MDEV-9112: Non-blocking client API missing on non-x86 platforms

The check for UCONTEXT in cmake was somehow become broken, disabling the
fallback to ucontext. This caused the non-blocking client API to not be
available for non-x86 platforms, on which no hand-crafted assembler
implementation of my_context is available.

config.h.cmake

@@ -95,6 +95,7 @@
 #cmakedefine HAVE_SYS_TYPES_H 1
 #cmakedefine HAVE_SYS_UN_H 1
 #cmakedefine HAVE_SYS_VADVISE_H 1
+#cmakedefine HAVE_UCONTEXT_H 1
 #cmakedefine HAVE_TERM_H 1
 #cmakedefine HAVE_TERMBITS_H 1
 #cmakedefine HAVE_TERMIOS_H 1
@@ -291,7 +292,6 @@
 #cmakedefine HAVE_THR_YIELD 1
 #cmakedefine HAVE_TIME 1
 #cmakedefine HAVE_TIMES 1
-#cmakedefine HAVE_UCONTEXT 1
 #cmakedefine HAVE_VALLOC 1
 #cmakedefine HAVE_VIDATTR 1
 #define HAVE_VIO_READ_BUFF 1

configure.cmake

@@ -1098,3 +1098,6 @@ CHECK_STRUCT_HAS_MEMBER("struct dirent" d_ino "dirent.h"  STRUCT_DIRENT_HAS_D_IN
 CHECK_STRUCT_HAS_MEMBER("struct dirent" d_namlen "dirent.h"  STRUCT_DIRENT_HAS_D_NAMLEN)
 SET(SPRINTF_RETURNS_INT 1)
 CHECK_INCLUDE_FILE(ucontext.h HAVE_UCONTEXT_H)
+IF(NOT HAVE_UCONTEXT_H)
+  CHECK_INCLUDE_FILE(sys/ucontext.h HAVE_UCONTEXT_H)
+ENDIF()

include/my_context.h

@@ -31,7 +31,7 @@
 #define MY_CONTEXT_USE_X86_64_GCC_ASM
 #elif defined(__GNUC__) && __GNUC__ >= 3 && defined(__i386__)
 #define MY_CONTEXT_USE_I386_GCC_ASM
-#elif defined(HAVE_UCONTEXT)
+#elif defined(HAVE_UCONTEXT_H)
 #define MY_CONTEXT_USE_UCONTEXT
 #else
 #define MY_CONTEXT_DISABLE

sql-common/client.c

@@ -4293,6 +4293,7 @@ mysql_options(MYSQL *mysql,enum mysql_option option, const void *arg)
       stacksize= ASYNC_CONTEXT_DEFAULT_STACK_SIZE;
     if (my_context_init(&ctxt->async_context, stacksize))
     {
+      set_mysql_error(mysql, CR_OUT_OF_MEMORY, unknown_sqlstate);
       my_free(ctxt);
       DBUG_RETURN(1);
     }