avformat/libopenmpt: Probe file format from file data if possible

When building with libopenmpt 0.3, use the libopenmpt file header
probing functions for probing. libopenmpt probing functions are
allocation-free and designed to be as fast as possible.

For libopenmpt 0.2, or when libopenmpt 0.3 file header probing cannot
probe successfully due to too small probe buffer, test the filename
against the file extensions supported by the libopenmpt library that
is actually linked, instead of relying on a hard-coded file extension
list. File extension testing is also allocation-free and designed to
be fast in libopenmpt. Avoiding a hard-coded file extension list is
useful because later libopenmpt versions will likely add support for
more module file formats.

libopenmpt file header probing is tested regularly against the FATE
suite and other diverse file collections by libopenmpt upstream in
order to avoid false positives.

FATE passes with './configure --enable-libopenmpt' as well as with
'./configure --enable-libopenmpt --enable-libmodplug'.

libopenmpt probing adds about 5%..10% cpu time (depending on precise
usage pattern and host CPU and compiler version used for libopenmpt)
compared to all current internal FFmpeg probing functions combined in
tools/probetest for all of its module formats combined (currently 41
modules formats in libopenmpt 0.3.4 and 234 file formats in FFmpeg).
Signed-off-by: Jörn Heusipp <osmanx@problemloesungsmaschine.de>
Reviewed-by: Josh de Kock <josh@itanimul.li>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/libopenmpt.c

@@ -218,6 +218,62 @@ static int read_seek_openmpt(AVFormatContext *s, int stream_idx, int64_t ts, int
     return 0;
 }
 
+static int probe_openmpt_extension(AVProbeData *p)
+{
+    const char *ext;
+    if (p->filename) {
+        ext = strrchr(p->filename, '.');
+        if (ext && strlen(ext + 1) > 0) {
+            ext++;  /* skip '.' */
+            if (openmpt_is_extension_supported(ext) == 1)
+                return AVPROBE_SCORE_EXTENSION;
+        }
+    }
+    return 0;
+}
+
+static int read_probe_openmpt(AVProbeData *p)
+{
+#if OPENMPT_API_VERSION_AT_LEAST(0,3,0)
+    int probe_result;
+    if (p->buf && p->buf_size > 0) {
+        probe_result = openmpt_probe_file_header_without_filesize(
+                           OPENMPT_PROBE_FILE_HEADER_FLAGS_DEFAULT,
+                           p->buf, p->buf_size,
+                           &openmpt_logfunc, NULL, NULL, NULL, NULL, NULL);
+        if (probe_result == OPENMPT_PROBE_FILE_HEADER_RESULT_SUCCESS) {
+            /* As probing here relies on code external to FFmpeg, do not return
+             * AVPROBE_SCORE_MAX in order to reduce the impact in the rare
+             * cases of false positives.
+             */
+            return AVPROBE_SCORE_MIME + 1;
+        } else if (probe_result == OPENMPT_PROBE_FILE_HEADER_RESULT_WANTMOREDATA) {
+            if (probe_openmpt_extension(p) > 0) {
+                return AVPROBE_SCORE_RETRY;
+            } else {
+                if (p->buf_size >= openmpt_probe_file_header_get_recommended_size()) {
+                    /* We have already received the recommended amount of data
+                     * and still cannot decide. Return a rather low score.
+                     */
+                    return AVPROBE_SCORE_RETRY / 2;
+                } else {
+                    /* The file extension is unknown and we have very few data
+                     * bytes available. libopenmpt cannot decide anything here,
+                     * and returning any score > 0 would result in successfull
+                     * probing of random data.
+                     */
+                    return 0;
+                }
+            }
+        } else if (probe_result == OPENMPT_PROBE_FILE_HEADER_RESULT_FAILURE) {
+            return 0;
+        }
+    }
+#endif
+    /* for older libopenmpt, fall back to file extension probing */
+    return probe_openmpt_extension(p);
+}
+
 static const AVClass class_openmpt = {
     .class_name = "libopenmpt",
     .item_name  = av_default_item_name,
@@ -229,6 +285,7 @@ AVInputFormat ff_libopenmpt_demuxer = {
     .name           = "libopenmpt",
     .long_name      = NULL_IF_CONFIG_SMALL("Tracker formats (libopenmpt)"),
     .priv_data_size = sizeof(OpenMPTContext),
+    .read_probe     = read_probe_openmpt,
     .read_header    = read_header_openmpt,
     .read_packet    = read_packet_openmpt,
     .read_close     = read_close_openmpt,