fix: 网关 Modbus 复用连接并加连接超时，避免单主机挂死拖垮其它网关

mb-102 connect 阻塞会导致 mb-104 轮询超时被标成读失败；0.3.12 非阻塞 connect + Session 复用后 fail-fast。

Co-authored-by: Cursor <cursoragent@cursor.com>

gateway/win-protocol/src/main.cpp

@@ -113,7 +113,7 @@ struct AppState {
   std::map<std::string, PointValue> values;  // key = point id
   std::map<std::string, bool> hostOk;
   std::map<std::string, int> hostLatencyMs;
-  std::string version = "0.3.9-keep-good";
+  std::string version = "0.3.12-connect-timeout";
   std::map<std::string, Session> sessions;
   bool pollRunning = false;
 };
@@ -601,15 +601,27 @@ void pollOnce() {
     int lastLat = 0;
     int unit = mine.empty() ? 1 : mine.front().unitId;
     std::map<int, uint16_t> flat;
+    // 同一主机整轮共用一条 TCP，避免每点新建连接把从站打满
+    modbus_tcp::Session sess;
+    const bool linked = sess.open(h.host, h.port);
+    if (!linked) {
+      hostOk[h.id] = false;
+      hostLatency[h.id] = 0;
+      std::cout << "[gateway] host " << h.id << " TCP connect failed " << h.host << ":" << h.port
+                << "\n"
+                << std::flush;
+      continue;
+    }
     // 只采真实 Modbus；连不上 / 读失败不造模拟数
     if (mine.empty()) {
       std::vector<uint16_t> tmp;
       int lat = 0;
       const int probeLogical = 1;
       const int probePdu = std::max(0, probeLogical + addrOffset);
-      bool ok = modbus_tcp::readHolding(h.host, h.port, unit, probePdu, 1, tmp, &lat);
+      bool ok = sess.readHolding(unit, probePdu, 1, tmp, &lat);
       hostOk[h.id] = ok;
       hostLatency[h.id] = lat;
+      sess.close();
       continue;
     }
     // flat 按点表逻辑地址索引；读请求地址 = 逻辑地址 + offset
@@ -626,7 +638,17 @@ void pollOnce() {
     std::sort(addrs.begin(), addrs.end());
     addrs.erase(std::unique(addrs.begin(), addrs.end()), addrs.end());
     int chunksOk = 0, chunksFail = 0;
+    int singleOk = 0;
+    int consecutiveFail = 0;
+    constexpr int kMaxSingleFallback = 20;  // 块失败后最多单点回退
+    constexpr int kAbortAfterFails = 3;     // 连续失败则放弃本主机，避免拖死其它网关
     for (size_t ai = 0; ai < addrs.size();) {
+      if (consecutiveFail >= kAbortAfterFails) {
+        std::cout << "[gateway] host " << h.id << " abort after " << consecutiveFail
+                  << " consecutive Modbus failures\n"
+                  << std::flush;
+        break;
+      }
       int start = addrs[ai];
       int end = start + 1;
       size_t aj = ai + 1;
@@ -640,33 +662,47 @@ void pollOnce() {
         }
       }
       int cnt = std::min(chunk, std::max(1, end - start));
+      if (cnt > 125) cnt = 125;
       const int pduStart = std::max(0, start + addrOffset);
       std::vector<uint16_t> regs;
       int lat = 0;
-      if (modbus_tcp::readHolding(h.host, h.port, unit, pduStart, cnt, regs, &lat) &&
+      if (sess.readHolding(unit, pduStart, cnt, regs, &lat, consecutiveFail == 0) &&
           static_cast<int>(regs.size()) == cnt) {
         anyOk = true;
         lastLat = lat;
         ++chunksOk;
+        consecutiveFail = 0;
         for (int i = 0; i < cnt; ++i) flat[start + i] = regs[static_cast<size_t>(i)];
       } else {
         ++chunksFail;
-        for (size_t k = ai; k < aj; ++k) {
+        ++consecutiveFail;
+        // 块读失败：有限次单点回退（复用同一连接）
+        bool gotOne = false;
+        for (size_t k = ai; k < aj && singleOk < kMaxSingleFallback; ++k) {
           const int one = addrs[k];
           const int onePdu = std::max(0, one + addrOffset);
           std::vector<uint16_t> oneReg;
           int oneLat = 0;
-          if (modbus_tcp::readHolding(h.host, h.port, unit, onePdu, 1, oneReg, &oneLat) &&
-              oneReg.size() == 1) {
+          if (sess.readHolding(unit, onePdu, 1, oneReg, &oneLat, false) && oneReg.size() == 1) {
             anyOk = true;
             lastLat = oneLat;
             flat[one] = oneReg[0];
             ++chunksOk;
+            ++singleOk;
+            gotOne = true;
+            consecutiveFail = 0;
+          }
         }
+        if (!gotOne && consecutiveFail >= kAbortAfterFails) {
+          std::cout << "[gateway] host " << h.id << " abort after " << consecutiveFail
+                    << " consecutive Modbus failures\n"
+                    << std::flush;
+          break;
         }
       }
       ai = aj > ai ? aj : ai + 1;
     }
+    sess.close();
     hostOk[h.id] = anyOk;
     hostLatency[h.id] = lastLat;
     std::cout << "[gateway] host " << h.id << " range=[" << minAddr << "," << maxEnd