Hash tables backed by DSA shared memory.

Add general purpose chaining hash tables for DSA memory.  Unlike
DynaHash in shared memory mode, these hash tables can grow as
required, and cope with being mapped into different addresses in
different backends.

There is a wide range of potential users for such a hash table, though
it's very likely the interface will need to evolve as we come to
understand the needs of different kinds of users.  E.g support for
iterators and incremental resizing is planned for later commits and
the details of the callback signatures are likely to change.

Author: Thomas Munro
Reviewed-By: John Gorman, Andres Freund, Dilip Kumar, Robert Haas
Discussion:
	https://postgr.es/m/CAEepm=3d8o8XdVwYT6O=bHKsKAM2pu2D6sV1S_=4d+jStVCE7w@mail.gmail.com
	https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

src/backend/lib/Makefile

@@ -12,7 +12,7 @@ subdir = src/backend/lib
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = binaryheap.o bipartite_match.o hyperloglog.o ilist.o knapsack.o \
-       pairingheap.o rbtree.o stringinfo.o
+OBJS = binaryheap.o bipartite_match.o dshash.o hyperloglog.o ilist.o \
+	   knapsack.o pairingheap.o rbtree.o stringinfo.o
 
 include $(top_srcdir)/src/backend/common.mk

src/include/lib/dshash.h

+/*-------------------------------------------------------------------------
+ *
+ * dshash.h
+ *	  Concurrent hash tables backed by dynamic shared memory areas.
+ *
+ * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/include/lib/dshash.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef DSHASH_H
+#define DSHASH_H
+
+#include "utils/dsa.h"
+
+/* The opaque type representing a hash table. */
+struct dshash_table;
+typedef struct dshash_table dshash_table;
+
+/* A handle for a dshash_table which can be shared with other processes. */
+typedef dsa_pointer dshash_table_handle;
+
+/* The type for hash values. */
+typedef uint32 dshash_hash;
+
+/*
+ * A function used for comparing keys.  This version is compatible with
+ * HashCompareFunction in hsearch.h and memcmp.
+ */
+typedef int (*dshash_compare_function) (const void *a, const void *b, size_t size);
+
+/*
+ * A function type used for comparing keys.  This version allows compare
+ * functions to receive a pointer to arbitrary user data that was given to the
+ * create or attach function.  Similar to qsort_arg_comparator.
+ */
+typedef int (*dshash_compare_arg_function) (const void *a, const void *b, void *arg);
+
+/*
+ * A function type for computing hash values for keys.  This version is
+ * compatible with HashValueFunc in hsearch.h and hash functions like
+ * tag_hash.
+ */
+typedef dshash_hash (*dshash_hash_function) (const void *v, size_t size);
+
+/*
+ * A function type for computing hash values for keys.  This version allows
+ * hash functions to receive a pointer to arbitrary user data that was given
+ * to the create or attach function.
+ */
+typedef dshash_hash (*dshash_hash_arg_function) (const void *v, void *arg);
+
+/*
+ * The set of parameters needed to create or attach to a hash table.  The
+ * members tranche_id and tranche_name do not need to be initialized when
+ * attaching to an existing hash table.
+ *
+ * Compare and hash functions mus be supplied even when attaching, because we
+ * can't safely share function pointers between backends in general.  Either
+ * the arg variants or the non-arg variants should be supplied; the other
+ * function pointers should be NULL.  If the arg varants are supplied then the
+ * user data pointer supplied to the create and attach functions will be
+ * passed to the hash and compare functions.
+ */
+typedef struct dshash_parameters
+{
+	size_t		key_size;		/* Size of the key (initial bytes of entry) */
+	size_t		entry_size;		/* Total size of entry */
+	dshash_compare_function compare_function;	/* Compare function */
+	dshash_compare_arg_function compare_arg_function;	/* Arg version */
+	dshash_hash_function hash_function; /* Hash function */
+	dshash_hash_arg_function hash_arg_function; /* Arg version */
+	int			tranche_id;		/* The tranche ID to use for locks */
+} dshash_parameters;
+
+/* Forward declaration of private types for use only by dht.c. */
+struct dshash_table_item;
+typedef struct dshash_table_item dshash_table_item;
+
+/* Creating, sharing and destroying from hash tables. */
+extern dshash_table *dshash_create(dsa_area *area,
+			  const dshash_parameters *params,
+			  void *arg);
+extern dshash_table *dshash_attach(dsa_area *area,
+			  const dshash_parameters *params,
+			  dshash_table_handle handle,
+			  void *arg);
+extern void dshash_detach(dshash_table *hash_table);
+extern dshash_table_handle dshash_get_hash_table_handle(dshash_table *hash_table);
+extern void dshash_destroy(dshash_table *hash_table);
+
+/* Finding, creating, deleting entries. */
+extern void *dshash_find(dshash_table *hash_table,
+			const void *key, bool exclusive);
+extern void *dshash_find_or_insert(dshash_table *hash_table,
+					  const void *key, bool *found);
+extern bool dshash_delete_key(dshash_table *hash_table, const void *key);
+extern void dshash_delete_entry(dshash_table *hash_table, void *entry);
+extern void dshash_release_lock(dshash_table *hash_table, void *entry);
+
+/* Debugging support. */
+extern void dshash_dump(dshash_table *hash_table);
+
+#endif							/* DSHASH_H */

src/tools/pgindent/typedefs.list

@@ -2604,6 +2604,13 @@ dsa_pointer
 dsa_segment_header
 dsa_segment_index
 dsa_segment_map
+dshash_hash
+dshash_parameters
+dshash_partition
+dshash_table
+dshash_table_control
+dshash_table_handle
+dshash_table_item
 dsm_control_header
 dsm_control_item
 dsm_handle