Consolidate the function pointer types used by dshash.c.

Commit 8c0d7bafad introduced dshash with hash
and compare functions like DynaHash's, and also variants that take a user
data pointer instead of size.  Simplify the interface by merging them into
a single pair of function pointer types that take both size and a user data
pointer.

Since it is anticipated that memcmp and tag_hash behavior will be a common
requirement, provide wrapper functions dshash_memcmp and dshash_memhash that
conform to the new function types.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/20170823054644.efuzftxjpfi6wwqs%40alap3.anarazel.de

src/backend/lib/dshash.c

@@ -35,6 +35,7 @@
 #include "storage/ipc.h"
 #include "storage/lwlock.h"
 #include "utils/dsa.h"
+#include "utils/hsearch.h"
 #include "utils/memutils.h"
 
 /*
@@ -188,9 +189,8 @@ static inline bool equal_keys(dshash_table *hash_table,
 /*
  * Create a new hash table backed by the given dynamic shared area, with the
  * given parameters.  The returned object is allocated in backend-local memory
- * using the current MemoryContext.  If 'arg' is non-null, the arg variants of
- * hash and compare functions must be provided in 'params' and 'arg' will be
- * passed down to them.
+ * using the current MemoryContext.  'arg' will be passed through to the
+ * compare and hash functions.
  */
 dshash_table *
 dshash_create(dsa_area *area, const dshash_parameters *params, void *arg)
@@ -198,14 +198,6 @@ dshash_create(dsa_area *area, const dshash_parameters *params, void *arg)
 	dshash_table *hash_table;
 	dsa_pointer control;
 
-	/* Sanity checks on the set of supplied functions. */
-	Assert((params->compare_function != NULL) ^
-		   (params->compare_arg_function != NULL));
-	Assert((params->hash_function != NULL) ^
-		   (params->hash_arg_function != NULL));
-	Assert(arg == NULL || (params->compare_arg_function != NULL));
-	Assert(arg == NULL || (params->hash_arg_function != NULL));
-
 	/* Allocate the backend-local object representing the hash table. */
 	hash_table = palloc(sizeof(dshash_table));
 
@@ -263,9 +255,8 @@ dshash_create(dsa_area *area, const dshash_parameters *params, void *arg)
 
 /*
  * Attach to an existing hash table using a handle.  The returned object is
- * allocated in backend-local memory using the current MemoryContext.  If
- * 'arg' is non-null, the arg variants of hash and compare functions must be
- * provided in 'params' and 'arg' will be passed down to them.
+ * allocated in backend-local memory using the current MemoryContext.  'arg'
+ * will be passed through to the compare and hash functions.
  */
 dshash_table *
 dshash_attach(dsa_area *area, const dshash_parameters *params,
@@ -274,14 +265,6 @@ dshash_attach(dsa_area *area, const dshash_parameters *params,
 	dshash_table *hash_table;
 	dsa_pointer control;
 
-	/* Sanity checks on the set of supplied functions. */
-	Assert((params->compare_function != NULL) ^
-		   (params->compare_arg_function != NULL));
-	Assert((params->hash_function != NULL) ^
-		   (params->hash_arg_function != NULL));
-	Assert(arg == NULL || (params->compare_arg_function != NULL));
-	Assert(arg == NULL || (params->hash_arg_function != NULL));
-
 	/* Allocate the backend-local object representing the hash table. */
 	hash_table = palloc(sizeof(dshash_table));
 
@@ -582,6 +565,24 @@ dshash_release_lock(dshash_table *hash_table, void *entry)
 	LWLockRelease(PARTITION_LOCK(hash_table, partition_index));
 }
 
+/*
+ * A compare function that forwards to memcmp.
+ */
+int
+dshash_memcmp(const void *a, const void *b, size_t size, void *arg)
+{
+	return memcmp(a, b, size);
+}
+
+/*
+ * A hash function that forwards to tag_hash.
+ */
+dshash_hash
+dshash_memhash(const void *v, size_t size, void *arg)
+{
+	return tag_hash(v, size);
+}
+
 /*
  * Print debugging information about the internal state of the hash table to
  * stderr.  The caller must hold no partition locks.
@@ -874,11 +875,9 @@ delete_item_from_bucket(dshash_table *hash_table,
 static inline dshash_hash
 hash_key(dshash_table *hash_table, const void *key)
 {
-	if (hash_table->params.hash_arg_function != NULL)
-		return hash_table->params.hash_arg_function(key, hash_table->arg);
-	else
 	return hash_table->params.hash_function(key,
-												hash_table->params.key_size);
+											hash_table->params.key_size,
+											hash_table->arg);
 }
 
 /*
@@ -887,13 +886,7 @@ hash_key(dshash_table *hash_table, const void *key)
 static inline bool
 equal_keys(dshash_table *hash_table, const void *a, const void *b)
 {
-	int			r;
-
-	if (hash_table->params.compare_arg_function != NULL)
-		r = hash_table->params.compare_arg_function(a, b, hash_table->arg);
-	else
-		r = hash_table->params.compare_function(a, b,
-												hash_table->params.key_size);
-
-	return r == 0;
+	return hash_table->params.compare_function(a, b,
+											   hash_table->params.key_size,
+											   hash_table->arg) == 0;
 }

src/include/lib/dshash.h

@@ -26,32 +26,13 @@ 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 comparing keys. */
+typedef int (*dshash_compare_function) (const void *a, const void *b,
+										size_t size, void *arg);
 
-/*
- * 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);
+/* A function type for computing hash values for keys. */
+typedef dshash_hash (*dshash_hash_function) (const void *v, size_t size,
+											 void *arg);
 
 /*
  * The set of parameters needed to create or attach to a hash table.  The
@@ -70,9 +51,7 @@ 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;
 
@@ -101,6 +80,10 @@ 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);
 
+/* Convenience hash and compare functions wrapping memcmp and tag_hash. */
+extern int dshash_memcmp(const void *a, const void *b, size_t size, void *arg);
+extern dshash_hash dshash_memhash(const void *v, size_t size, void *arg);
+
 /* Debugging support. */
 extern void dshash_dump(dshash_table *hash_table);