Add foreach_hash macro

For lists we've had a new foreach style macros since 14dd0f2. This
adds a similar macro for hash tables. This new foreach_hash macro makes
iterating over the items in an HTAB as simple as iterating over the
items in a List. The only additional thing to keep in mind is that when
exiting the loop early you need to call foreach_hash_term.

Author: JelteF

src/backend/utils/hash/dynahash.c

@@ -1462,7 +1462,7 @@ hash_get_num_entries(HTAB *hashp)
 }
 
 /*
- * hash_seq_init/_search/_term
+ * hash_seq_init/_new/_search/_term
  *			Sequentially search through hash table and return
  *			all the elements one by one, return NULL when no more.
  *
@@ -1496,6 +1496,19 @@ hash_seq_init(HASH_SEQ_STATUS *status, HTAB *hashp)
 		register_seq_scan(hashp);
 }
 
+/*
+ * Same as hash_seq_init(), but returns the status struct instead of taking a
+ * pointer.
+ */
+HASH_SEQ_STATUS
+hash_seq_new(HTAB *hashp)
+{
+	HASH_SEQ_STATUS status;
+
+	hash_seq_init(&status, hashp);
+	return status;
+}
+
 /*
  * Same as above but scan by the given hash value.
  * See also hash_seq_search().

src/include/utils/hsearch.h

@@ -273,6 +273,51 @@ typedef struct
 	uint32		hashvalue;		/* hashvalue to start seqscan over hash */
 } HASH_SEQ_STATUS;
 
+/*
+ * foreach_hash - iterate over all entries in a hash table
+ *
+ * This macro simplifies hash table iteration by combining hash_seq_init
+ * and hash_seq_search into a single for-loop construct.
+ *
+ * Usage:
+ *   foreach_hash(MyEntry, entry, my_hashtable)
+ *   {
+ *       // use entry
+ *   }
+ *
+ * This replaces the more verbose pattern:
+ *   HASH_SEQ_STATUS status;
+ *   MyEntry *entry;
+ *   hash_seq_init(&status, my_hashtable);
+ *   while ((entry = (MyEntry *) hash_seq_search(&status)) != NULL)
+ *   {
+ *       // use entry
+ *   }
+ *
+ * For early termination, use foreach_hash_term() before break:
+ *   foreach_hash(MyEntry, entry, my_hashtable)
+ *   {
+ *       if (found_it)
+ *       {
+ *           foreach_hash_term(entry);
+ *           break;
+ *       }
+ *   }
+ */
+#define foreach_hash(type, var, htab) \
+	for (type *var = 0, *var##__outerloop = (type *) 1; \
+		 var##__outerloop; \
+		 var##__outerloop = 0) \
+		for (HASH_SEQ_STATUS var##__status = hash_seq_new(htab); \
+			 (var = (type *) hash_seq_search(&var##__status)) != NULL; )
+
+/*
+ * foreach_hash_term - terminate a foreach_hash loop early
+ *
+ * Call this before 'break' to properly clean up the hash scan.
+ */
+#define foreach_hash_term(var) hash_seq_term(&var##__status)
+
 /*
  * prototypes for functions in dynahash.c
  */
@@ -293,6 +338,7 @@ extern bool hash_update_hash_key(HTAB *hashp, void *existingEntry,
 								 const void *newKeyPtr);
 extern int64 hash_get_num_entries(HTAB *hashp);
 extern void hash_seq_init(HASH_SEQ_STATUS *status, HTAB *hashp);
+extern HASH_SEQ_STATUS hash_seq_new(HTAB *hashp);
 extern void hash_seq_init_with_hash_value(HASH_SEQ_STATUS *status,
 										  HTAB *hashp,
 										  uint32 hashvalue);