Fix calculation related to temporary WAL segment name in basic_archive

The file name used for its temporary destination, before renaming it to
the real deal, has been using a microseconds in a timestamp aimed to be
originally in milli-seconds.  This is harmless as this is aimed at being
a safeguard against name collisions (note MyProcPid in the name), but
let's be correct with the maths.

While on it, add a note in the module's makefile to document why
installcheck is not supported.

Author: Nathan Bossart
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/20221014044106.GA1673343@nathanxps13
Backpatch-through: 15

contrib/basic_archive/Makefile

@@ -5,7 +5,8 @@ PGFILEDESC = "basic_archive - basic archive module"
 
 REGRESS = basic_archive
 REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/basic_archive/basic_archive.conf
-
+# Disabled because these tests require "shared_preload_libraries=basic_archive",
+# which typical installcheck users do not have (e.g. buildfarm clients).
 NO_INSTALLCHECK = 1
 
 ifdef USE_PGXS

contrib/basic_archive/basic_archive.c

@@ -221,7 +221,7 @@ basic_archive_file_internal(const char *file, const char *path)
 	char		temp[MAXPGPATH + 256];
 	struct stat st;
 	struct timeval tv;
-	uint64		epoch;
+	uint64		epoch;			/* milliseconds */
 
 	ereport(DEBUG3,
 			(errmsg("archiving \"%s\" via basic_archive", file)));
@@ -268,7 +268,7 @@ basic_archive_file_internal(const char *file, const char *path)
 	 */
 	gettimeofday(&tv, NULL);
 	if (pg_mul_u64_overflow((uint64) 1000, (uint64) tv.tv_sec, &epoch) ||
-		pg_add_u64_overflow(epoch, (uint64) tv.tv_usec, &epoch))
+		pg_add_u64_overflow(epoch, (uint64) (tv.tv_usec / 1000), &epoch))
 		elog(ERROR, "could not generate temporary file name for archiving");
 
 	snprintf(temp, sizeof(temp), "%s/%s.%s.%d." UINT64_FORMAT,