Add TCP store

Like UDSRemoteStore, but over a TCP connection. Example usage:

  $ nix path-info --store tcp://example.org:1234 ...

src/libstore/tcp-store.cc

@@ -0,0 +1,107 @@
+#include "remote-store.hh"
+#include "finally.hh"
+
+#include <fcntl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/un.h>
+#include <unistd.h>
+#include <arpa/inet.h>
+#include <netdb.h>
+
+namespace nix {
+
+struct TCPStoreConfig : virtual RemoteStoreConfig
+{
+    using RemoteStoreConfig::RemoteStoreConfig;
+
+    const std::string name() override { return "TCP Store"; }
+};
+
+struct TCPStore : public virtual TCPStoreConfig, public virtual RemoteStore
+{
+    std::string host;
+    uint16_t port = 0;
+
+    TCPStore(const std::string scheme, std::string path, const Params & params)
+        : StoreConfig(params)
+        , RemoteStoreConfig(params)
+        , TCPStoreConfig(params)
+        , Store(params)
+        , RemoteStore(params)
+    {
+        // FIXME: use parsed URL.
+
+        auto p = path.find(':');
+        if (p == std::string::npos)
+            throw UsageError("tcp:// stores require a port number (e.g. 'tcp://example.org:1234'), in '%s'", path);
+
+        host = std::string(path, 0, p);
+        if (auto port2 = string2Int<uint16_t>(std::string(path, p + 1)))
+            port = *port2;
+        else
+            throw UsageError("invalid TCP port number, in '%s'", path);
+    }
+
+    std::string getUri() override
+    { return fmt("tcp://"); }
+
+    static std::set<std::string> uriSchemes()
+    { return {"tcp"}; }
+
+    bool sameMachine() override
+    { return false; }
+
+    ref<RemoteStore::Connection> openConnection() override
+    {
+        auto conn = make_ref<Connection>();
+
+        struct addrinfo hints;
+        memset(&hints, 0, sizeof(hints));
+        hints.ai_family = AF_UNSPEC;
+        hints.ai_socktype = SOCK_STREAM;
+        hints.ai_flags = 0;
+        hints.ai_protocol = IPPROTO_TCP;
+
+        struct addrinfo * result;
+        if (auto s = getaddrinfo(host.c_str(), fmt("%d", port).c_str(), &hints, &result))
+            throw Error("DNS lookup of '%s' failed: %s", host, gai_strerror(s));
+
+        Finally cleanup([&]() { freeaddrinfo(result); });
+
+        std::string err;
+
+        for (auto rp = result; rp; rp = rp->ai_next) {
+            AutoCloseFD fd = socket(
+                rp->ai_family,
+                rp->ai_socktype
+                #ifdef SOCK_CLOEXEC
+                | SOCK_CLOEXEC
+                #endif
+                , rp->ai_protocol);
+            if (!fd) {
+                err = strerror(errno);
+                continue;
+            }
+            closeOnExec(fd.get());
+
+            if (::connect(fd.get(), rp->ai_addr, rp->ai_addrlen) == -1) {
+                err = strerror(errno);
+                continue;
+            }
+
+            conn->fd = std::move(fd);
+            conn->from.fd = conn->fd.get();
+            conn->to.fd = conn->fd.get();
+            conn->startTime = std::chrono::steady_clock::now();
+            return conn;
+        }
+
+        throw Error("could not connect to daemon at '%s': %s", host, err);
+    }
+};
+
+static RegisterStoreImplementation<TCPStore, TCPStoreConfig> regTCPStore;
+
+}