resolves #19: laminard crashes under load

reading into a static buffer is a race condition that is only
manifested under load. There's no guarantee the clause in then()
will run before another task overwrites the buffer. Allocating
a local string is the only correct solution

src/laminar.cpp

@@ -371,7 +371,7 @@ void Laminar::run() {
     sigaddset(&mask, SIGCHLD);
     sigprocmask(SIG_BLOCK, &mask, NULL);
     int sigchld = signalfd(-1, &mask, 0);
-    srv->addDescriptor(sigchld, [this](char* buf, size_t sz){
+    srv->addDescriptor(sigchld, [this](const char* buf, size_t sz){
         struct signalfd_siginfo* siginfo = (struct signalfd_siginfo*) buf;
         // TODO: re-enable assertion when the cause for its triggering
         // is discovered and solved
@@ -505,7 +505,7 @@ std::shared_ptr<Run> Laminar::queueJob(std::string name, ParamMap params) {
 bool Laminar::stepRun(std::shared_ptr<Run> run) {
     bool complete = run->step();
     if(!complete) {
-        srv->addDescriptor(run->fd, [=](char* b,size_t n){
+        srv->addDescriptor(run->fd, [=](const char* b,size_t n){
             std::string s(b,n);
             run->log += s;
             for(LaminarClient* c : clients) {

src/server.cpp

@@ -420,7 +420,7 @@ void Server::stop() {
     eventfd_write(efd_quit, 1);
 }
 
-void Server::addDescriptor(int fd, std::function<void(char*,size_t)> cb) {
+void Server::addDescriptor(int fd, std::function<void(const char*,size_t)> cb) {
     auto event = this->ioContext.lowLevelProvider->wrapInputFd(fd, kj::LowLevelAsyncIoProvider::TAKE_OWNERSHIP);
     tasks.add(handleFdRead(event, cb).attach(std::move(event)));
 }
@@ -455,13 +455,14 @@ void Server::acceptRpcClient(kj::Own<kj::ConnectionReceiver>&& listener) {
 // returns a promise which will read a chunk of data from the file descriptor
 // wrapped by stream and invoke the provided callback with the read data.
 // Repeats until ::read returns <= 0
-kj::Promise<void> Server::handleFdRead(kj::AsyncInputStream* stream, std::function<void(char*,size_t)> cb) {
-    static char* buffer = new char[PROC_IO_BUFSIZE];
-    return stream->tryRead(buffer, 1, PROC_IO_BUFSIZE).then([this,stream,cb](size_t sz) {
+kj::Promise<void> Server::handleFdRead(kj::AsyncInputStream* stream, std::function<void(const char*,size_t)> cb) {
+    std::string buffer;
+    buffer.reserve(PROC_IO_BUFSIZE);
+    return stream->tryRead((void*)buffer.data(), 1, PROC_IO_BUFSIZE).then(kj::mvCapture(kj::mv(buffer), [this,stream,cb](std::string&& buffer, size_t sz) {
         if(sz > 0) {
-            cb(buffer, sz);
+            cb(buffer.data(), sz);
             return handleFdRead(stream, cb);
         }
         return kj::Promise<void>(kj::READY_NOW);
-    });
+    })).attach(kj::mv(buffer));
 }

src/server.h

@@ -41,12 +41,12 @@ public:
 
     // add a file descriptor to be monitored for output. The callback will be
     // invoked with the read data
-    void addDescriptor(int fd, std::function<void(char*,size_t)> cb);
+    void addDescriptor(int fd, std::function<void(const char*,size_t)> cb);
 
 private:
     void acceptHttpClient(kj::Own<kj::ConnectionReceiver>&& listener);
     void acceptRpcClient(kj::Own<kj::ConnectionReceiver>&& listener);
-    kj::Promise<void> handleFdRead(kj::AsyncInputStream* stream, std::function<void(char*,size_t)> cb);
+    kj::Promise<void> handleFdRead(kj::AsyncInputStream* stream, std::function<void(const char*,size_t)> cb);
 
     void taskFailed(kj::Exception&& exception) override {
         kj::throwFatalException(kj::mv(exception));