Allow discovery of whether a dynamic background worker is running.

Using the infrastructure provided by this patch, it's possible either
to wait for the startup of a dynamically-registered background worker,
or to poll the status of such a worker without waiting.  In either
case, the current PID of the worker process can also be obtained.
As usual, worker_spi is updated to demonstrate the new functionality.

Patch by me.  Review by Andres Freund.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index a6f77c105c2..c4b5d0196b3 100644
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -21,6 +21,7 @@
 #include "miscadmin.h"
 #include "storage/latch.h"
 #include "storage/ipc.h"
+#include "storage/proc.h"
 #include "storage/shmem.h"
 #include "storage/sinval.h"
 #include "tcop/tcopprot.h"
@@ -57,6 +58,14 @@ typedef struct
  */
 #define NumProcSignalSlots	(MaxBackends + NUM_AUXPROCTYPES)
 
+/*
+ * If this flag is set, the process latch will be set whenever SIGUSR1
+ * is received.  This is useful when waiting for a signal from the postmaster.
+ * Spurious wakeups must be expected.  Make sure that the flag is cleared
+ * in the error path.
+ */
+bool set_latch_on_sigusr1;
+
 static ProcSignalSlot *ProcSignalSlots = NULL;
 static volatile ProcSignalSlot *MyProcSignalSlot = NULL;
 
@@ -276,6 +285,9 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN))
 		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
 
+	if (set_latch_on_sigusr1)
+		SetLatch(&MyProc->procLatch);
+
 	latch_sigusr1_handler();
 
 	errno = save_errno;