liblvm: allow config settings to be read without full lvm cmd

A program may be using liblvm2app for simply checking a config
setting in lvm.conf.  In this case, a full lvm context is not
needed, only cmd->cft (which are the config settings read from
lvm.conf).

lvm_config_find_bool() can now be passed a NULL lvm context
in which case it will only create cmd->cft, check the config
setting asked for, and destroy the cmd.

3 files changed, 71 insertions, 3 deletions

diff --git a/lib/commands/toolcontext.c b/lib/commands/toolcontext.c
index 90e5d5adc..1e3f14a79 100644
--- a/lib/commands/toolcontext.c
+++ b/lib/commands/toolcontext.c
@@ -1776,6 +1776,49 @@ bad:
 	return 0;
 }
 
+void destroy_config_context(struct cmd_context *cmd)
+{
+	_destroy_config(cmd);
+
+	if (cmd->mem)
+		dm_pool_destroy(cmd->mem);
+	if (cmd->libmem)
+		dm_pool_destroy(cmd->libmem);
+
+	dm_free(cmd);
+}
+
+/*
+ * A "config context" is a very light weight toolcontext that
+ * is only used for reading config settings from lvm.conf.
+ */
+struct cmd_context *create_config_context(void)
+{
+	struct cmd_context *cmd;
+
+	if (!(cmd = dm_zalloc(sizeof(*cmd))))
+		goto_out;
+
+	strcpy(cmd->system_dir, DEFAULT_SYS_DIR);
+
+	if (!_get_env_vars(cmd))
+		goto_out;
+
+	if (!(cmd->libmem = dm_pool_create("library", 4 * 1024)))
+		goto_out;
+
+	dm_list_init(&cmd->config_files);
+
+	if (!_init_lvm_conf(cmd))
+		goto_out;
+
+	return cmd;
+out:
+	if (cmd)
+		destroy_config_context(cmd);
+	return NULL;
+}
+
 /* Entry point */
 struct cmd_context *create_toolcontext(unsigned is_long_lived,
 				       const char *system_dir,
diff --git a/lib/commands/toolcontext.h b/lib/commands/toolcontext.h
index c3b9b2ea5..2cecf2751 100644
--- a/lib/commands/toolcontext.h
+++ b/lib/commands/toolcontext.h
@@ -218,6 +218,14 @@ int init_lvmcache_orphans(struct cmd_context *cmd);
 int init_filters(struct cmd_context *cmd, unsigned load_persistent_cache);
 int init_connections(struct cmd_context *cmd);
 
+/*
+ * A config context is a very light weight cmd struct that
+ * is only used for reading config settings from lvm.conf,
+ * which are at cmd->cft.
+ */
+struct cmd_context *create_config_context(void);
+void destroy_config_context(struct cmd_context *cmd);
+
 struct format_type *get_format_by_name(struct cmd_context *cmd, const char *format);
 
 const char *system_id_from_string(struct cmd_context *cmd, const char *str);
diff --git a/liblvm/lvm_base.c b/liblvm/lvm_base.c
index 8b4def271..fce994cfd 100644
--- a/liblvm/lvm_base.c
+++ b/liblvm/lvm_base.c
@@ -126,14 +126,31 @@ int lvm_config_override(lvm_t libh, const char *config_settings)
 	return rc;
 }
 
+/*
+ * When full lvm connection is not being used, libh can be NULL
+ * and this command will internally create a single-use, light-weight
+ * cmd struct that only has cmd->cft populated from lvm.conf.
+ */
 int lvm_config_find_bool(lvm_t libh, const char *config_path, int fail)
 {
 	int rc = 0;
-	struct cmd_context *cmd = (struct cmd_context *)libh;
-	struct saved_env e = store_user_env((struct cmd_context *)libh);
+	struct cmd_context *cmd;
+	struct saved_env e;
+
+	if (libh) {
+		cmd = (struct cmd_context *)libh;
+		e = store_user_env((struct cmd_context *)libh);
+	} else {
+		if (!(cmd = create_config_context()))
+			return 0;
+	}
 
 	rc = dm_config_tree_find_bool(cmd->cft, config_path, fail);
-	restore_user_env(&e);
+
+	if (libh)
+		restore_user_env(&e);
+	else
+		destroy_config_context(cmd);
 	return rc;
 }