Make it possible for aug_init to report initialization errors

Sometimes initialization fails for mundane reasons, like syntax errors in
lens files. So far, it was not possible for callers to report details for
these errors, because they always got a NULL augeas handle back.

The new flag AUG_NO_ERR_CLOSE causes aug_init to return a handle even if
initialization fails, so that callers can use the aug_error* functions to
extract error details before calling aug_close.

  * src/augeas.c (aug_init): return a partially initialized struct augeas
    if AUG_NO_ERR_CLOSE is set
  * src/augeas.h (aug_flags): add AUG_NO_ERR_CLOSE
  * src/augtool.c: call aug_init with AUG_NO_ERR_CLOSE and print errors on
    initialization failure

src/augeas.c

@@ -412,6 +412,7 @@ struct augeas *aug_init(const char *root, const char *loadpath,
     struct augeas *result;
     struct tree *tree_root = make_tree(NULL, NULL, NULL, NULL);
     int r;
+    bool close_on_error = true;
 
     if (tree_root == NULL)
         return NULL;
@@ -442,6 +443,10 @@ struct augeas *aug_init(const char *root, const char *loadpath,
 
     result->origin->children->label = strdup(s_augeas);
 
+    /* We are now initialized enough that we can dare return RESULT even
+     * when we encounter errors if the caller so wishes */
+    close_on_error = !(flags & AUG_NO_ERR_CLOSE);
+
     result->modpathz = NULL;
     result->nmodpath = 0;
     if (loadpath != NULL) {
@@ -538,8 +543,11 @@ struct augeas *aug_init(const char *root, const char *loadpath,
 
  error:
     api_exit(result);
-    aug_close(result);
-    return NULL;
+    if (close_on_error) {
+        aug_close(result);
+        result = NULL;
+    }
+    return result;
 }
 
 void tree_unlink_children(struct augeas *aug, struct tree *tree) {

src/augeas.h

@@ -49,7 +49,9 @@ enum aug_flags {
                                      what would have changed */
     AUG_NO_LOAD      = (1 << 5),  /* Do not load the tree from AUG_INIT */
     AUG_NO_MODL_AUTOLOAD = (1 << 6),
-    AUG_ENABLE_SPAN  = (1 << 7)   /* Track the span in the input of nodes */
+    AUG_ENABLE_SPAN  = (1 << 7),  /* Track the span in the input of nodes */
+    AUG_NO_ERR_CLOSE = (1 << 8)   /* Do not close automatically when
+                                     encountering error during aug_init */
 };
 
 /* Function: aug_init
@@ -63,11 +65,20 @@ enum aug_flags {
  * searched in. This is in addition to the standard load path and the
  * directories in AUGEAS_LENS_LIB
  *
- * FLAGS is a bitmask made up of values from AUG_FLAGS.
+ * FLAGS is a bitmask made up of values from AUG_FLAGS. The flag
+ * AUG_NO_ERR_CLOSE can be used to get more information on why
+ * initialization failed. If it is set in FLAGS, the caller must check that
+ * aug_error returns AUG_NOERROR before using the returned augeas handle
+ * for any other operation. If the handle reports any error, the caller
+ * should only call the aug_error functions an aug_close on this handle.
  *
  * Returns:
  * a handle to the Augeas tree upon success. If initialization fails,
- * returns NULL.
+ * returns NULL if AUG_NO_ERR_CLOSE is not set in FLAGS. If
+ * AUG_NO_ERR_CLOSE is set, might return an Augeas handle even on
+ * failure. In that case, caller must check for errors using augeas_error,
+ * and, if an error is reported, only use the handle with the aug_error
+ * functions and aug_close.
  */
 augeas *aug_init(const char *root, const char *loadpath, unsigned int flags);
 

src/augtool.c

@@ -398,6 +398,21 @@ static int run_command(const char *line) {
     return result;
 }
 
+static void print_aug_error(void) {
+    if (aug_error(aug) == AUG_ENOMEM) {
+        fprintf(stderr, "Out of memory.\n");
+        return;
+    }
+    if (aug_error(aug) != AUG_NOERROR) {
+        fprintf(stderr, "error: %s\n", aug_error_message(aug));
+        if (aug_error_minor_message(aug) != NULL)
+            fprintf(stderr, "error: %s\n",
+                    aug_error_minor_message(aug));
+        if (aug_error_details(aug) != NULL)
+            fputs(aug_error_details(aug), stderr);
+    }
+}
+
 static int main_loop(void) {
     char *line = NULL;
     int ret = 0;
@@ -478,18 +493,7 @@ static int main_loop(void) {
             return ret;
         if (code < 0) {
             ret = -1;
-            if (aug_error(aug) == AUG_ENOMEM) {
-                fprintf(stderr, "Out of memory.\n");
-                return -1;
-            }
-            if (aug_error(aug) != AUG_NOERROR) {
-                fprintf(stderr, "error: %s\n", aug_error_message(aug));
-                if (aug_error_minor_message(aug) != NULL)
-                    fprintf(stderr, "error: %s\n",
-                            aug_error_minor_message(aug));
-                if (aug_error_details(aug) != NULL)
-                    fprintf(stderr, "error: %s\n", aug_error_details(aug));
-            }
+            print_aug_error();
         }
     }
 }
@@ -521,9 +525,11 @@ int main(int argc, char **argv) {
 
     parse_opts(argc, argv);
 
-    aug = aug_init(root, loadpath, flags);
-    if (aug == NULL) {
+    aug = aug_init(root, loadpath, flags|AUG_NO_ERR_CLOSE);
+    if (aug == NULL || aug_error(aug) != AUG_NOERROR) {
         fprintf(stderr, "Failed to initialize Augeas\n");
+        if (aug != NULL)
+            print_aug_error();
         exit(EXIT_FAILURE);
     }
     if (print_version) {