[http/client] Support unix domain sockets as a backend

Предыдущий заход: https://a.yandex-team.ru/review/2811077/details#comment--4489184

После изменений можно будет обращаться к HTTP серверу, слушающему на unix domain socket. Пример такого сервера:
```js
require('http').createServer((req, res) => {
    console.log(req);

    res.writeHead(200, {'Content-Type': 'application/json'});
    res.end(JSON.stringify({
        'metrics': [
            {
                'labels': {
                    'name': 'metric',
                    'some_key': 'some_value'
                },
                'type': 'GAUGE',
                'value': 123
            }
        ]
    }, null, 4));
})
.listen('/home/ivanzhukov/test_ua/ua.sock');
// вместо .listen(7777);
```

4 files changed, 62 insertions, 0 deletions

diff --git a/util/network/address.cpp b/util/network/address.cpp
index 140ad0d3d0a..47badf504f2 100644
--- a/util/network/address.cpp
+++ b/util/network/address.cpp
@@ -6,6 +6,12 @@
     #include <sys/un.h>
 #endif
 
+#ifndef UNIX_PATH_MAX
+inline constexpr size_t UNIX_PATH_LIMIT = 108;
+#else
+inline constexpr size_t UNIX_PATH_LIMIT = UNIX_PATH_MAX;
+#endif
+
 using namespace NAddr;
 
 template <bool printPort>
@@ -201,3 +207,17 @@ socklen_t NAddr::SockAddrLength(const sockaddr* addr) {
 
     ythrow yexception() << "unsupported address family: " << addr->sa_family;
 }
+
+TUnixSocketAddr::TUnixSocketAddr(TStringBuf path) {
+    Y_ENSURE(path.Size() <= UNIX_PATH_LIMIT - 1,
+             "Unix path \"" << path << "\" is longer than " << UNIX_PATH_LIMIT);
+    SockAddr_.Set(path);
+}
+
+const sockaddr* TUnixSocketAddr::Addr() const {
+    return SockAddr_.SockAddr();
+}
+
+socklen_t TUnixSocketAddr::Len() const {
+    return SockAddr_.Len();
+}
diff --git a/util/network/address.h b/util/network/address.h
index 448fcac0c9d..fac61ce665a 100644
--- a/util/network/address.h
+++ b/util/network/address.h
@@ -5,6 +5,7 @@
 
 #include <util/generic/ptr.h>
 #include <util/generic/string.h>
+#include <util/network/sock.h>
 
 namespace NAddr {
     class IRemoteAddr {
@@ -133,4 +134,16 @@ namespace NAddr {
     private:
         const sockaddr_in6 A_;
     };
+
+    class TUnixSocketAddr: public IRemoteAddr {
+    public:
+        explicit TUnixSocketAddr(TStringBuf path);
+
+        const sockaddr* Addr() const override;
+
+        socklen_t Len() const override;
+
+    private:
+        TSockAddrLocal SockAddr_;
+    };
 }
diff --git a/util/network/sock.h b/util/network/sock.h
index b10be2f7156..d5ebeccbb0f 100644
--- a/util/network/sock.h
+++ b/util/network/sock.h
@@ -77,6 +77,14 @@ struct TSockAddrLocal: public ISockAddr {
         strlcpy(Path, path, PathSize);
     }
 
+    inline void Set(TStringBuf path) noexcept {
+        Clear();
+        in.sin_family = AF_INET;
+        in.sin_addr.s_addr = IpFromString("127.0.0.1");
+        in.sin_port = 0;
+        strlcpy(Path, path.data(), Min(PathSize, path.size() + 1));
+    }
+
     sockaddr* SockAddr() {
         return (struct sockaddr*)(&in);
     }
@@ -148,6 +156,10 @@ struct TSockAddrLocal: public sockaddr_un, public ISockAddr {
         Clear();
     }
 
+    TSockAddrLocal(TStringBuf path) {
+        Set(path);
+    }
+
     TSockAddrLocal(const char* path) {
         Set(path);
     }
@@ -170,6 +182,12 @@ struct TSockAddrLocal: public sockaddr_un, public ISockAddr {
         strlcpy(sun_path, path, sizeof(sun_path));
     }
 
+    inline void Set(TStringBuf path) noexcept {
+        Clear();
+        sun_family = AF_UNIX;
+        strlcpy(sun_path, path.data(), Min(sizeof(sun_path), path.size() + 1));
+    }
+
     sockaddr* SockAddr() override {
         return (struct sockaddr*)(struct sockaddr_un*)this;
     }
diff --git a/util/network/sock_ut.cpp b/util/network/sock_ut.cpp
index fd8c7837478..c702e1ba04d 100644
--- a/util/network/sock_ut.cpp
+++ b/util/network/sock_ut.cpp
@@ -3,6 +3,7 @@
 #include <library/cpp/testing/unittest/registar.h>
 #include <library/cpp/threading/future/legacy_future.h>
 
+#include <util/string/split.h>
 #include <util/system/fs.h>
 
 Y_UNIT_TEST_SUITE(TSocketTest) {
@@ -165,4 +166,14 @@ Y_UNIT_TEST_SUITE(TSocketTest) {
         NFs::Remove(localServerSockName);
     }
 
+    Y_UNIT_TEST(DetermingPath) {
+        const TString connectionString = "/var/run/some.sock http://localhost/endpoint";
+
+        TStringBuf sockPath, endpoint;
+        StringSplitter(connectionString).Split(' ').SkipEmpty().CollectInto(&sockPath, &endpoint);
+
+        TSockAddrLocal sal;
+        sal.Set(sockPath);
+        UNIT_ASSERT_STRINGS_EQUAL(sal.ToString(), "/var/run/some.sock");
+    }
 }