From 93a81386f61e9973ef5cae5e007a48c9bf042b3e Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Thu, 27 Sep 2018 11:15:19 +0900
Subject: [PATCH 2/7] Add conditional lock feature to dshash

Dshash currently waits for lock unconditionally. This commit adds new
interfaces for dshash_find and dshash_find_or_insert. The new
interfaces have an extra parameter "nowait" taht commands not to wait
for lock.
---
 src/backend/lib/dshash.c | 58 ++++++++++++++++++++++++++++++++++++++++++++----
 src/include/lib/dshash.h |  6 ++++-
 2 files changed, 59 insertions(+), 5 deletions(-)

diff --git a/src/backend/lib/dshash.c b/src/backend/lib/dshash.c
index af904c034e..d8bdaecae5 100644
--- a/src/backend/lib/dshash.c
+++ b/src/backend/lib/dshash.c
@@ -394,6 +394,17 @@ 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, NULL, exclusive, false);
+}
+
+/*
+ * Addition to dshash_find, returns immediately when nowait is true and lock
+ * was not acquired. Lock status is set to *lock_failed if any.
+ */
+void *
+dshash_find_extended(dshash_table *hash_table, const void *key,
+					 bool *lock_acquired, bool exclusive, bool nowait)
 {
 	dshash_hash hash;
 	size_t		partition;
@@ -405,8 +416,23 @@ dshash_find(dshash_table *hash_table, const void *key, bool exclusive)
 	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 +467,22 @@ 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);
+}
+
+/*
+ * Addition to dshash_find_or_insert, returns NULL if nowait is true and lock
+ * was not acquired.
+ *
+ * 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 +497,16 @@ 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 8ab1a21f3e..475d22ab55 100644
--- a/src/include/lib/dshash.h
+++ b/src/include/lib/dshash.h
@@ -90,8 +90,12 @@ 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 *lock_acquired, bool exclusive, bool nowait);
 extern void *dshash_find_or_insert(dshash_table *hash_table,
-					  const void *key, bool *found);
+			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);
-- 
2.16.3