ADDED: Support engines in single-threaded version

This patch revives an old proof-of-concept option to include engines
in the single threaded version.   The code compiles and tests again.
Added Prolog flag `engines` to indicate the system has engine support.

CMakeLists.txt

@@ -271,10 +271,13 @@ set(SWIPL_TEST_DIRS unprotected core db attvar debug tabling library
     xsb/wfs_tests xsb/table_tests xsb/incremental_tests xsb/nonmt_tests
     xsb/sub_tests)
 if(MULTI_THREADED)
-  set(SWIPL_TEST_DIRS ${SWIPL_TEST_DIRS} thread thread_wait signals)
+  list(APPEND SWIPL_TEST_DIRS thread thread_wait signals)
+endif()
+if(ENGINES)
+  list(APPEND SWIPL_TEST_DIRS engines)
 endif()
 if(O_BIGNUM)
-  set(SWIPL_TEST_DIRS ${SWIPL_TEST_DIRS} rational)
+  list(APPEND SWIPL_TEST_DIRS rational)
 endif()
 
 add_test(NAME swipl:abi-version

library/prolog_colour.pl

@@ -2255,6 +2255,7 @@
 known_flag(conda).
 known_flag(dde).
 known_flag(emscripten).
+known_flag(engines).
 known_flag(executable_format).
 known_flag(gc_thread).
 known_flag(gmp_version).

man/overview.doc

@@ -1667,6 +1667,11 @@ default emulator.
 Format of the SWI-Prolog executable, e.g. \const{elf} for when
 \file{swipl} is an ELF binary file.
 
+    \prologflagitem{engines}{bool}{r}
+True if engines are supported.  This is always the case on the
+multi-threaded versions.   Engines may be enabled on single
+threaded versions of SWI-Prolog.
+
     \prologflagitem{exit_status}{integer}{r}
 Set by halt/1 to its argument, making the exit status available to
 hooks registered with at_halt/1.

src/os/pl-prologflag.c

@@ -3,7 +3,7 @@
     Author:        Jan Wielemaker
     E-mail:        [email protected]
     WWW:           http://www.swi-prolog.org
-    Copyright (c)  2011-2024, University of Amsterdam
+    Copyright (c)  2011-2025, University of Amsterdam
 			      VU University Amsterdam
 			      CWI, Amsterdam
 			      SWI-Prolog Solutions b.v.
@@ -1925,6 +1925,7 @@ initPrologFlags(void)
 #endif
 #ifdef O_PLMT
   setPrologFlag("threads",	FT_BOOL, !GD->options.nothreads, 0);
+  setPrologFlag("engines",	FT_BOOL, true, 0);
   if ( GD->options.xpce >= 0 )
     setPrologFlag("xpce",	FT_BOOL, GD->options.xpce, 0);
   setPrologFlag("system_thread_id", FT_INTEGER|FF_READONLY, (intptr_t)0);
@@ -1934,6 +1935,9 @@ initPrologFlags(void)
 #else
   setPrologFlag("threads",	FT_BOOL|FF_READONLY, false, 0);
   setPrologFlag("gc_thread",    FT_BOOL|FF_READONLY, false, PLFLAG_GCTHREAD);
+#ifdef O_ENGINES
+  setPrologFlag("engines",	FT_BOOL, true, 0);
+#endif
 #endif
 #ifdef O_DDE
   setPrologFlag("dde", FT_BOOL|FF_READONLY, true, 0);

src/pl-global.h

@@ -776,12 +776,12 @@ struct PL_local_data
     DefinitionChain local_definitions;	/* P_THREAD_LOCAL predicates */
     int magic;				/* PL_THREAD_MAGIC (checking) */
     int exit_requested;			/* Thread is asked to exit */
+    struct _PL_thread_info_t *creator;	/* Thread that created me */
+    uint64_t creator_seq_id;		/* Seq id of creater */
 #ifdef O_PLMT
     simpleMutex scan_lock;		/* Hold for asynchronous scans */
     thread_wait_for *waiting_for;	/* thread_wait/2 info */
     alert_channel alert;		/* How to alert the thread */
-    struct _PL_thread_info_t *creator;	/* Thread that created me */
-    uint64_t creator_seq_id;		/* Seq id of creater */
     double child_cputime;		/* Time of completed children */
     uint64_t child_inferences;		/* Inferences in children */
 #endif

src/pl-incl.h

@@ -2955,6 +2955,10 @@ static inline int __ptr_to_bool(const intptr_t ptr) { return ptr != 0; }
 #define WEAK_TRY_CALL_VOID(Name, ...) \
 	(WEAK_FUNC(Name) != NULL ? WEAK_FUNC(Name)(__VA_ARGS__) : (void)0)
 
+#if defined(O_ENGINES) && !defined(O_PLMT)
+extern struct PL_local_data *PL_current_engine_ptr;
+#endif
+
 #include "pl-util.h"			/* (Debug) utilities */
 #include "pl-alloc.h"			/* Allocation primitives */
 #include "pl-init.h"			/* Declarations needed by pl-init.c */

src/pl-thread.c

@@ -1107,6 +1107,7 @@ dummy_handler(int sig)
 }
 #endif
 
+#ifdef O_PLMT
 static double
 halt_grace_time(void)
 { GET_LD
@@ -1125,6 +1126,7 @@ halt_grace_time(void)
 
   return t;
 }
+#endif
 
 int
 exitPrologThreads(void)

tests/test.pl

@@ -3,7 +3,7 @@
     Author:        Jan Wielemaker and Anjo Anjewierden
     E-mail:        [email protected]
     WWW:           http://www.swi-prolog.org
-    Copyright (c)  1996-2023, University of Amsterdam
+    Copyright (c)  1996-2025, University of Amsterdam
                               VU University Amsterdam
                               CWI, Amsterdam
                               SWI-Prolog Solutions b.v.
@@ -1944,6 +1944,8 @@
     wide_character_types.
 testset(thread) :-
     current_prolog_flag(threads, true).
+testset(engines) :-
+    current_prolog_flag(engines, true).
 testset(mutex) :-
     current_prolog_flag(threads, true).