horse.Provider.Socket.WebSocket.pas does not compile for Delphi on Linux since 3.3.3

[JasonDFS]

Summary

Every Horse release from 3.3.3 through 3.3.5 fails to compile under Delphi when the target platform is Linux (dcclinux64), because the Delphi/POSIX branch of THorseWebSocketSocketTransport.WaitReadable calls FD_SET(ASocket, LSet). In Delphi's POSIX RTL FD_SET is not a procedure: identifiers are case-insensitive, Posix.SysSelect declares the record type fd_set, and the RTL therefore names the macro _FD_SET. The compiler reads the call as a one-argument typecast and stops at the comma.

The unit is pulled in by Horse.Provider.Epoll, so any Delphi project using HORSE_PROVIDER_EPOLL on Linux is affected. 3.3.2 is the last release that builds. The fix is one character.

Environment

Horse	3.3.5 (tag 3.3.5, commit f3821dd); also 3.3.3 and 3.3.4
Compiler	Embarcadero Delphi for Linux 64 bit compiler version 37.0 (RAD Studio 37.0)
Target	Linux64
Defines	HORSE_PROVIDER_EPOLL (the unit fails on its own regardless of provider)
Host	Windows 11, cross-compiling

Steps to reproduce

Compiling the single unit is enough; no project or SDK is needed:

cd <horse>/src
"C:\Program Files (x86)\Embarcadero\Studio\37.0\bin\dcclinux64.exe" -B -Q --no-config ^
  -NSSystem;Posix;Data;Web;Xml ^
  -U"C:\Program Files (x86)\Embarcadero\Studio\37.0\lib\linux64\release;." ^
  Horse.Provider.Socket.WebSocket.pas

Actual result

Embarcadero Delphi for Linux 64 bit compiler version 37.0
Horse.Provider.Socket.WebSocket.pas(299) Error: E2029 ')' expected but ',' found

When compiled as part of an application the error surfaces as
Fatal: F2063 Could not compile used unit 'Horse.Provider.Socket.WebSocket.pas'.

Expected result

The unit compiles. It does on Win64 (dcc64, same source, 0 errors), and it did in 3.3.2 before this code existed.

Cause

src/Horse.Provider.Socket.WebSocket.pas, Delphi/POSIX branch of WaitReadable (lines 293-303 in 3.3.5):

  {$ELSE}
  var
    LSet: fd_set;
    LTimeout: timeval;
  begin
    FD_ZERO(LSet);
    FD_SET(ASocket, LSet);        // <-- line 299
    LTimeout.tv_sec := ATimeoutMS div 1000;
    LTimeout.tv_usec := (ATimeoutMS mod 1000) * 1000;
    Result := select(ASocket + 1, @LSet, nil, nil, @LTimeout) > 0;
  end;
  {$ENDIF}

Delphi identifiers are case-insensitive, so FD_SET and fd_set are the same identifier, and fd_set is already the record type declared two lines above. FD_SET(ASocket, LSet) is therefore parsed as fd_set(ASocket, LSet), a typecast, which takes exactly one argument. Hence ')' expected but ',' found.

The RTL anticipates this. Posix.SysSelect.pas declares:

procedure FD_ZERO(var fdset: fd_set); inline;
procedure _FD_SET(fd: Integer; var fdset: fd_set); inline;   // leading underscore: avoids the fd_set type
procedure FD_CLR(fd: Integer; var fdset: fd_set); inline;
function FD_ISSET(fd: Integer; const fdset: fd_set): Boolean; inline;

FD_ZERO, FD_CLR and FD_ISSET have no colliding type and are fine; only FD_SET needs the underscore.

Why it was not caught

• The FPC branch uses fpFD_SET, so Lazarus on Linux is unaffected.
• The Windows branch fills fd_array by hand (and its comment notes that Winapi.WinSock2 exposes FD_SET as a type, which is the same hazard in different clothing).
• The compile matrix (tests/run_compile_matrix.ps1) and the Docker test images cover Delphi/Windows and FPC/Linux, but not Delphi/Linux, so the Delphi/POSIX branch was never compiled before release.

Introduced by commit cfacc5e ("fix: treat EAGAIN as no-data rather than disconnect in WebSocket socket transport", PR #549), first released in 3.3.3.

Fix

--- a/src/Horse.Provider.Socket.WebSocket.pas
+++ b/src/Horse.Provider.Socket.WebSocket.pas
@@ -296,7 +296,7 @@ function THorseWebSocketSocketTransport.WaitReadable(const ASocket: TSocket;
     LTimeout: timeval;
   begin
     FD_ZERO(LSet);
-    FD_SET(ASocket, LSet);
+    _FD_SET(ASocket, LSet);
     LTimeout.tv_sec := ATimeoutMS div 1000;
     LTimeout.tv_usec := (ATimeoutMS mod 1000) * 1000;
     Result := select(ASocket + 1, @LSet, nil, nil, @LTimeout) > 0;

Verified with that change applied to 3.3.5: the unit compiles for Linux64 (1413 lines, 0 errors) and for Win64, and a Delphi application using HORSE_PROVIDER_EPOLL links for Linux64 and runs.

It would also be worth adding a dcclinux64 compile of Horse.Provider.Socket.WebSocket.pas (and Horse.Provider.Epoll.pas) to the compile matrix so the Delphi/POSIX branches are exercised before a release. Compiling units for Linux needs only the compiler and lib\linux64\release; no Linux SDK is required unless something is linked.