Fix //test/syscalls:tcp_socket_test_native

The data written was larger than the write buffer, and nobody was reading the
other end.

PiperOrigin-RevId: 371436084

test/syscalls/linux/BUILD

@@ -3710,6 +3710,7 @@ cc_binary(
     deps = [
         ":socket_test_util",
         "//test/util:file_descriptor",
+        "@com_google_absl//absl/strings",
         "@com_google_absl//absl/time",
         gtest,
         "//test/util:posix_error",

test/syscalls/linux/tcp_socket.cc

@@ -27,6 +27,7 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/strings/str_split.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 #include "test/syscalls/linux/socket_test_util.h"
@@ -1144,6 +1145,17 @@ TEST_P(SimpleTcpSocketTest, SelfConnectSendRecv) {
 }
 
 TEST_P(SimpleTcpSocketTest, SelfConnectSend) {
+  // Ensure the write size is not larger than the write buffer.
+  size_t write_size = 512 << 10;  // 512 KiB.
+  constexpr char kWMem[] = "/proc/sys/net/ipv4/tcp_wmem";
+  std::string wmem = ASSERT_NO_ERRNO_AND_VALUE(GetContents(kWMem));
+  std::vector<std::string> vals = absl::StrSplit(wmem, absl::ByAnyChar("\t "));
+  size_t max_wmem;
+  ASSERT_TRUE(absl::SimpleAtoi(vals.back(), &max_wmem));
+  if (write_size > max_wmem) {
+    write_size = max_wmem;
+  }
+
   // Initialize address to the loopback one.
   sockaddr_storage addr =
       ASSERT_NO_ERRNO_AND_VALUE(InetLoopbackAddr(GetParam()));
@@ -1164,7 +1176,7 @@ TEST_P(SimpleTcpSocketTest, SelfConnectSend) {
   ASSERT_THAT(RetryEINTR(connect)(s.get(), AsSockAddr(&addr), addrlen),
               SyscallSucceeds());
 
-  std::vector<char> writebuf(512 << 10);  // 512 KiB.
+  std::vector<char> writebuf(write_size);
 
   // Try to send the whole thing.
   int n;