m_config: introduce basic mechanism to synchronize global option updates

The way option runtime changes are handled is pretty bad in the current
codebase. There's a big option struct (MPOpts), which contains almost
everything, and for which no synchronization mechanism exists. This was
handled by either making some options read-only after initialization,
duplicating the option struct, using sub-options (in the VO), and so on.

Introduce a mechanism that creates a copy of the global options (or
parts of it), and provides a well-defined way to update them in a
thread-safe way.

Most code can remain the same, just that all the component glue code has
to explicitly make use of it first.

There is still lots of room for improvement. For example, the update
mechanism could be better.

1 files changed, 54 insertions, 0 deletions

diff --git a/options/m_config.h b/options/m_config.h
index c8225706ee..101565745f 100644
--- a/options/m_config.h
+++ b/options/m_config.h
@@ -19,6 +19,7 @@
 #define MPLAYER_M_CONFIG_H
 
 #include <stddef.h>
+#include <stdint.h>
 #include <stdbool.h>
 
 #include "misc/bstr.h"
@@ -41,6 +42,8 @@ struct m_config_option {
     bool is_set_from_cmdline : 1;   // Set by user from command line
     bool is_set_locally : 1;        // Has a backup entry
     bool warning_was_printed : 1;
+    int16_t shadow_offset;          // Offset into m_config_shadow.data
+    int16_t group;                  // Index into m_config.groups
     const char *name;               // Full name (ie option-subopt)
     const struct m_option *opt;     // Option description
     void *data;                     // Raw value of the option
@@ -80,6 +83,16 @@ typedef struct m_config {
     bool subopt_deprecation_warning;
 
     void *optstruct; // struct mpopts or other
+
+    int shadow_size;
+
+    // List of m_sub_options instances.
+    // Index 0 is the top-level and is always present.
+    struct m_config_group *groups;
+    int num_groups;
+
+    // Thread-safe shadow memory; only set for the main m_config.
+    struct m_config_shadow *shadow;
 } m_config_t;
 
 // Create a new config object.
@@ -96,6 +109,10 @@ struct m_config *m_config_new(void *talloc_ctx, struct mp_log *log,
                               size_t size, const void *defaults,
                               const struct m_option *options);
 
+// Creates "backup" shadow memory for use with m_config_cache. Sets it on
+// mpv_global. Expected to be called at early init on the main m_config.
+void m_config_create_shadow(struct m_config *config);
+
 // (Warning: new object references config->log and others.)
 struct m_config *m_config_dup(void *talloc_ctx, struct m_config *config);
 
@@ -187,6 +204,10 @@ const char *m_config_get_positional_option(const struct m_config *config, int n)
 // Returns: error code (<0), or number of expected params (0, 1)
 int m_config_option_requires_param(struct m_config *config, bstr name);
 
+// Notify m_config_cache users that the option has (probably) changed its value.
+void m_config_notify_change_co(struct m_config *config,
+                               struct m_config_option *co);
+
 // Return all (visible) option names as NULL terminated string list.
 char **m_config_list_options(void *ta_parent, const struct m_config *config);
 
@@ -252,4 +273,37 @@ void *m_config_alloc_struct(void *talloc_ctx,
 void *m_sub_options_copy(void *talloc_ctx, const struct m_sub_options *opts,
                          const void *ptr);
 
+// This can be used to create and synchronize per-thread option structs,
+// which then can be read without synchronization. No concurrent access to
+// the cache itself is allowed.
+struct m_config_cache {
+    // The struct as indicated by m_config_cache_alloc's group parameter.
+    void *opts;
+
+    // Internal.
+    struct m_config_shadow *shadow;
+    struct m_config *shadow_config;
+    long long ts;
+    int group;
+};
+
+// Create a mirror copy from the global options.
+//  ta_parent: parent for the returned allocation
+//  global: option data source
+//  group: the option group to return. This can be NULL for the global option
+//         struct (MPOpts), or m_sub_options used in a certain OPT_SUBSTRUCT()
+//         item.
+struct m_config_cache *m_config_cache_alloc(void *ta_parent,
+                                            struct mpv_global *global,
+                                            const struct m_sub_options *group);
+
+// Update the options in cache->opts to current global values. Return whether
+// there was an update notification at all (which may or may not indicate that
+// some options have changed).
+// Keep in mind that while the cache->opts pointer does not change, the option
+// data itself will (e.g. string options might be reallocated).
+bool m_config_cache_update(struct m_config_cache *cache);
+
+struct m_config *mp_get_root_config(struct mpv_global *global);
+
 #endif /* MPLAYER_M_CONFIG_H */