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.

3 files changed, 23 insertions, 1 deletions

diff --git a/src/include/postmaster/bgworker.h b/src/include/postmaster/bgworker.h
index 0bb897b8f3..8bbbeb492c 100644
--- a/src/include/postmaster/bgworker.h
+++ b/src/include/postmaster/bgworker.h
@@ -80,13 +80,32 @@ typedef struct BackgroundWorker
 	char		bgw_library_name[BGW_MAXLEN];	/* only if bgw_main is NULL */
 	char		bgw_function_name[BGW_MAXLEN];	/* only if bgw_main is NULL */
 	Datum		bgw_main_arg;
+	pid_t		bgw_notify_pid;		/* SIGUSR1 this backend on start/stop */
 } BackgroundWorker;
 
+typedef enum BgwHandleStatus
+{
+	BGWH_STARTED,				/* worker is running */
+	BGWH_NOT_YET_STARTED,		/* worker hasn't been started yet */
+	BGWH_STOPPED,				/* worker has exited */
+	BGWH_POSTMASTER_DIED		/* postmaster died; worker status unclear */
+} BgwHandleStatus;
+
+struct BackgroundWorkerHandle;
+typedef struct BackgroundWorkerHandle BackgroundWorkerHandle;
+
 /* Register a new bgworker during shared_preload_libraries */
 extern void RegisterBackgroundWorker(BackgroundWorker *worker);
 
 /* Register a new bgworker from a regular backend */
-extern bool RegisterDynamicBackgroundWorker(BackgroundWorker *worker);
+extern bool RegisterDynamicBackgroundWorker(BackgroundWorker *worker,
+											BackgroundWorkerHandle **handle);
+
+/* Query the status of a bgworker */
+extern BgwHandleStatus GetBackgroundWorkerPid(BackgroundWorkerHandle *handle,
+					   pid_t *pidp);
+extern BgwHandleStatus WaitForBackgroundWorkerStartup(BackgroundWorkerHandle *
+							   handle, pid_t *pid);
 
 /* This is valid in a running worker */
 extern BackgroundWorker *MyBgworkerEntry;
diff --git a/src/include/postmaster/bgworker_internals.h b/src/include/postmaster/bgworker_internals.h
index 3b53027fd4..6d5ee20dc3 100644
--- a/src/include/postmaster/bgworker_internals.h
+++ b/src/include/postmaster/bgworker_internals.h
@@ -40,6 +40,8 @@ extern Size BackgroundWorkerShmemSize(void);
 extern void BackgroundWorkerShmemInit(void);
 extern void BackgroundWorkerStateChange(void);
 extern void ForgetBackgroundWorker(slist_mutable_iter *cur);
+extern void ReportBackgroundWorkerPID(RegisteredBgWorker *);
+extern void BackgroundWorkerStopNotifications(pid_t pid);
 
 /* Function to start a background worker, called from postmaster.c */
 extern void StartBackgroundWorker(void);
diff --git a/src/include/postmaster/postmaster.h b/src/include/postmaster/postmaster.h
index c090595a45..5dc6dc5a5f 100644
--- a/src/include/postmaster/postmaster.h
+++ b/src/include/postmaster/postmaster.h
@@ -52,6 +52,7 @@ extern void ClosePostmasterPorts(bool am_syslogger);
 extern int	MaxLivePostmasterChildren(void);
 
 extern int	GetNumShmemAttachedBgworkers(void);
+extern bool PostmasterMarkPIDForWorkerNotify(int);
 
 #ifdef EXEC_BACKEND
 extern pid_t postmaster_forkexec(int argc, char *argv[]);