lavu/channel_layout: change av_get_channel_layout() behavior at the next bump

The new syntax is preferred since it allows backward syntax compatibility
with libswr when switching to the new option handling code with
AV_OPT_TYPE_CHANNEL_LAYOUT.

With the new parser the string:
1234

is interpreted as a channel layout mask, rather than as a number of
channels, and thus it's compatible with the current way to set a channel
layout as an integer (e.g. for the icl and ocl options) making use of
integer option values.

ff_get_channel_layout() with compat=0 will be used in the
AV_OPT_TYPE_CHANNEL handler code.

The user is encouraged to switch to the new forward compatible syntax,
which requires to put a trailing "c" when specifying a layout as a number
of channels.

libavutil/channel_layout.c

@@ -103,7 +103,11 @@ static const struct {
     { "downmix",     2,  AV_CH_LAYOUT_STEREO_DOWNMIX, },
 };
 
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+static uint64_t get_channel_layout_single(const char *name, int name_len, int compat)
+#else
 static uint64_t get_channel_layout_single(const char *name, int name_len)
+#endif
 {
     int i;
     char *end;
@@ -120,16 +124,40 @@ static uint64_t get_channel_layout_single(const char *name, int name_len)
             !memcmp(channel_names[i].name, name, name_len))
             return (int64_t)1 << i;
     i = strtol(name, &end, 10);
-    if (end - name == name_len ||
-        (end + 1 - name == name_len && *end  == 'c'))
+
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+    if (compat) {
+        if (end - name == name_len ||
+            (end + 1 - name == name_len && *end  == 'c')) {
+            layout = av_get_default_channel_layout(i);
+            if (end - name == name_len) {
+                av_log(NULL, AV_LOG_WARNING,
+                       "Single channel layout '%.*s' is interpreted as a number of channels, "
+                       "switch to the syntax '%.*sc' otherwise it will be interpreted as a "
+                       "channel layout number in a later version\n",
+                       name_len, name, name_len, name);
+                return layout;
+            }
+        }
+    } else {
+#endif
+    if ((end + 1 - name == name_len && *end  == 'c'))
         return av_get_default_channel_layout(i);
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+    }
+#endif
+
     layout = strtoll(name, &end, 0);
     if (end - name == name_len)
         return FFMAX(layout, 0);
     return 0;
 }
 
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+uint64_t ff_get_channel_layout(const char *name, int compat)
+#else
 uint64_t av_get_channel_layout(const char *name)
+#endif
 {
     const char *n, *e;
     const char *name_end = name + strlen(name);
@@ -137,7 +165,11 @@ uint64_t av_get_channel_layout(const char *name)
 
     for (n = name; n < name_end; n = e + 1) {
         for (e = n; e < name_end && *e != '+' && *e != '|'; e++);
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+        layout_single = get_channel_layout_single(n, e - n, compat);
+#else
         layout_single = get_channel_layout_single(n, e - n);
+#endif
         if (!layout_single)
             return 0;
         layout |= layout_single;
@@ -145,6 +177,13 @@ uint64_t av_get_channel_layout(const char *name)
     return layout;
 }
 
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+uint64_t av_get_channel_layout(const char *name)
+{
+    return ff_get_channel_layout(name, 1);
+}
+#endif
+
 void av_bprint_channel_layout(struct AVBPrint *bp,
                               int nb_channels, uint64_t channel_layout)
 {

libavutil/channel_layout.h

@@ -136,7 +136,12 @@ enum AVMatrixEncoding {
  * - a channel layout mask, in hexadecimal starting with "0x" (see the
  *   AV_CH_* macros).
  *
- * Example: "stereo+FC" = "2+FC" = "2c+1c" = "0x7"
+ * @warning Starting from the next major bump the trailing character
+ * 'c' to specify a number of channels will be required, while a
+ * channel layout mask could also be specified as a decimal number
+ * (if and only if not followed by "c").
+ *
+ * Example: "stereo+FC" = "2c+FC" = "2c+1c" = "0x7"
  */
 uint64_t av_get_channel_layout(const char *name);
 

libavutil/internal.h

@@ -39,6 +39,7 @@
 #include "timer.h"
 #include "cpu.h"
 #include "dict.h"
+#include "version.h"
 
 #if ARCH_X86
 #   include "x86/emms.h"
@@ -219,4 +220,8 @@ void avpriv_request_sample(void *avc,
  */
 int avpriv_open(const char *filename, int flags, ...);
 
+#if FF_API_GET_CHANNEL_LAYOUT_COMPAT
+uint64_t ff_get_channel_layout(const char *name, int compat);
+#endif
+
 #endif /* AVUTIL_INTERNAL_H */

libavutil/version.h

@@ -138,6 +138,9 @@
 #ifndef FF_API_VDPAU
 #define FF_API_VDPAU                    (LIBAVUTIL_VERSION_MAJOR < 53)
 #endif
+#ifndef FF_API_GET_CHANNEL_LAYOUT_COMPAT
+#define FF_API_GET_CHANNEL_LAYOUT_COMPAT (LIBAVUTIL_VERSION_MAJOR < 53)
+#endif
 
 /**
  * @}