Single-reader, single-writer, lightweight shared message queue.

This code provides infrastructure for user backends to communicate
relatively easily with background workers.  The message queue is
structured as a ring buffer and allows messages of arbitary length
to be sent and received.

Patch by me.  Review by KaiGai Kohei and Andres Freund.

src/include/storage/shm_mq.h

@@ -0,0 +1,70 @@
+/*-------------------------------------------------------------------------
+ *
+ * shm_mq.h
+ *	  single-reader, single-writer shared memory message queue
+ *
+ * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/shm_mq.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SHM_MQ_H
+#define SHM_MQ_H
+
+#include "postmaster/bgworker.h"
+#include "storage/dsm.h"
+#include "storage/proc.h"
+
+/* The queue itself, in shared memory. */
+struct shm_mq;
+typedef struct shm_mq shm_mq;
+
+/* Backend-private state. */
+struct shm_mq_handle;
+typedef struct shm_mq_handle shm_mq_handle;
+
+/* Possible results of a send or receive operation. */
+typedef enum
+{
+	SHM_MQ_SUCCESS,			/* Sent or received a message. */
+	SHM_MQ_WOULD_BLOCK,		/* Not completed; retry later. */
+	SHM_MQ_DETACHED			/* Other process has detached queue. */
+} shm_mq_result;
+
+/*
+ * Primitives to create a queue and set the sender and receiver.
+ *
+ * Both the sender and the receiver must be set before any messages are read
+ * or written, but they need not be set by the same process.  Each must be
+ * set exactly once.
+ */
+extern shm_mq *shm_mq_create(void *address, Size size);
+extern void shm_mq_set_receiver(shm_mq *mq, PGPROC *);
+extern void shm_mq_set_sender(shm_mq *mq, PGPROC *);
+
+/* Accessor methods for sender and receiver. */
+extern PGPROC *shm_mq_get_receiver(shm_mq *);
+extern PGPROC *shm_mq_get_sender(shm_mq *);
+
+/* Set up backend-local queue state. */
+extern shm_mq_handle *shm_mq_attach(shm_mq *mq, dsm_segment *seg,
+			  BackgroundWorkerHandle *handle);
+
+/* Break connection. */
+extern void shm_mq_detach(shm_mq *);
+
+/* Send or receive messages. */
+extern shm_mq_result shm_mq_send(shm_mq_handle *mqh,
+			uint64 nbytes, void *data, bool nowait);
+extern shm_mq_result shm_mq_receive(shm_mq_handle *mqh,
+			   uint64 *nbytesp, void **datap, bool nowait);
+
+/* Wait for our counterparty to attach to the queue. */
+extern shm_mq_result shm_mq_wait_for_attach(shm_mq_handle *mqh);
+
+/* Smallest possible queue. */
+extern const Size shm_mq_minimum_size;
+
+#endif   /* SHM_MQ_H */