decoder/Client: pass std::exception_ptr to method SeekError()

This allows decoder plugins to show detailed seek errors that will be
included in the seek error message in the MPD protocol.

Currently, this new parameter is optional, and some decoders may never
pass details because they don't have any.

Author: MaxKellermann

NEWS

@@ -2,6 +2,7 @@ ver 0.25 (not yet released)
 * protocol
   - implement "window" parameter for command "list"
   - new command "stringnormalization"
+  - show detailed seek errors
 * decoder
   - vgmstream: new plugin
 * output

src/decoder/Bridge.cxx

@@ -348,24 +348,27 @@ DecoderBridge::GetSeekFrame() noexcept
 }
 
 void
-DecoderBridge::SeekError() noexcept
+DecoderBridge::SeekError(std::exception_ptr &&_error) noexcept
 {
 	assert(dc.pipe != nullptr);
 
+	if (!_error)
+		_error = std::make_exception_ptr(std::runtime_error{"Decoder failed to seek"});
+
 	if (initial_seek_running) {
 		/* d'oh, we can't seek to the sub-song start position,
 		   what now? - no idea, ignoring the problem for now. */
 		initial_seek_running = false;
 
 		if (initial_seek_essential)
-			error = std::make_exception_ptr(std::runtime_error("Decoder failed to seek"));
+			error = std::move(_error);
 
 		return;
 	}
 
 	assert(dc.command == DecoderCommand::SEEK);
 
-	dc.seek_error = true;
+	dc.seek_error = std::move(_error);
 	seeking = false;
 
 	CommandFinished();

src/decoder/Bridge.hxx

@@ -157,7 +157,7 @@ public:
 	void CommandFinished() noexcept override;
 	SongTime GetSeekTime() noexcept override;
 	uint64_t GetSeekFrame() noexcept override;
-	void SeekError() noexcept override;
+	void SeekError(std::exception_ptr &&_error) noexcept override;
 	InputStreamPtr OpenUri(std::string_view uri) override;
 	size_t Read(InputStream &is,
 		    std::span<std::byte> dest) noexcept override;

src/decoder/Client.hxx

@@ -9,6 +9,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <exception>
 #include <span>
 #include <string_view>
 
@@ -71,8 +72,10 @@ public:
 	/**
 	 * Call this instead of CommandFinished() when seeking has
 	 * failed.
+	 *
+	 * @param error details about the seek error (optional)
 	 */
-	virtual void SeekError() noexcept = 0;
+	virtual void SeekError(std::exception_ptr &&error={}) noexcept = 0;
 
 	/**
 	 * Open a new #InputStream and wait until it's ready.

src/decoder/Control.cxx

@@ -119,7 +119,7 @@ DecoderControl::Seek(std::unique_lock<Mutex> &lock, SongTime t)
 		throw std::runtime_error("Not seekable");
 
 	seek_time = t;
-	seek_error = false;
+	seek_error = {};
 	SynchronousCommandLocked(lock, DecoderCommand::SEEK);
 
 	while (state == DecoderState::START)
@@ -135,7 +135,7 @@ DecoderControl::Seek(std::unique_lock<Mutex> &lock, SongTime t)
 		WaitForDecoder(lock);
 
 	if (seek_error)
-		throw std::runtime_error("Decoder failed to seek");
+		std::rethrow_exception(seek_error);
 }
 
 void

src/decoder/Control.hxx

@@ -90,11 +90,15 @@ public:
 	 */
 	std::exception_ptr error;
 
+	/**
+	 * The most recent seek error.
+	 */
+	std::exception_ptr seek_error;
+
 private:
 	bool quit;
 
 public:
-	bool seek_error;
 	bool seekable;
 
 	/**

src/decoder/plugins/DsdiffDecoderPlugin.cxx

@@ -385,7 +385,7 @@ dsdiff_decode_chunk(DecoderClient &client, InputStream &is,
 				} else
 					client.SeekError();
 			} catch (...) {
-				client.SeekError();
+				client.SeekError(std::current_exception());
 			}
 		}
 

src/decoder/plugins/DsfDecoderPlugin.cxx

@@ -265,7 +265,7 @@ dsf_decode_chunk(DecoderClient &client, InputStream &is,
 				} else
 					client.SeekError();
 			} catch (...) {
-				client.SeekError();
+				client.SeekError(std::current_exception());
 			}
 		}
 

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -543,9 +543,10 @@ FfmpegDecode(DecoderClient &client, InputStream *input,
 			/* AVSEEK_FLAG_BACKWARD asks FFmpeg to seek to
 			   the packet boundary before the seek time
 			   stamp, not after */
-			if (av_seek_frame(&format_context, audio_stream, where,
-					  AVSEEK_FLAG_ANY|AVSEEK_FLAG_BACKWARD) < 0)
-				client.SeekError();
+			if (int error = av_seek_frame(&format_context, audio_stream, where,
+						      AVSEEK_FLAG_ANY|AVSEEK_FLAG_BACKWARD);
+			    error < 0)
+				client.SeekError(std::make_exception_ptr(MakeFfmpegError(error, "av_seek_frame() failed")));
 			else {
 				codec_context.FlushBuffers();
 				min_frame = client.GetSeekFrame();

src/decoder/plugins/GmeDecoderPlugin.cxx

@@ -13,6 +13,7 @@
 #include "fs/FileSystem.hxx"
 #include "fs/NarrowPath.hxx"
 #include "lib/fmt/PathFormatter.hxx"
+#include "lib/fmt/RuntimeError.hxx"
 #include "util/ScopeExit.hxx"
 #include "util/StringCompare.hxx"
 #include "util/Domain.hxx"
@@ -28,6 +29,8 @@
 
 #define SUBTUNE_PREFIX "tune_"
 
+using std::string_view_literals::operator""sv;
+
 static constexpr Domain gme_domain("gme");
 
 static constexpr unsigned GME_SAMPLE_RATE = 44100;
@@ -202,8 +205,7 @@ gme_file_decode(DecoderClient &client, Path path_fs)
 			unsigned where = client.GetSeekTime().ToMS();
 			gme_err = gme_seek(emu, where);
 			if (gme_err != nullptr) {
-				LogWarning(gme_domain, gme_err);
-				client.SeekError();
+				client.SeekError(std::make_exception_ptr(FmtRuntimeError("gme_see() failed: {}"sv, gme_err)));
 			} else
 				client.CommandFinished();
 		}