avformat/cafdec: Implement FLAC-in-CAF parsing

The afconvert utility shipped with MacOS supports muxing of FLAC
in CAF, see afconvert help output on a recent Mac here:
https://hydrogenaud.io/index.php?topic=122509.0 A file created
with afconvert free of copyright (licensed CC0) can be found here:
http://www.audiograaf.nl/misc_stuff/afconvert-FLAC-in-CAF.caf

libavformat/caf.c

@@ -46,6 +46,7 @@ const AVCodecTag ff_codec_caf_tags[] = {
     { AV_CODEC_ID_GSM,             MKTAG('a','g','s','m') },
     { AV_CODEC_ID_GSM_MS,          MKTAG('m','s', 0, '1') },
     { AV_CODEC_ID_ILBC,            MKTAG('i','l','b','c') },
+    { AV_CODEC_ID_FLAC,            MKTAG('f','l','a','c') },
     { AV_CODEC_ID_MACE3,           MKTAG('M','A','C','3') },
     { AV_CODEC_ID_MACE6,           MKTAG('M','A','C','6') },
     { AV_CODEC_ID_MP1,             MKTAG('.','m','p','1') },

libavformat/cafdec.c

@@ -32,6 +32,7 @@
 #include "internal.h"
 #include "isom.h"
 #include "mov_chan.h"
+#include "libavcodec/flac.h"
 #include "libavutil/intreadwrite.h"
 #include "libavutil/intfloat.h"
 #include "libavutil/dict.h"
@@ -170,6 +171,49 @@ static int read_kuki_chunk(AVFormatContext *s, int64_t size)
             }
             avio_skip(pb, size - ALAC_NEW_KUKI);
         }
+    } else if (st->codecpar->codec_id == AV_CODEC_ID_FLAC) {
+        int last, type, flac_metadata_size;
+        uint8_t buf[4];
+        /* The magic cookie format for FLAC consists mostly of an mp4 dfLa atom. */
+        if (size < (16 + FLAC_STREAMINFO_SIZE)) {
+            av_log(s, AV_LOG_ERROR, "invalid FLAC magic cookie\n");
+            return AVERROR_INVALIDDATA;
+        }
+        /* Check cookie version. */
+        if (avio_r8(pb) != 0) {
+            av_log(s, AV_LOG_ERROR, "unknown FLAC magic cookie\n");
+            return AVERROR_INVALIDDATA;
+        }
+        avio_rb24(pb); /* Flags */
+        /* read dfLa fourcc */
+        if (avio_read(pb, buf, 4) != 4) {
+            av_log(s, AV_LOG_ERROR, "failed to read FLAC magic cookie\n");
+            return pb->error < 0 ? pb->error : AVERROR_INVALIDDATA;
+        }
+        if (memcmp(buf, "dfLa", 4)) {
+            av_log(s, AV_LOG_ERROR, "invalid FLAC magic cookie\n");
+            return AVERROR_INVALIDDATA;
+        }
+        /* Check dfLa version. */
+        if (avio_r8(pb) != 0) {
+            av_log(s, AV_LOG_ERROR, "unknown dfLa version\n");
+            return AVERROR_INVALIDDATA;
+        }
+        avio_rb24(pb); /* Flags */
+        if (avio_read(pb, buf, sizeof(buf)) != sizeof(buf)) {
+            av_log(s, AV_LOG_ERROR, "failed to read FLAC metadata block header\n");
+            return pb->error < 0 ? pb->error : AVERROR_INVALIDDATA;
+        }
+        flac_parse_block_header(buf, &last, &type, &flac_metadata_size);
+        if (type != FLAC_METADATA_TYPE_STREAMINFO || flac_metadata_size != FLAC_STREAMINFO_SIZE) {
+            av_log(s, AV_LOG_ERROR, "STREAMINFO must be first FLACMetadataBlock\n");
+            return AVERROR_INVALIDDATA;
+        }
+        ret = ff_get_extradata(s, st->codecpar, pb, FLAC_STREAMINFO_SIZE);
+        if (ret < 0)
+            return ret;
+        if (!last)
+            av_log(s, AV_LOG_WARNING, "non-STREAMINFO FLACMetadataBlock(s) ignored\n");
     } else if (st->codecpar->codec_id == AV_CODEC_ID_OPUS) {
         // The data layout for Opus is currently unknown, so we do not export
         // extradata at all. Multichannel streams are not supported.