avfilter/f_metadata: allow direct flushing when printing to file

Useful for monitoring sparse data in realtime

doc/filters.texi

@@ -22731,6 +22731,9 @@ plain filename any writable url can be specified. Filename ``-'' is a shorthand
 for standard output. If @code{file} option is not set, output is written to the log
 with AV_LOG_INFO loglevel.
 
+@item direct
+Reduces buffering in print mode when output is written to a URL set using @var{file}.
+
 @end table
 
 @subsection Examples

libavfilter/f_metadata.c

@@ -88,6 +88,8 @@ typedef struct MetadataContext {
     int (*compare)(struct MetadataContext *s,
                    const char *value1, const char *value2);
     void (*print)(AVFilterContext *ctx, const char *msg, ...) av_printf_format(2, 3);
+
+    int direct;    // reduces buffering when printing to user-supplied URL
 } MetadataContext;
 
 #define OFFSET(x) offsetof(MetadataContext, x)
@@ -111,6 +113,7 @@ static const AVOption filt_name##_options[] = { \
     {   "ends_with",   NULL, 0, AV_OPT_TYPE_CONST, {.i64 = METADATAF_ENDS_WITH },   0, 0, FLAGS, "function" }, \
     { "expr", "set expression for expr function", OFFSET(expr_str), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, FLAGS }, \
     { "file", "set file where to print metadata information", OFFSET(file_str), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, FLAGS }, \
+    { "direct", "reduce buffering when printing to user-set file or pipe", OFFSET(direct), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, FLAGS }, \
     { NULL } \
 }
 
@@ -274,6 +277,9 @@ static av_cold int init(AVFilterContext *ctx)
                    s->file_str, buf);
             return ret;
         }
+
+        if (s->direct)
+            s->avio_context->direct = AVIO_FLAG_DIRECT;
     }
 
     return 0;