Allow dynamic allocation of shared memory segments.

Patch by myself and Amit Kapila.  Design help from Noah Misch.  Review
by Andres Freund.

src/include/storage/dsm_impl.h

@@ -0,0 +1,75 @@
+/*-------------------------------------------------------------------------
+ *
+ * dsm_impl.h
+ *	  low-level dynamic shared memory primitives
+ *
+ * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/dsm_impl.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef DSM_IMPL_H
+#define DSM_IMPL_H
+
+/* Dynamic shared memory implementations. */
+#define DSM_IMPL_NONE			0
+#define DSM_IMPL_POSIX			1
+#define DSM_IMPL_SYSV			2
+#define DSM_IMPL_WINDOWS		3
+#define DSM_IMPL_MMAP			4
+
+/*
+ * Determine which dynamic shared memory implementations will be supported
+ * on this platform, and which one will be the default.
+ */
+#ifdef WIN32
+#define USE_DSM_WINDOWS
+#define DEFAULT_DYNAMIC_SHARED_MEMORY_TYPE		DSM_IMPL_WINDOWS
+#else
+#ifdef HAVE_SHM_OPEN
+#define USE_DSM_POSIX
+#define DEFAULT_DYNAMIC_SHARED_MEMORY_TYPE		DSM_IMPL_POSIX
+#endif
+#define USE_DSM_SYSV
+#ifndef DEFAULT_DYNAMIC_SHARED_MEMORY_TYPE
+#define DEFAULT_DYNAMIC_SHARED_MEMORY_TYPE		DSM_IMPL_SYSV
+#endif
+#define USE_DSM_MMAP
+#endif
+
+/* GUC. */
+extern int dynamic_shared_memory_type;
+
+/*
+ * Directory for on-disk state.
+ *
+ * This is used by all implementations for crash recovery and by the mmap
+ * implementation for storage.
+ */
+#define PG_DYNSHMEM_DIR					"pg_dynshmem"
+#define PG_DYNSHMEM_MMAP_FILE_PREFIX	"mmap."
+
+/* A "name" for a dynamic shared memory segment. */
+typedef uint32 dsm_handle;
+
+/* All the shared-memory operations we know about. */
+typedef enum
+{
+	DSM_OP_CREATE,
+	DSM_OP_ATTACH,
+	DSM_OP_DETACH,
+	DSM_OP_RESIZE,
+	DSM_OP_DESTROY
+} dsm_op;
+
+/* Create, attach to, detach from, resize, or destroy a segment. */
+extern bool dsm_impl_op(dsm_op op, dsm_handle handle, uint64 request_size,
+			void **impl_private, void **mapped_address, uint64 *mapped_size,
+			int elevel);
+
+/* Some implementations cannot resize segments.  Can this one? */
+extern bool dsm_impl_can_resize(void);
+
+#endif   /* DSM_IMPL_H */