evhttpd implementation

- *Replace usage of boost::asio with [libevent2](http://libevent.org/)*.
boost::asio is not part of C++11, so unlike other boost there is no
forwards-compatibility reason to stick with it. Together with #4738 (convert
json_spirit to UniValue), this rids Bitcoin Core of the worst offenders with
regard to compile-time slowness.

- *Replace spit-and-duct-tape http server with evhttp*. Front-end http handling
is handled by libevent, a work queue (with configurable depth and parallelism)
is used to handle application requests.

- *Wrap HTTP request in C++ class*; this makes the application code mostly
HTTP-server-neutral

- *Refactor RPC to move all http-specific code to a separate file*.
Theoreticaly this can allow building without HTTP server but with another RPC
backend, e.g. Qt's debug console (currently not implemented) or future RPC
mechanisms people may want to use.

- *HTTP dispatch mechanism*; services (e.g., RPC, REST) register which URL
paths they want to handle.

By using a proven, high-performance asynchronous networking library (also used
by Tor) and HTTP server, problems such as #5674, #5655, #344 should be avoided.

What works? bitcoind, bitcoin-cli, bitcoin-qt. Unit tests and RPC/REST tests
pass. The aim for now is everything but SSL support.

Configuration options:

- `-rpcthreads`: repurposed as "number of  work handler threads". Still
defaults to 4.

- `-rpcworkqueue`: maximum depth of work queue. When this is reached, new
requests will return a 500 Internal Error.

- `-rpctimeout`: inactivity time, in seconds, after which to disconnect a
client.

- `-debug=http`: low-level http activity logging

1 files changed, 586 insertions, 0 deletions

diff --git a/src/httpserver.cpp b/src/httpserver.cpp
new file mode 100644
index 0000000000..89366b2e4e
--- /dev/null
+++ b/src/httpserver.cpp
@@ -0,0 +1,586 @@
+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "httpserver.h"
+
+#include "chainparamsbase.h"
+#include "compat.h"
+#include "util.h"
+#include "netbase.h"
+#include "rpcprotocol.h" // For HTTP status codes
+#include "sync.h"
+#include "ui_interface.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <signal.h>
+
+#include <event2/event.h>
+#include <event2/http.h>
+#include <event2/thread.h>
+#include <event2/buffer.h>
+#include <event2/util.h>
+#include <event2/keyvalq_struct.h>
+
+#ifdef EVENT__HAVE_NETINET_IN_H
+#include <netinet/in.h>
+#ifdef _XOPEN_SOURCE_EXTENDED
+#include <arpa/inet.h>
+#endif
+#endif
+
+#include <boost/algorithm/string/case_conv.hpp> // for to_lower()
+#include <boost/foreach.hpp>
+#include <boost/scoped_ptr.hpp>
+
+/** HTTP request work item */
+class HTTPWorkItem : public HTTPClosure
+{
+public:
+    HTTPWorkItem(HTTPRequest* req, const std::string &path, const HTTPRequestHandler& func):
+        req(req), path(path), func(func)
+    {
+    }
+    void operator()()
+    {
+        func(req.get(), path);
+    }
+
+    boost::scoped_ptr<HTTPRequest> req;
+
+private:
+    std::string path;
+    HTTPRequestHandler func;
+};
+
+/** Simple work queue for distributing work over multiple threads.
+ * Work items are simply callable objects.
+ */
+template <typename WorkItem>
+class WorkQueue
+{
+private:
+    /** Mutex protects entire object */
+    CWaitableCriticalSection cs;
+    CConditionVariable cond;
+    /* XXX in C++11 we can use std::unique_ptr here and avoid manual cleanup */
+    std::deque<WorkItem*> queue;
+    bool running;
+    size_t maxDepth;
+
+public:
+    WorkQueue(size_t maxDepth) : running(true),
+                                 maxDepth(maxDepth)
+    {
+    }
+    /* Precondition: worker threads have all stopped */
+    ~WorkQueue()
+    {
+        while (!queue.empty()) {
+            delete queue.front();
+            queue.pop_front();
+        }
+    }
+    /** Enqueue a work item */
+    bool Enqueue(WorkItem* item)
+    {
+        boost::unique_lock<boost::mutex> lock(cs);
+        if (queue.size() >= maxDepth) {
+            return false;
+        }
+        queue.push_back(item);
+        cond.notify_one();
+        return true;
+    }
+    /** Thread function */
+    void Run()
+    {
+        while (running) {
+            WorkItem* i = 0;
+            {
+                boost::unique_lock<boost::mutex> lock(cs);
+                while (running && queue.empty())
+                    cond.wait(lock);
+                if (!running)
+                    break;
+                i = queue.front();
+                queue.pop_front();
+            }
+            (*i)();
+            delete i;
+        }
+    }
+    /** Interrupt and exit loops */
+    void Interrupt()
+    {
+        boost::unique_lock<boost::mutex> lock(cs);
+        running = false;
+        cond.notify_all();
+    }
+
+    /** Return current depth of queue */
+    size_t Depth()
+    {
+        boost::unique_lock<boost::mutex> lock(cs);
+        return queue.size();
+    }
+};
+
+struct HTTPPathHandler
+{
+    HTTPPathHandler() {}
+    HTTPPathHandler(std::string prefix, bool exactMatch, HTTPRequestHandler handler):
+        prefix(prefix), exactMatch(exactMatch), handler(handler)
+    {
+    }
+    std::string prefix;
+    bool exactMatch;
+    HTTPRequestHandler handler;
+};
+
+/** HTTP module state */
+
+//! libevent event loop
+static struct event_base* eventBase = 0;
+//! HTTP server
+struct evhttp* eventHTTP = 0;
+//! List of subnets to allow RPC connections from
+static std::vector<CSubNet> rpc_allow_subnets;
+//! Work queue for handling longer requests off the event loop thread
+static WorkQueue<HTTPClosure>* workQueue = 0;
+//! Handlers for (sub)paths
+std::vector<HTTPPathHandler> pathHandlers;
+
+/** Check if a network address is allowed to access the HTTP server */
+static bool ClientAllowed(const CNetAddr& netaddr)
+{
+    if (!netaddr.IsValid())
+        return false;
+    BOOST_FOREACH (const CSubNet& subnet, rpc_allow_subnets)
+        if (subnet.Match(netaddr))
+            return true;
+    return false;
+}
+
+/** Initialize ACL list for HTTP server */
+static bool InitHTTPAllowList()
+{
+    rpc_allow_subnets.clear();
+    rpc_allow_subnets.push_back(CSubNet("127.0.0.0/8")); // always allow IPv4 local subnet
+    rpc_allow_subnets.push_back(CSubNet("::1"));         // always allow IPv6 localhost
+    if (mapMultiArgs.count("-rpcallowip")) {
+        const std::vector<std::string>& vAllow = mapMultiArgs["-rpcallowip"];
+        BOOST_FOREACH (std::string strAllow, vAllow) {
+            CSubNet subnet(strAllow);
+            if (!subnet.IsValid()) {
+                uiInterface.ThreadSafeMessageBox(
+                    strprintf("Invalid -rpcallowip subnet specification: %s. Valid are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24).", strAllow),
+                    "", CClientUIInterface::MSG_ERROR);
+                return false;
+            }
+            rpc_allow_subnets.push_back(subnet);
+        }
+    }
+    std::string strAllowed;
+    BOOST_FOREACH (const CSubNet& subnet, rpc_allow_subnets)
+        strAllowed += subnet.ToString() + " ";
+    LogPrint("http", "Allowing HTTP connections from: %s\n", strAllowed);
+    return true;
+}
+
+/** HTTP request method as string - use for logging only */
+static std::string RequestMethodString(HTTPRequest::RequestMethod m)
+{
+    switch (m) {
+    case HTTPRequest::GET:
+        return "GET";
+        break;
+    case HTTPRequest::POST:
+        return "POST";
+        break;
+    case HTTPRequest::HEAD:
+        return "HEAD";
+        break;
+    case HTTPRequest::PUT:
+        return "PUT";
+        break;
+    default:
+        return "unknown";
+    }
+}
+
+/** HTTP request callback */
+static void http_request_cb(struct evhttp_request* req, void* arg)
+{
+    std::auto_ptr<HTTPRequest> hreq(new HTTPRequest(req));
+
+    LogPrint("http", "Received a %s request for %s from %s\n",
+             RequestMethodString(hreq->GetRequestMethod()), hreq->GetURI(), hreq->GetPeer().ToString());
+
+    // Early address-based allow check
+    if (!ClientAllowed(hreq->GetPeer())) {
+        hreq->WriteReply(HTTP_FORBIDDEN);
+        return;
+    }
+
+    // Early reject unknown HTTP methods
+    if (hreq->GetRequestMethod() == HTTPRequest::UNKNOWN) {
+        hreq->WriteReply(HTTP_BADMETHOD);
+        return;
+    }
+
+    // Find registered handler for prefix
+    std::string strURI = hreq->GetURI();
+    std::string path;
+    std::vector<HTTPPathHandler>::const_iterator i = pathHandlers.begin();
+    std::vector<HTTPPathHandler>::const_iterator iend = pathHandlers.end();
+    for (; i != iend; ++i) {
+        bool match = false;
+        if (i->exactMatch)
+            match = (strURI == i->prefix);
+        else
+            match = (strURI.substr(0, i->prefix.size()) == i->prefix);
+        if (match) {
+            path = strURI.substr(i->prefix.size());
+            break;
+        }
+    }
+
+    // Dispatch to worker thread
+    if (i != iend) {
+        std::auto_ptr<HTTPWorkItem> item(new HTTPWorkItem(hreq.release(), path, i->handler));
+        assert(workQueue);
+        if (workQueue->Enqueue(item.get()))
+            item.release(); /* if true, queue took ownership */
+        else
+            item->req->WriteReply(HTTP_INTERNAL, "Work queue depth exceeded");
+    } else {
+        hreq->WriteReply(HTTP_NOTFOUND);
+    }
+}
+
+/** Event dispatcher thread */
+static void ThreadHTTP(struct event_base* base, struct evhttp* http)
+{
+    RenameThread("bitcoin-http");
+    LogPrint("http", "Entering http event loop\n");
+    event_base_dispatch(base);
+    // Event loop will be interrupted by InterruptHTTPServer()
+    LogPrint("http", "Exited http event loop\n");
+}
+
+/** Bind HTTP server to specified addresses */
+static bool HTTPBindAddresses(struct evhttp* http)
+{
+    int defaultPort = GetArg("-rpcport", BaseParams().RPCPort());
+    int nBound = 0;
+    std::vector<std::pair<std::string, uint16_t> > endpoints;
+
+    // Determine what addresses to bind to
+    if (!mapArgs.count("-rpcallowip")) { // Default to loopback if not allowing external IPs
+        endpoints.push_back(std::make_pair("::1", defaultPort));
+        endpoints.push_back(std::make_pair("127.0.0.1", defaultPort));
+        if (mapArgs.count("-rpcbind")) {
+            LogPrintf("WARNING: option -rpcbind was ignored because -rpcallowip was not specified, refusing to allow everyone to connect\n");
+        }
+    } else if (mapArgs.count("-rpcbind")) { // Specific bind address
+        const std::vector<std::string>& vbind = mapMultiArgs["-rpcbind"];
+        for (std::vector<std::string>::const_iterator i = vbind.begin(); i != vbind.end(); ++i) {
+            int port = defaultPort;
+            std::string host;
+            SplitHostPort(*i, port, host);
+            endpoints.push_back(std::make_pair(host, port));
+        }
+    } else { // No specific bind address specified, bind to any
+        endpoints.push_back(std::make_pair("::", defaultPort));
+        endpoints.push_back(std::make_pair("0.0.0.0", defaultPort));
+    }
+
+    // Bind addresses
+    for (std::vector<std::pair<std::string, uint16_t> >::iterator i = endpoints.begin(); i != endpoints.end(); ++i) {
+        LogPrint("http", "Binding RPC on address %s port %i\n", i->first, i->second);
+        if (evhttp_bind_socket(http, i->first.empty() ? NULL : i->first.c_str(), i->second) == 0) {
+            nBound += 1;
+        } else {
+            LogPrintf("Binding RPC on address %s port %i failed.\n", i->first, i->second);
+        }
+    }
+    return nBound > 0;
+}
+
+/** Simple wrapper to set thread name and run work queue */
+static void HTTPWorkQueueRun(WorkQueue<HTTPClosure>* queue)
+{
+    RenameThread("bitcoin-httpworker");
+    queue->Run();
+}
+
+bool StartHTTPServer(boost::thread_group& threadGroup)
+{
+    struct evhttp* http = 0;
+    struct event_base* base = 0;
+
+    if (!InitHTTPAllowList())
+        return false;
+
+    if (GetBoolArg("-rpcssl", false)) {
+        uiInterface.ThreadSafeMessageBox(
+            "SSL mode for RPC (-rpcssl) is no longer supported.",
+            "", CClientUIInterface::MSG_ERROR);
+        return false;
+    }
+
+#ifdef WIN32
+    evthread_use_windows_threads();
+#else
+    evthread_use_pthreads();
+#endif
+
+    base = event_base_new(); // XXX RAII
+    if (!base) {
+        LogPrintf("Couldn't create an event_base: exiting\n");
+        return false;
+    }
+
+    /* Create a new evhttp object to handle requests. */
+    http = evhttp_new(base); // XXX RAII
+    if (!http) {
+        LogPrintf("couldn't create evhttp. Exiting.\n");
+        event_base_free(base);
+        return false;
+    }
+
+    evhttp_set_timeout(http, GetArg("-rpctimeout", 30));
+    evhttp_set_max_body_size(http, MAX_SIZE);
+    evhttp_set_gencb(http, http_request_cb, NULL);
+
+    if (!HTTPBindAddresses(http)) {
+        LogPrintf("Unable to bind any endpoint for RPC server\n");
+        evhttp_free(http);
+        event_base_free(base);
+        return false;
+    }
+
+    LogPrint("http", "Starting HTTP server\n");
+    int workQueueDepth = std::max((long)GetArg("-rpcworkqueue", 16), 1L);
+    int rpcThreads = std::max((long)GetArg("-rpcthreads", 4), 1L);
+    LogPrintf("HTTP: creating work queue of depth %d and %d worker threads\n", workQueueDepth, rpcThreads);
+    workQueue = new WorkQueue<HTTPClosure>(workQueueDepth);
+
+    threadGroup.create_thread(boost::bind(&ThreadHTTP, base, http));
+
+    for (int i = 0; i < rpcThreads; i++)
+        threadGroup.create_thread(boost::bind(&HTTPWorkQueueRun, workQueue));
+
+    eventBase = base;
+    eventHTTP = http;
+    return true;
+}
+
+void InterruptHTTPServer()
+{
+    LogPrint("http", "Interrupting HTTP server\n");
+    if (eventBase)
+        event_base_loopbreak(eventBase);
+    if (workQueue)
+        workQueue->Interrupt();
+}
+
+void StopHTTPServer()
+{
+    LogPrint("http", "Stopping HTTP server\n");
+    delete workQueue;
+    if (eventHTTP) {
+        evhttp_free(eventHTTP);
+        eventHTTP = 0;
+    }
+    if (eventBase) {
+        event_base_free(eventBase);
+        eventBase = 0;
+    }
+}
+
+struct event_base* EventBase()
+{
+    return eventBase;
+}
+
+static void httpevent_callback_fn(evutil_socket_t, short, void* data)
+{
+    // Static handler simply passes through execution flow to _handle method
+    ((HTTPEvent*)data)->_handle();
+}
+
+void HTTPEvent::_handle()
+{
+    (*handler)();
+    if (deleteWhenTriggered)
+        delete this;
+}
+
+HTTPEvent::HTTPEvent(struct event_base* base, bool deleteWhenTriggered, HTTPClosure* handler) : deleteWhenTriggered(deleteWhenTriggered), handler(handler)
+{
+    ev = event_new(base, -1, 0, httpevent_callback_fn, this);
+    assert(ev);
+}
+HTTPEvent::~HTTPEvent()
+{
+    event_free(ev);
+}
+void HTTPEvent::trigger(struct timeval* tv)
+{
+    if (tv == NULL)
+        event_active(ev, 0, 0); // immediately trigger event in main thread
+    else
+        evtimer_add(ev, tv); // trigger after timeval passed
+}
+HTTPRequest::HTTPRequest(struct evhttp_request* req) : req(req),
+                                                       replySent(false)
+{
+}
+HTTPRequest::~HTTPRequest()
+{
+    if (!replySent) {
+        // Keep track of whether reply was sent to avoid request leaks
+        LogPrintf("%s: Unhandled request\n", __func__);
+        WriteReply(HTTP_INTERNAL, "Unhandled request");
+    }
+    // evhttpd cleans up the request, as long as a reply was sent.
+}
+
+std::pair<bool, std::string> HTTPRequest::GetHeader(const std::string& hdr)
+{
+    const struct evkeyvalq* headers = evhttp_request_get_input_headers(req);
+    assert(headers);
+    const char* val = evhttp_find_header(headers, hdr.c_str());
+    if (val)
+        return std::make_pair(true, val);
+    else
+        return std::make_pair(false, "");
+}
+
+std::string HTTPRequest::ReadBody()
+{
+    struct evbuffer* buf = evhttp_request_get_input_buffer(req);
+    if (!buf)
+        return "";
+    size_t size = evbuffer_get_length(buf);
+    /** Trivial implementation: if this is ever a performance bottleneck,
+     * internal copying can be avoided in multi-segment buffers by using
+     * evbuffer_peek and an awkward loop. Though in that case, it'd be even
+     * better to not copy into an intermediate string but use a stream
+     * abstraction to consume the evbuffer on the fly in the parsing algorithm.
+     */
+    const char* data = (const char*)evbuffer_pullup(buf, size);
+    if (!data) // returns NULL in case of empty buffer
+        return "";
+    std::string rv(data, size);
+    evbuffer_drain(buf, size);
+    return rv;
+}
+
+void HTTPRequest::WriteHeader(const std::string& hdr, const std::string& value)
+{
+    struct evkeyvalq* headers = evhttp_request_get_output_headers(req);
+    assert(headers);
+    evhttp_add_header(headers, hdr.c_str(), value.c_str());
+}
+
+/** Closure sent to main thread to request a reply to be sent to
+ * a HTTP request.
+ * Replies must be sent in the main loop in the main http thread,
+ * this cannot be done from worker threads.
+ */
+struct HTTPSendReplyHandler : HTTPClosure {
+public:
+    HTTPSendReplyHandler(struct evhttp_request* req, int nStatus) : req(req), nStatus(nStatus)
+    {
+    }
+    void operator()()
+    {
+        evhttp_send_reply(req, nStatus, NULL, NULL);
+    }
+private:
+    struct evhttp_request* req;
+    int nStatus;
+};
+
+void HTTPRequest::WriteReply(int nStatus, const std::string& strReply)
+{
+    assert(!replySent && req);
+    // Send event to main http thread to send reply message
+    struct evbuffer* evb = evhttp_request_get_output_buffer(req);
+    assert(evb);
+    evbuffer_add(evb, strReply.data(), strReply.size());
+    HTTPEvent* ev = new HTTPEvent(eventBase, true,
+                                  new HTTPSendReplyHandler(req, nStatus));
+    ev->trigger(0);
+    replySent = true;
+    req = 0; // transferred back to main thread
+}
+
+CService HTTPRequest::GetPeer()
+{
+    evhttp_connection* con = evhttp_request_get_connection(req);
+    CService peer;
+    if (con) {
+        // evhttp retains ownership over returned address string
+        const char* address = "";
+        uint16_t port = 0;
+        evhttp_connection_get_peer(con, (char**)&address, &port);
+        peer = CService(address, port);
+    }
+    return peer;
+}
+
+std::string HTTPRequest::GetURI()
+{
+    return evhttp_request_get_uri(req);
+}
+
+HTTPRequest::RequestMethod HTTPRequest::GetRequestMethod()
+{
+    switch (evhttp_request_get_command(req)) {
+    case EVHTTP_REQ_GET:
+        return GET;
+        break;
+    case EVHTTP_REQ_POST:
+        return POST;
+        break;
+    case EVHTTP_REQ_HEAD:
+        return HEAD;
+        break;
+    case EVHTTP_REQ_PUT:
+        return PUT;
+        break;
+    default:
+        return UNKNOWN;
+        break;
+    }
+}
+
+void RegisterHTTPHandler(const std::string &prefix, bool exactMatch, const HTTPRequestHandler &handler)
+{
+    LogPrint("http", "Registering HTTP handler for %s (exactmath %d)\n", prefix, exactMatch);
+    pathHandlers.push_back(HTTPPathHandler(prefix, exactMatch, handler));
+}
+
+void UnregisterHTTPHandler(const std::string &prefix, bool exactMatch)
+{
+    std::vector<HTTPPathHandler>::iterator i = pathHandlers.begin();
+    std::vector<HTTPPathHandler>::iterator iend = pathHandlers.end();
+    for (; i != iend; ++i)
+        if (i->prefix == prefix && i->exactMatch == exactMatch)
+            break;
+    if (i != iend)
+    {
+        LogPrint("http", "Unregistering HTTP handler for %s (exactmath %d)\n", prefix, exactMatch);
+        pathHandlers.erase(i);
+    }
+}
+