Improved error message when user attempts to decode a non-FLAC file (SF#2222789: https://sourceforge.net/tracker2/?func=detail&aid=2222789&group_id=13478&atid=113478

doc/html/changelog.html

@@ -83,8 +83,9 @@
 				<ul>
 					<li>Added support for encoding from and decoding to the RF64 format, and a new corresponding option <span class="argument"><a href="#flac_options_force_rf64_format" />--force-rf64-format</a></span>.  (<a href="https://sourceforge.net/tracker/index.php?func=detail&amp;aid=1762502&amp;group_id=13478&amp;atid=363478">SF #1762502</a>).  <span class="argument"><a href="documentation_tools_flac.html#flac_options_keep_foreign_metadata">--keep-foreign-metadata</a></span> is also supported.</li>
 					<li>Added support for encoding from and decoding to the Sony Wave64 format, and a new corresponding option <span class="argument"><a href="#flac_options_force_wave64_format" />--force-wave64-format</a></span>.  (<a href="https://sourceforge.net/tracker/index.php?func=detail&amp;aid=1769582&amp;group_id=13478&amp;atid=363478">SF #1769582</a>).  <span class="argument"><a href="documentation_tools_flac.html#flac_options_keep_foreign_metadata">--keep-foreign-metadata</a></span> is also supported.</li>
-					<li>Added a new options <span class="argument"><a href="documentation_tools_flac.html#flac_options_preserve_modtime">--preserve-modtime</a></span> and <span class="argument"><a href="documentation_tools_flac.html#flac_options_no_preserve_modtime">--no-preserve-modtime</a></span> to specify whether or not output files should copy the timestamp and permissions from their input files.  The default is <span class="argument"><a href="documentation_tools_flac.html#flac_options_preserve_modtime">--preserve-modtime</a></span> as in previous versions.  (<a href="https://sourceforge.net/tracker/index.php?func=detail&amp;aid=1805428&amp;group_id=13478&amp;atid=363478">SF #1805428</a>).</li>
+					<li>Added new options <span class="argument"><a href="documentation_tools_flac.html#flac_options_preserve_modtime">--preserve-modtime</a></span> and <span class="argument"><a href="documentation_tools_flac.html#flac_options_no_preserve_modtime">--no-preserve-modtime</a></span> to specify whether or not output files should copy the timestamp and permissions from their input files.  The default is <span class="argument"><a href="documentation_tools_flac.html#flac_options_preserve_modtime">--preserve-modtime</a></span> as in previous versions.  (<a href="https://sourceforge.net/tracker/index.php?func=detail&amp;aid=1805428&amp;group_id=13478&amp;atid=363478">SF #1805428</a>).</li>
 					<li>The <span class="argument"><a href="documentation_tools_flac.html#flac_options_sector_align">--sector-align</a></span> option of <span class="commandname">flac</span> has been deprecated and may not exist in future versions.  <a href="http://www.etree.org/shnutils/shntool/">shntool</a> provides similar functionality. (<a href="https://sourceforge.net/tracker/index.php?func=detail&amp;aid=1805946&amp;group_id=13478&amp;atid=363478">SF #1805946</a>)</li>
+					<li>Improved error message when user attempts to decode a non-FLAC file (<a href="https://sourceforge.net/tracker2/?func=detail&aid=2222789&group_id=13478&atid=113478">SF #2222789</a>).</li>
 					<li>Fix bug where <span class="commandname">flac</span> was disallowing use of <span class="argument">--replay-gain</span> when encoding from stdin (<a href="https://sourceforge.net/tracker2/?func=detail&aid=1840124&group_id=13478&atid=113478">SF #1840124</a>).</li>
 					<li>Fix bug with fractional seconds on some locales (<a href="https://sourceforge.net/tracker2/?func=detail&aid=1815517&group_id=13478&atid=113478">SF #1815517</a>, <a href="https://sourceforge.net/tracker2/?func=detail&aid=1858012&group_id=13478&atid=113478">SF #1858012</a>).</li>
 				</ul>

src/flac/decode.c

@@ -77,6 +77,7 @@ typedef struct {
 	FLAC__bool abort_flag;
 	FLAC__bool aborting_due_to_until; /* true if we intentionally abort decoding prematurely because we hit the --until point */
 	FLAC__bool aborting_due_to_unparseable; /* true if we abort decoding because we hit an unparseable frame */
+	FLAC__bool error_callback_suppress_messages; /* turn on to prevent repeating messages from the error callback */
 
 	FLAC__bool iff_headers_need_fixup;
 
@@ -228,6 +229,7 @@ FLAC__bool DecoderSession_construct(DecoderSession *d, FLAC__bool is_ogg, FLAC__
 	d->abort_flag = false;
 	d->aborting_due_to_until = false;
 	d->aborting_due_to_unparseable = false;
+	d->error_callback_suppress_messages = false;
 
 	d->iff_headers_need_fixup = false;
 
@@ -1357,11 +1359,26 @@ void error_callback(const FLAC__StreamDecoder *decoder, FLAC__StreamDecoderError
 {
 	DecoderSession *decoder_session = (DecoderSession*)client_data;
 	(void)decoder;
-	flac__utils_printf(stderr, 1, "%s: *** Got error code %d:%s\n", decoder_session->inbasefilename, status, FLAC__StreamDecoderErrorStatusString[status]);
+	if(!decoder_session->error_callback_suppress_messages)
+		flac__utils_printf(stderr, 1, "%s: *** Got error code %d:%s\n", decoder_session->inbasefilename, status, FLAC__StreamDecoderErrorStatusString[status]);
 	if(!decoder_session->continue_through_decode_errors) {
-		decoder_session->abort_flag = true;
+		/* if we got a sync error while looking for metadata, either it's not a FLAC file (more likely) or the file is corrupted */
-		if(status == FLAC__STREAM_DECODER_ERROR_STATUS_UNPARSEABLE_STREAM)
+		if(
+			!decoder_session->error_callback_suppress_messages &&
+			status == FLAC__STREAM_DECODER_ERROR_STATUS_LOST_SYNC &&
+			FLAC__stream_decoder_get_state(decoder) == FLAC__STREAM_DECODER_SEARCH_FOR_METADATA
+		) {
+			flac__utils_printf(stderr, 1,
+				"\n"
+				"The input file is either not a FLAC file or is corrupted.  If you are\n"
+				"convinced it is a FLAC file, you can rerun the same command and add the\n"
+				"-F parameter to try and recover as much as possible from the file.\n"
+			);
+			decoder_session->error_callback_suppress_messages = true;
+		}
+		else if(status == FLAC__STREAM_DECODER_ERROR_STATUS_UNPARSEABLE_STREAM)
 			decoder_session->aborting_due_to_unparseable = true;
+		decoder_session->abort_flag = true;
 	}
 }