msg: add a mechanism to output messages to a ringbuffer

Until now, mp_msg output always went to the terminal. There was no way
to grab the stream of output messages. But this will be needed by
various future changes: Lua scripts, slave mode, client library...

This commit allows registering a ring buffer. A callback would be more
straight-forward, but since msg.c sits at the bottom of the lock
hierarchy (it's used by virtually everything), this would probably be a
nightmare. A ring buffer will be simpler and more predictable in the
long run.

We allocate new memory for each ringbuffer entry, which is probably a
bit expensive. We could try to be clever and somehow pack the data
directly into the buffer, but I felt like this wouldn't be worth the
complexity. You'd have to copy the data a bunch of times anyway. I'm
hoping that we can get away with using the ringbuffer mechanism for
low frequency important messages only (and not e.g. for high volume
debug messages), so the cost doesn't matter that much.

A ringbuffer has a simple, single log level. I considered allowing
--msglevel style per-prefix configuration for each ringbuffer, but
that would have been pretty complicated to implement, and wouldn't
have been that useful either.

common/msg.c

@@ -30,6 +30,7 @@
 #include "compat/atomics.h"
 #include "common/common.h"
 #include "common/global.h"
+#include "misc/ring.h"
 #include "options/options.h"
 #include "osdep/terminal.h"
 #include "osdep/io.h"
@@ -53,6 +54,8 @@ struct mp_log_root {
     bool color;
     int verbose;
     bool force_stderr;
+    struct mp_log_buffer **buffers;
+    int num_buffers;
     // --- semi-atomic access
     bool mute;
     // --- must be accessed atomically
@@ -66,10 +69,17 @@ struct mp_log {
     struct mp_log_root *root;
     const char *prefix;
     const char *verbose_prefix;
-    int level;
+    int level;                  // minimum log level for any outputs
+    int terminal_level;         // minimum log level for terminal output
     int64_t reload_counter;
 };
 
+struct mp_log_buffer {
+    struct mp_log_root *root;
+    struct mp_ring *ring;
+    int level;
+};
+
 // Protects some (not all) state in mp_log_root
 static pthread_mutex_t mp_msg_lock = PTHREAD_MUTEX_INITIALIZER;
 
@@ -99,6 +109,9 @@ static void update_loglevel(struct mp_log *log)
         if (match_mod(log->verbose_prefix, mod))
             log->level = level;
     }
+    log->terminal_level = log->level;
+    for (int n = 0; n < log->root->num_buffers; n++)
+        log->level = MPMAX(log->level, log->root->buffers[n]->level);
     log->reload_counter = log->root->reload_counter;
     pthread_mutex_unlock(&mp_msg_lock);
 }
@@ -110,11 +123,6 @@ bool mp_msg_test(struct mp_log *log, int lev)
     mp_memory_barrier();
     if (!log->root || log->root->mute)
         return false;
-    if (lev == MSGL_STATUS) {
-        // skip status line output if stderr is a tty but in background
-        if (terminal_in_background())
-            return false;
-    }
     if (log->reload_counter != log->root->reload_counter)
         update_loglevel(log);
     return lev <= log->level || (log->root->smode && lev == MSGL_SMODE);
@@ -183,22 +191,13 @@ static void set_msg_color(FILE* stream, int lev)
     terminal_set_foreground_color(stream, v_colors[lev]);
 }
 
-void mp_msg_va(struct mp_log *log, int lev, const char *format, va_list va)
+static void print_msg_on_terminal(struct mp_log *log, int lev, char *text)
 {
-    if (!mp_msg_test(log, lev))
-        return; // do not display
-
-    pthread_mutex_lock(&mp_msg_lock);
-
     struct mp_log_root *root = log->root;
     FILE *stream = (root->force_stderr || lev == MSGL_STATUS) ? stderr : stdout;
 
-    char tmp[MSGSIZE_MAX];
-    if (vsnprintf(tmp, MSGSIZE_MAX, format, va) < 0)
-        snprintf(tmp, MSGSIZE_MAX, "[fprintf error]\n");
-    tmp[MSGSIZE_MAX - 2] = '\n';
-    tmp[MSGSIZE_MAX - 1] = 0;
-    char *text = tmp;
+    if (!(lev <= log->terminal_level || (root->smode && lev == MSGL_SMODE)))
+        return;
 
     bool header = root->header;
     const char *prefix = log->prefix;
@@ -208,6 +207,9 @@ void mp_msg_va(struct mp_log *log, int lev, const char *format, va_list va)
         prefix = log->verbose_prefix;
 
     if (lev == MSGL_STATUS) {
+        // skip status line output if stderr is a tty but in background
+        if (terminal_in_background())
+            return;
         if (root->termosd) {
             prepare_status_line(root, text);
             terminate = "\r";
@@ -242,6 +244,54 @@ void mp_msg_va(struct mp_log *log, int lev, const char *format, va_list va)
     if (root->color)
         terminal_set_foreground_color(stream, -1);
     fflush(stream);
+}
+
+static void write_msg_to_buffers(struct mp_log *log, int lev, char *text)
+{
+    struct mp_log_root *root = log->root;
+    for (int n = 0; n < root->num_buffers; n++) {
+        struct mp_log_buffer *buffer = root->buffers[n];
+        if (lev >= buffer->level) {
+            // Assuming a single writer (serialized by msg lock)
+            int avail = mp_ring_available(buffer->ring) / sizeof(void *);
+            if (avail < 1)
+                continue;
+            struct mp_log_buffer_entry *entry = talloc_ptrtype(NULL, entry);
+            if (avail > 1) {
+                *entry = (struct mp_log_buffer_entry) {
+                    .prefix = talloc_strdup(entry, log->verbose_prefix),
+                    .level = lev,
+                    .text = talloc_strdup(entry, text),
+                };
+            } else {
+                // write overflow message to signal that messages might be lost
+                *entry = (struct mp_log_buffer_entry) {
+                    .prefix = "overflow",
+                    .level = MSGL_FATAL,
+                    .text = "",
+                };
+            }
+            mp_ring_write(buffer->ring, (unsigned char *)&entry, sizeof(entry));
+        }
+    }
+}
+
+void mp_msg_va(struct mp_log *log, int lev, const char *format, va_list va)
+{
+    if (!mp_msg_test(log, lev))
+        return; // do not display
+
+    pthread_mutex_lock(&mp_msg_lock);
+
+    char tmp[MSGSIZE_MAX];
+    if (vsnprintf(tmp, MSGSIZE_MAX, format, va) < 0)
+        snprintf(tmp, MSGSIZE_MAX, "[fprintf error]\n");
+    tmp[MSGSIZE_MAX - 2] = '\n';
+    tmp[MSGSIZE_MAX - 1] = 0;
+    char *text = tmp;
+
+    print_msg_on_terminal(log, lev, text);
+    write_msg_to_buffers(log, lev, text);
 
     pthread_mutex_unlock(&mp_msg_lock);
 }
@@ -344,6 +394,78 @@ void mp_msg_uninit(struct mpv_global *global)
     global->log = NULL;
 }
 
+struct mp_log_buffer *mp_msg_log_buffer_new(struct mpv_global *global,
+                                            int size, int level)
+{
+    struct mp_log_root *root = global->log->root;
+
+    pthread_mutex_lock(&mp_msg_lock);
+
+    struct mp_log_buffer *buffer = talloc_ptrtype(NULL, buffer);
+    *buffer = (struct mp_log_buffer) {
+        .root = root,
+        .level = level,
+        .ring = mp_ring_new(buffer, sizeof(void *) * size),
+    };
+    if (!buffer->ring)
+        abort();
+
+    MP_TARRAY_APPEND(root, root->buffers, root->num_buffers, buffer);
+
+    mp_atomic_add_and_fetch(&root->reload_counter, 1);
+    mp_memory_barrier();
+
+    pthread_mutex_unlock(&mp_msg_lock);
+
+    return buffer;
+}
+
+void mp_msg_log_buffer_destroy(struct mp_log_buffer *buffer)
+{
+    if (!buffer)
+        return;
+
+    pthread_mutex_lock(&mp_msg_lock);
+
+    struct mp_log_root *root = buffer->root;
+    for (int n = 0; n < root->num_buffers; n++) {
+        if (root->buffers[n] == buffer) {
+            MP_TARRAY_REMOVE_AT(root->buffers, root->num_buffers, n);
+            goto found;
+        }
+    }
+
+    abort();
+
+found:
+
+    while (1) {
+        struct mp_log_buffer_entry *e = mp_msg_log_buffer_read(buffer);
+        if (!e)
+            break;
+        talloc_free(e);
+    }
+    talloc_free(buffer);
+
+    mp_atomic_add_and_fetch(&root->reload_counter, 1);
+    mp_memory_barrier();
+
+    pthread_mutex_unlock(&mp_msg_lock);
+}
+
+// Return a queued message, or if the buffer is empty, NULL.
+// Thread-safety: one buffer can be read by a single thread only.
+struct mp_log_buffer_entry *mp_msg_log_buffer_read(struct mp_log_buffer *buffer)
+{
+    void *ptr = NULL;
+    int read = mp_ring_read(buffer->ring, (unsigned char *)&ptr, sizeof(ptr));
+    if (read == 0)
+        return NULL;
+    if (read != sizeof(ptr))
+        abort();
+    return ptr;
+}
+
 // Thread-safety: fully thread-safe, but keep in mind that the lifetime of
 //                log must be guaranteed during the call.
 //                Never call this from signal handlers.

common/msg_control.h

@@ -12,6 +12,18 @@ void mp_msg_force_stderr(struct mpv_global *global, bool force_stderr);
 void mp_msg_flush_status_line(struct mpv_global *global);
 bool mp_msg_has_status_line(struct mpv_global *global);
 
+struct mp_log_buffer_entry {
+    char *prefix;
+    int level;
+    char *text;
+};
+
+struct mp_log_buffer;
+struct mp_log_buffer *mp_msg_log_buffer_new(struct mpv_global *global,
+                                            int size, int level);
+void mp_msg_log_buffer_destroy(struct mp_log_buffer *buffer);
+struct mp_log_buffer_entry *mp_msg_log_buffer_read(struct mp_log_buffer *buffer);
+
 struct bstr;
 int mp_msg_split_msglevel(struct bstr *s, struct bstr *out_mod, int *out_level);