feat: update current remote description with fingerprint

Adds a `currentRemoteDescription` field to `PeerConnection` which
can be updated independently of `remoteDescription`, the idea being
the `remoteDescription` is the offer received from the remote peer
and the `currentRemoteDescription` is what has actually been
negotiated with them.

The `currentRemoteDescription` is initialised when the
`remoteDescription` is set.

Currently the `checkFingerprint` method updates the certificate
fingerprint used by the connection to the remote peer but I guess
other modifications may be useful.

Exposes the `currentRemoteDescription` via a getter similar to
`remoteDescription`.

DOC.md

@@ -278,6 +278,24 @@ Return value: the length of the string copied in buffer (including the terminati
 
 If `buffer` is `NULL`, the description is not copied but the size is still returned.
 
+#### rtcGetCurrentRemoteDescription
+
+```
+int rtcGetCurrentRemoteDescription(int pc, char *buffer, int size)
+```
+
+Retrieves the currently negotiated remote description in SDP format.
+
+Arguments:
+
+- `pc`: the Peer Connection identifier
+- `buffer`: a user-supplied buffer to store the description
+- `size`: the size of `buffer`
+
+Return value: the length of the string copied in buffer (including the terminating null character) or a negative error code
+
+If `buffer` is `NULL`, the description is not copied but the size is still returned.
+
 #### rtcGetLocalDescriptionType
 
 ```

include/rtc/peerconnection.hpp

@@ -84,6 +84,7 @@ class RTC_CPP_EXPORT PeerConnection final : CheshireCat<impl::PeerConnection> {
 	bool hasMedia() const;
 	optional<Description> localDescription() const;
 	optional<Description> remoteDescription() const;
+	optional<Description> currentRemoteDescription() const;
 	size_t remoteMaxMessageSize() const;
 	optional<string> localAddress() const;
 	optional<string> remoteAddress() const;

include/rtc/rtc.h

@@ -216,6 +216,7 @@ RTC_C_EXPORT int rtcAddRemoteCandidate(int pc, const char *cand, const char *mid
 
 RTC_C_EXPORT int rtcGetLocalDescription(int pc, char *buffer, int size);
 RTC_C_EXPORT int rtcGetRemoteDescription(int pc, char *buffer, int size);
+RTC_C_EXPORT int rtcGetCurrentRemoteDescription(int pc, char *buffer, int size);
 
 RTC_C_EXPORT int rtcGetLocalDescriptionType(int pc, char *buffer, int size);
 RTC_C_EXPORT int rtcGetRemoteDescriptionType(int pc, char *buffer, int size);

src/capi.cpp

@@ -609,6 +609,17 @@ int rtcGetRemoteDescription(int pc, char *buffer, int size) {
 	});
 }
 
+int rtcGetCurrentRemoteDescription(int pc, char *buffer, int size) {
+	return wrap([&] {
+		auto peerConnection = getPeerConnection(pc);
+
+		if (auto desc = peerConnection->currentRemoteDescription())
+			return copyAndReturn(string(*desc), buffer, size);
+		else
+			return RTC_ERR_NOT_AVAIL;
+	});
+}
+
 int rtcGetLocalDescriptionType(int pc, char *buffer, int size) {
 	return wrap([&] {
 		auto peerConnection = getPeerConnection(pc);

src/impl/peerconnection.cpp

@@ -118,6 +118,11 @@ optional<Description> PeerConnection::remoteDescription() const {
 	return mRemoteDescription;
 }
 
+optional<Description> PeerConnection::currentRemoteDescription() const {
+	std::lock_guard lock(mCurrentRemoteDescriptionMutex);
+	return mCurrentRemoteDescription;
+}
+
 size_t PeerConnection::remoteMaxMessageSize() const {
 	const size_t localMax = config.maxMessageSize.value_or(DEFAULT_LOCAL_MAX_MESSAGE_SIZE);
 
@@ -239,7 +244,7 @@ shared_ptr<DtlsTransport> PeerConnection::initDtlsTransport() {
 			throw std::logic_error("No underlying ICE transport for DTLS transport");
 
 		auto certificate = mCertificate.get();
-		auto verifierCallback = weak_bind(&PeerConnection::checkFingerprint, this, _1);
+		auto verifierCallback = weak_bind(&PeerConnection::checkFingerprint, this, _1, fingerprintAlgorithm);
 		auto dtlsStateChangeCallback =
 		    [this, weak_this = weak_from_this()](DtlsTransport::State transportState) {
 			    auto shared_this = weak_this.lock();
@@ -439,17 +444,21 @@ void PeerConnection::rollbackLocalDescription() {
 	}
 }
 
-bool PeerConnection::checkFingerprint(const std::string &fingerprint) const {
+bool PeerConnection::checkFingerprint(const std::string &fingerprint, const CertificateFingerprint::Algorithm algorithm) {
 	std::lock_guard lock(mRemoteDescriptionMutex);
 	if (!mRemoteDescription || !mRemoteDescription->fingerprint())
 		return false;
 
-	if (config.disableFingerprintVerification)
-		return true;
-
 	auto expectedFingerprint = mRemoteDescription->fingerprint()->value;
-	if (expectedFingerprint  == fingerprint) {
+	if (config.disableFingerprintVerification || expectedFingerprint == fingerprint) {
 		PLOG_VERBOSE << "Valid fingerprint \"" << fingerprint << "\"";
+
+		if (mCurrentRemoteDescription) {
+			std::lock_guard lock(mCurrentRemoteDescriptionMutex);
+			// update current remote description with incoming fingerprint
+			mCurrentRemoteDescription->setFingerprint(CertificateFingerprint{algorithm, fingerprint});
+		}
+
 		return true;
 	}
 
@@ -1078,6 +1087,20 @@ void PeerConnection::processLocalCandidate(Candidate candidate) {
 	                   &localCandidateCallback, std::move(candidate));
 }
 
+void PeerConnection::processCurrentRemoteDescription(Description description) {
+	{
+		// Set as current remote description
+		std::lock_guard lock(mCurrentRemoteDescriptionMutex);
+
+		std::vector<Candidate> existingCandidates;
+		if (mCurrentRemoteDescription)
+			existingCandidates = mCurrentRemoteDescription->extractCandidates();
+
+		mCurrentRemoteDescription.emplace(description);
+		mCurrentRemoteDescription->addCandidates(std::move(existingCandidates));
+	}
+}
+
 void PeerConnection::processRemoteDescription(Description description) {
 	// Update the SSRC cache for existing tracks
 	updateTrackSsrcCache(description);
@@ -1124,6 +1147,14 @@ void PeerConnection::processRemoteCandidate(Candidate candidate) {
 		candidate.resolve(Candidate::ResolveMode::Simple);
 		mRemoteDescription->addCandidate(candidate);
 	}
+	{
+		// Set as remote candidate on current remote description too
+		std::lock_guard lock(mCurrentRemoteDescriptionMutex);
+		if (mCurrentRemoteDescription->hasCandidate(candidate))
+			return; // already in description, ignore
+
+		mCurrentRemoteDescription->addCandidate(candidate);
+	}
 
 	if (candidate.isResolved()) {
 		iceTransport->addRemoteCandidate(std::move(candidate));

src/impl/peerconnection.hpp

@@ -41,6 +41,7 @@ struct PeerConnection : std::enable_shared_from_this<PeerConnection> {
 
 	optional<Description> localDescription() const;
 	optional<Description> remoteDescription() const;
+	optional<Description> currentRemoteDescription() const;
 	size_t remoteMaxMessageSize() const;
 
 	shared_ptr<IceTransport> initIceTransport();
@@ -53,7 +54,7 @@ struct PeerConnection : std::enable_shared_from_this<PeerConnection> {
 
 	void endLocalCandidates();
 	void rollbackLocalDescription();
-	bool checkFingerprint(const std::string &fingerprint) const;
+	bool checkFingerprint(const std::string &fingerprint, const CertificateFingerprint::Algorithm algorithm);
 	void forwardMessage(message_ptr message);
 	void forwardMedia(message_ptr message);
 	void forwardBufferedAmount(uint16_t stream, size_t amount);
@@ -77,6 +78,7 @@ struct PeerConnection : std::enable_shared_from_this<PeerConnection> {
 	void processLocalDescription(Description description);
 	void processLocalCandidate(Candidate candidate);
 	void processRemoteDescription(Description description);
+	void processCurrentRemoteDescription(Description description);
 	void processRemoteCandidate(Candidate candidate);
 	string localBundleMid() const;
 
@@ -135,8 +137,8 @@ struct PeerConnection : std::enable_shared_from_this<PeerConnection> {
 
 	Processor mProcessor;
 	optional<Description> mLocalDescription, mRemoteDescription;
-	optional<Description> mCurrentLocalDescription;
-	mutable std::mutex mLocalDescriptionMutex, mRemoteDescriptionMutex;
+	optional<Description> mCurrentLocalDescription, mCurrentRemoteDescription;
+	mutable std::mutex mLocalDescriptionMutex, mRemoteDescriptionMutex, mCurrentRemoteDescriptionMutex;
 
 	shared_ptr<MediaHandler> mMediaHandler;
 

src/peerconnection.cpp

@@ -69,6 +69,10 @@ optional<Description> PeerConnection::remoteDescription() const {
 	return impl()->remoteDescription();
 }
 
+optional<Description> PeerConnection::currentRemoteDescription() const {
+	return impl()->currentRemoteDescription();
+}
+
 size_t PeerConnection::remoteMaxMessageSize() const { return impl()->remoteMaxMessageSize(); }
 
 bool PeerConnection::hasMedia() const {
@@ -243,6 +247,7 @@ void PeerConnection::setRemoteDescription(Description description) {
 	iceTransport->setRemoteDescription(description); // ICE transport might reject the description
 
 	impl()->processRemoteDescription(std::move(description));
+	impl()->processCurrentRemoteDescription(std::move(description));
 	impl()->changeSignalingState(newSignalingState);
 	signalingLock.unlock();