VFD SWMR sparse readers failed to open the .h5 file because the sparse writer

had finished its work and closed the .h5 file, thus removing the shadow file.
Make the sparse writer wait to close the .h5 file for a signal from
testvfdswmr.sh.  In testvfdswmr.sh, send the signal when the readers have all
finished.

test/testvfdswmr.sh.in

@@ -464,7 +464,9 @@ fi
         $IFDEBUG ps
 
         # Wait for the readers and the writer to finish.
-        wait
+        wait $pid_readers
+	kill -USR1 $pid_writer
+	wait $pid_writer
 
         # Collect exit codes of the readers
         n=0

test/vfd_swmr_sparse_writer.c

@@ -28,6 +28,8 @@
 /***********/
 /* Headers */
 /***********/
+#include <err.h>
+#include <signal.h>
 
 #include "h5test.h"
 #include "vfd_swmr_common.h"
@@ -383,6 +385,48 @@ error:
     return -1;
 } /* add_records() */
 
+static volatile sig_atomic_t got_sigusr1 = 0;
+
+static void
+sigusr1_handler(int H5_ATTR_UNUSED signo)
+{
+    got_sigusr1 = 1;
+}
+
+static void
+await_signal(void)
+{
+    sigset_t sleepset, fullset, oldset;
+    struct sigaction sa, osa;
+
+    memset(&sa, '\0', sizeof(sa));
+    sa.sa_handler = sigusr1_handler;
+    if (sigemptyset(&sa.sa_mask) == -1 ||
+        sigfillset(&fullset) == -1 ||
+        sigemptyset(&sleepset) == -1) {
+        err(EXIT_FAILURE, "%s.%d: could not initialize signal masks",
+            __func__, __LINE__);
+    }
+
+    if (sigprocmask(SIG_BLOCK, &fullset, &oldset) == -1)
+        err(EXIT_FAILURE, "%s.%d: sigprocmask", __func__, __LINE__);
+
+    if (sigaction(SIGUSR1, &sa, &osa) == -1)
+        err(EXIT_FAILURE, "%s.%d: sigaction", __func__, __LINE__);
+
+    if (sigsuspend(&sleepset) == -1 && errno != EINTR)
+        err(EXIT_FAILURE, "%s.%d: sigsuspend", __func__, __LINE__);
+
+    if (got_sigusr1 != 0)
+        printf("Cancelled by SIGUSR1.\n");
+
+    if (sigaction(SIGUSR1, &osa, NULL) == -1)
+        err(EXIT_FAILURE, "%s.%d: sigaction", __func__, __LINE__);
+
+    if (sigprocmask(SIG_SETMASK, &oldset, NULL) == -1)
+        err(EXIT_FAILURE, "%s.%d: sigprocmask", __func__, __LINE__);
+}
+
 static void
 usage(void)
 {
@@ -499,6 +543,8 @@ int main(int argc, const char *argv[])
         HDexit(1);
     } /* end if */
 
+    await_signal();
+
     /* Emit informational message */
     if(verbose)
         HDfprintf(stderr, "WRITER: Closing objects/file\n");