From 2d6ee4deb8277d055b983a527dc4a8afb209d22d Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Thu, 19 Mar 2020 13:27:23 +1300
Subject: [PATCH v25 2/5] Add conditional lock facility to dshash.

Provide a way to avoid waiting for a partition lock in a dshash hash
table.  This is useful for later commits that prefer to buffer
insertions for later rather than sleep waiting for a lock.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgre.es/m/20180629.173418.190173462.horiguchi.kyotaro%40lab.ntt.co.jp
---
 src/backend/lib/dshash.c | 64 +++++++++++++++++++++++++++++++++++++---
 src/include/lib/dshash.h |  6 ++++
 2 files changed, 66 insertions(+), 4 deletions(-)

diff --git a/src/backend/lib/dshash.c b/src/backend/lib/dshash.c
index afaccb123b..4ba63549da 100644
--- a/src/backend/lib/dshash.c
+++ b/src/backend/lib/dshash.c
@@ -394,19 +394,51 @@ dshash_get_hash_table_handle(dshash_table *hash_table)
  */
 void *
 dshash_find(dshash_table *hash_table, const void *key, bool exclusive)
+{
+	return dshash_find_extended(hash_table, key, exclusive, false, NULL);
+}
+
+/*
+ * The version of dshash_find, which is allowed to return immediately on lock
+ * failure. Lock status is set to *lock_failed in that case.
+ */
+void *
+dshash_find_extended(dshash_table *hash_table, const void *key,
+					 bool exclusive, bool nowait, bool *lock_acquired)
 {
 	dshash_hash hash;
 	size_t		partition;
 	dshash_table_item *item;
 
+	/*
+	 * No need to return lock resut when !nowait. Otherwise the caller may
+	 * omit the lock result when NULL is returned.
+	 */
+	Assert(nowait || !lock_acquired);
+
 	hash = hash_key(hash_table, key);
 	partition = PARTITION_FOR_HASH(hash);
 
 	Assert(hash_table->control->magic == DSHASH_MAGIC);
 	Assert(!hash_table->find_locked);
 
-	LWLockAcquire(PARTITION_LOCK(hash_table, partition),
-				  exclusive ? LW_EXCLUSIVE : LW_SHARED);
+	if (nowait)
+	{
+		if (!LWLockConditionalAcquire(PARTITION_LOCK(hash_table, partition),
+									  exclusive ? LW_EXCLUSIVE : LW_SHARED))
+		{
+			if (lock_acquired)
+				*lock_acquired = false;
+			return NULL;
+		}
+	}
+	else
+		LWLockAcquire(PARTITION_LOCK(hash_table, partition),
+					  exclusive ? LW_EXCLUSIVE : LW_SHARED);
+
+	if (lock_acquired)
+		*lock_acquired = true;
+
 	ensure_valid_bucket_pointers(hash_table);
 
 	/* Search the active bucket. */
@@ -441,6 +473,22 @@ void *
 dshash_find_or_insert(dshash_table *hash_table,
 					  const void *key,
 					  bool *found)
+{
+	return dshash_find_or_insert_extended(hash_table, key, found, false);
+}
+
+/*
+ * The version of dshash_find_or_insert, which is allowed to return immediately
+ * on lock failure.
+ *
+ * Notes above dshash_find_extended() regarding locking and error handling
+ * equally apply here.
+ */
+void *
+dshash_find_or_insert_extended(dshash_table *hash_table,
+							   const void *key,
+							   bool *found,
+							   bool nowait)
 {
 	dshash_hash hash;
 	size_t		partition_index;
@@ -455,8 +503,16 @@ dshash_find_or_insert(dshash_table *hash_table,
 	Assert(!hash_table->find_locked);
 
 restart:
-	LWLockAcquire(PARTITION_LOCK(hash_table, partition_index),
-				  LW_EXCLUSIVE);
+	if (nowait)
+	{
+		if (!LWLockConditionalAcquire(
+				PARTITION_LOCK(hash_table, partition_index),
+				LW_EXCLUSIVE))
+			return NULL;
+	}
+	else
+		LWLockAcquire(PARTITION_LOCK(hash_table, partition_index),
+					  LW_EXCLUSIVE);
 	ensure_valid_bucket_pointers(hash_table);
 
 	/* Search the active bucket. */
diff --git a/src/include/lib/dshash.h b/src/include/lib/dshash.h
index ad88f32cdd..a7d19c6a85 100644
--- a/src/include/lib/dshash.h
+++ b/src/include/lib/dshash.h
@@ -89,8 +89,14 @@ 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_extended(dshash_table *hash_table, const void *key,
+								  bool exclusive, bool nowait,
+								  bool *lock_acquired);
 extern void *dshash_find_or_insert(dshash_table *hash_table,
 								   const void *key, bool *found);
+extern void *dshash_find_or_insert_extended(dshash_table *hash_table,
+											const void *key, bool *found,
+											bool nowait);
 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);
-- 
2.20.1