avfilter/avfiltergraph: add logging for filter formats

There is no convenient way, from the command line, to figure out which
formats a filter actually supports. This commit changes that by adding a
log output, at debug level, to simply print the list of formats each filter
advertises on its links, before any negotiation.

Furthermore, we can use the exact same helper function to also print out the
corresponding filter links when there is an error during format negotiation.

We need to use AV_BRINT_SIZE_UNLIMITED because the default format list for
filters like vf_scale is about 1700 characters long, significantly larger than
the the 1 kB default buffer.

libavfilter/avfiltergraph.c

@@ -440,6 +440,62 @@ static int formats_declared(AVFilterContext *f)
     return 1;
 }
 
+static void print_formats(void *log_ctx, int level, enum AVMediaType type,
+                          const AVFilterFormats *formats)
+{
+    AVBPrint bp;
+    av_bprint_init(&bp, 0, AV_BPRINT_SIZE_UNLIMITED);
+
+    switch (type) {
+    case AVMEDIA_TYPE_VIDEO:
+        for (unsigned i = 0; i < formats->nb_formats; i++)
+            av_bprintf(&bp, "%s%s", bp.len ? " " : "", av_get_pix_fmt_name(formats->formats[i]));
+        break;
+    case AVMEDIA_TYPE_AUDIO:
+        for (unsigned i = 0; i < formats->nb_formats; i++)
+            av_bprintf(&bp, "%s%s", bp.len ? " " : "", av_get_sample_fmt_name(formats->formats[i]));
+        break;
+    default:
+        av_bprintf(&bp, "(unknown)");
+        break;
+    }
+
+    if (av_bprint_is_complete(&bp)) {
+        av_log(log_ctx, level, "%s\n", bp.str);
+    } else {
+        av_log(log_ctx, level, "(out of memory)\n");
+    }
+    av_bprint_finalize(&bp, NULL);
+}
+
+static void print_link_formats(void *log_ctx, int level, const AVFilterLink *l)
+{
+    if (av_log_get_level() < level)
+        return;
+
+    av_log(log_ctx, level, "Link '%s.%s' -> '%s.%s':\n"
+                           "  src: ", l->src->name, l->srcpad->name, l->dst->name, l->dstpad->name);
+    print_formats(log_ctx, level, l->type, l->incfg.formats);
+    av_log(log_ctx, level, "  dst: ");
+    print_formats(log_ctx, level, l->type, l->outcfg.formats);
+}
+
+static void print_filter_formats(void *log_ctx, int level, const AVFilterContext *f)
+{
+    if (av_log_get_level() < level)
+        return;
+
+    av_log(log_ctx, level, "Filter '%s' formats:\n", f->name);
+    for (int i = 0; i < f->nb_inputs; i++) {
+        av_log(log_ctx, level, "  in[%d] '%s': ", i, f->input_pads[i].name);
+        print_formats(log_ctx, level, f->inputs[i]->type, f->inputs[i]->outcfg.formats);
+    }
+    for (int i = 0; i < f->nb_outputs; i++) {
+        av_log(log_ctx, level, "  out[%d] '%s': ", i, f->output_pads[i].name);
+        print_formats(log_ctx, level, f->outputs[i]->type, f->outputs[i]->incfg.formats);
+    }
+}
+
 /**
  * Perform one round of query_formats() and merging formats lists on the
  * filter graph.
@@ -467,7 +523,10 @@ static int query_formats(AVFilterGraph *graph, void *log_ctx)
         if (ret < 0 && ret != AVERROR(EAGAIN))
             return ret;
         /* note: EAGAIN could indicate a partial success, not counted yet */
-        count_queried += ret >= 0;
+        if (ret >= 0) {
+            print_filter_formats(log_ctx, AV_LOG_DEBUG, f);
+            count_queried++;
+        }
     }
 
     /* go through and merge as many format lists as possible */
@@ -524,6 +583,7 @@ static int query_formats(AVFilterGraph *graph, void *log_ctx)
                            "The filters '%s' and '%s' do not have a common format "
                            "and automatic conversion is disabled.\n",
                            link->src->name, link->dst->name);
+                    print_link_formats(log_ctx, AV_LOG_ERROR, link);
                     return AVERROR(EINVAL);
                 }
 
@@ -532,6 +592,7 @@ static int query_formats(AVFilterGraph *graph, void *log_ctx)
                     av_log(log_ctx, AV_LOG_ERROR,
                            "'%s' filter not present, cannot convert formats.\n",
                            neg->conversion_filter);
+                    print_link_formats(log_ctx, AV_LOG_ERROR, link);
                     return AVERROR(EINVAL);
                 }
                 snprintf(inst_name, sizeof(inst_name), "auto_%s_%d",
@@ -583,6 +644,7 @@ static int query_formats(AVFilterGraph *graph, void *log_ctx)
                         av_log(log_ctx, AV_LOG_ERROR,
                                "Impossible to convert between the formats supported by the filter "
                                "'%s' and the filter '%s'\n", link->src->name, link->dst->name);
+                        print_link_formats(log_ctx, AV_LOG_ERROR, link);
                         return AVERROR(ENOSYS);
                     }
                 }