Once again allow LWLocks to be used within DSM segments.

Prior to commit 7882c3b0b9, it was
possible to use LWLocks within DSM segments, but that commit broke
this use case by switching from a doubly linked list to a circular
linked list.  Switch back, using a new bit of general infrastructure
for maintaining lists of PGPROCs.

Thomas Munro, reviewed by me.

src/include/storage/proclist.h

@@ -0,0 +1,154 @@
+/*-------------------------------------------------------------------------
+ *
+ * proclist.h
+ *		operations on doubly-linked lists of pgprocnos
+ *
+ * The interface is similar to dlist from ilist.h, but uses pgprocno instead
+ * of pointers.  This allows proclist_head to be mapped at different addresses
+ * in different backends.
+ *
+ * See proclist_types.h for the structs that these functions operate on.  They
+ * are separated to break a header dependency cycle with proc.h.
+ *
+ * Portions Copyright (c) 2016, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		src/include/storage/proclist.h
+ *-------------------------------------------------------------------------
+ */
+#ifndef PROCLIST_H
+#define PROCLIST_H
+
+#include "storage/proc.h"
+#include "storage/proclist_types.h"
+
+/*
+ * Initialize a proclist.
+ */
+static inline void
+proclist_init(proclist_head *list)
+{
+	list->head = list->tail = INVALID_PGPROCNO;
+}
+
+/*
+ * Is the list empty?
+ */
+static inline bool
+proclist_is_empty(proclist_head *list)
+{
+	return list->head == INVALID_PGPROCNO;
+}
+
+/*
+ * Get a pointer to a proclist_node inside a given PGPROC, given a procno and
+ * an offset.
+ */
+static inline proclist_node *
+proclist_node_get(int procno, size_t node_offset)
+{
+	char	   *entry = (char *) GetPGProcByNumber(procno);
+
+	return (proclist_node *) (entry + node_offset);
+}
+
+/*
+ * Insert a node at the beginning of a list.
+ */
+static inline void
+proclist_push_head_offset(proclist_head *list, int procno, size_t node_offset)
+{
+	proclist_node *node = proclist_node_get(procno, node_offset);
+
+	if (list->head == INVALID_PGPROCNO)
+	{
+		Assert(list->tail == INVALID_PGPROCNO);
+		node->next = node->prev = INVALID_PGPROCNO;
+		list->head = list->tail = procno;
+	}
+	else
+	{
+		Assert(list->tail != INVALID_PGPROCNO);
+		node->next = list->head;
+		proclist_node_get(node->next, node_offset)->prev = procno;
+		node->prev = INVALID_PGPROCNO;
+		list->head = procno;
+	}
+}
+
+/*
+ * Insert a node a the end of a list.
+ */
+static inline void
+proclist_push_tail_offset(proclist_head *list, int procno, size_t node_offset)
+{
+	proclist_node *node = proclist_node_get(procno, node_offset);
+
+	if (list->tail == INVALID_PGPROCNO)
+	{
+		Assert(list->head == INVALID_PGPROCNO);
+		node->next = node->prev = INVALID_PGPROCNO;
+		list->head = list->tail = procno;
+	}
+	else
+	{
+		Assert(list->head != INVALID_PGPROCNO);
+		node->prev = list->tail;
+		proclist_node_get(node->prev, node_offset)->next = procno;
+		node->next = INVALID_PGPROCNO;
+		list->tail = procno;
+	}
+}
+
+/*
+ * Delete a node.  The node must be in the list.
+ */
+static inline void
+proclist_delete_offset(proclist_head *list, int procno, size_t node_offset)
+{
+	proclist_node *node = proclist_node_get(procno, node_offset);
+
+	if (node->prev == INVALID_PGPROCNO)
+		list->head = node->next;
+	else
+		proclist_node_get(node->prev, node_offset)->next = node->next;
+
+	if (node->next == INVALID_PGPROCNO)
+		list->tail = node->prev;
+	else
+		proclist_node_get(node->next, node_offset)->prev = node->prev;
+}
+
+/*
+ * Helper macros to avoid repetition of offsetof(PGPROC, <member>).
+ * 'link_member' is the name of a proclist_node member in PGPROC.
+ */
+#define proclist_delete(list, procno, link_member) \
+	proclist_delete_offset((list), (procno), offsetof(PGPROC, link_member))
+#define proclist_push_head(list, procno, link_member) \
+	proclist_push_head_offset((list), (procno), offsetof(PGPROC, link_member))
+#define proclist_push_tail(list, procno, link_member) \
+	proclist_push_tail_offset((list), (procno), offsetof(PGPROC, link_member))
+
+/*
+ * Iterate through the list pointed at by 'lhead', storing the current
+ * position in 'iter'.  'link_member' is the name of a proclist_node member in
+ * PGPROC.  Access the current position with iter.cur.
+ *
+ * The only list modification allowed while iterating is deleting the current
+ * node with proclist_delete(list, iter.cur, node_offset).
+ */
+#define proclist_foreach_modify(iter, lhead, link_member)					\
+	for (AssertVariableIsOfTypeMacro(iter, proclist_mutable_iter),			\
+		 AssertVariableIsOfTypeMacro(lhead, proclist_head *),				\
+		 (iter).cur = (lhead)->head,										\
+		 (iter).next = (iter).cur == INVALID_PGPROCNO ? INVALID_PGPROCNO :	\
+			 proclist_node_get((iter).cur,									\
+							   offsetof(PGPROC, link_member))->next;		\
+		 (iter).cur != INVALID_PGPROCNO;									\
+		 (iter).cur = (iter).next,											\
+		 (iter).next = (iter).cur == INVALID_PGPROCNO ? INVALID_PGPROCNO :	\
+			 proclist_node_get((iter).cur,									\
+							   offsetof(PGPROC, link_member))->next)
+
+#endif