Make EXEC_BACKEND more convenient on Linux and FreeBSD.

Try to disable ASLR when building in EXEC_BACKEND mode, to avoid random
memory mapping failures while testing.  For developer use only, no
effect on regular builds.

This has been originally applied as of f3e7806 for v15~, but
recently-added buildfarm member gokiburi tests this configuration on
older branches as well, causing it to fail randomly as ASLR would be
enabled.

Suggested-by: Andres Freund <andres@anarazel.de>
Tested-by: Bossart, Nathan <bossartn@amazon.com>
Discussion: https://postgr.es/m/20210806032944.m4tz7j2w47mant26%40alap3.anarazel.de
Backpatch-through: 12

src/common/exec.c

@@ -25,6 +25,14 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
+#ifdef EXEC_BACKEND
+#if defined(HAVE_SYS_PERSONALITY_H)
+#include <sys/personality.h>
+#elif defined(HAVE_SYS_PROCCTL_H)
+#include <sys/procctl.h>
+#endif
+#endif
+
 /* Inhibit mingw CRT's auto-globbing of command line arguments */
 #if defined(WIN32) && !defined(_MSC_VER)
 extern int _CRT_glob = 0; /* 0 turns off globbing; 1 turns it on */
@@ -492,6 +500,31 @@ set_pglocale_pgservice(const char *argv0, const char *app)
 	}
 }
 
+#ifdef EXEC_BACKEND
+/*
+ * For the benefit of PostgreSQL developers testing EXEC_BACKEND on Unix
+ * systems (code paths normally exercised only on Windows), provide a way to
+ * disable address space layout randomization, if we know how on this platform.
+ * Otherwise, backends may fail to attach to shared memory at the fixed address
+ * chosen by the postmaster.  (See also the macOS-specific hack in
+ * sysv_shmem.c.)
+ */
+int
+pg_disable_aslr(void)
+{
+#if defined(HAVE_SYS_PERSONALITY_H)
+	return personality(ADDR_NO_RANDOMIZE);
+#elif defined(HAVE_SYS_PROCCTL_H) && defined(PROC_ASLR_FORCE_DISABLE)
+	int			data = PROC_ASLR_FORCE_DISABLE;
+
+	return procctl(P_PID, 0, PROC_ASLR_CTL, &data);
+#else
+	errno = ENOSYS;
+	return -1;
+#endif
+}
+#endif
+
 #ifdef WIN32
 
 /*