	      "^([^ ]+) \\(([^)]*)\\)$",
+		   "^([^ ]+) \\(([^)]*)\\)$",
-	      REG_EXTENDED);
+		   REG_EXTENDED) )
+	INTERNAL_ERROR("compile time constant regex failed.");
 
     while ((line_len = getline (&line, &line_size, input)) != -1) {
 	regmatch_t match[3];

util/xutil.c

@@ -99,7 +99,7 @@ xstrndup (const char *s, size_t n)
     return ret;
 }
 
-void
+int
 xregcomp (regex_t *preg, const char *regex, int cflags)
 {
     int rerr;
@@ -110,9 +110,12 @@ xregcomp (regex_t *preg, const char *regex, int cflags)
 	char *error = xmalloc (error_size);
 
 	regerror (rerr, preg, error, error_size);
-	INTERNAL_ERROR ("compiling regex %s: %s\n",
+	fprintf (stderr, "compiling regex %s: %s\n",
 			regex, error);
+	free (error);
+	return 1;
     }
+    return 0;
 }
 
 int

util/xutil.h

@@ -43,7 +43,8 @@ xstrdup (const char *s);
 char *
 xstrndup (const char *s, size_t n);
 
-void
+/* Returns 0 for successful compilation, 1 otherwise */
+int
 xregcomp (regex_t *preg, const char *regex, int cflags);
 
 int