[PATCH 2/3] Introduce a reusable memory pool type

[jameson.miller81@gmail.com]

From: Jameson Miller <jamill@microsoft.com>

Extract the existing memory pool logic used by fast-import into a
generalized component. This memory pool component can then be used by
other components that need this functionality.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
---
 Makefile   |   1 +
 mem-pool.c | 130 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 mem-pool.h |  48 +++++++++++++++++++++++
 3 files changed, 179 insertions(+)
 create mode 100644 mem-pool.c
 create mode 100644 mem-pool.h

diff --git a/Makefile b/Makefile
index a1d8775adb..1e142b1dd9 100644
--- a/Makefile
+++ b/Makefile
@@ -832,6 +832,7 @@ LIB_OBJS += lockfile.o
 LIB_OBJS += log-tree.o
 LIB_OBJS += mailinfo.o
 LIB_OBJS += mailmap.o
+LIB_OBJS += mem-pool.o
 LIB_OBJS += match-trees.o
 LIB_OBJS += merge.o
 LIB_OBJS += merge-blobs.o
diff --git a/mem-pool.c b/mem-pool.c
new file mode 100644
index 0000000000..3028bc3c67
--- /dev/null
+++ b/mem-pool.c
@@ -0,0 +1,130 @@
+/*
+ * Memory Pool implementation logic.
+ */
+
+#include "cache.h"
+#include "mem-pool.h"
+
+#define MIN_ALLOC_GROWTH_SIZE 1024 * 1024
+
+struct mp_block {
+	struct mp_block *next_block;
+	char *next_free;
+	char *end;
+	uintmax_t space[FLEX_ARRAY];
+};
+
+static struct mp_block *mem_pool_alloc_block(struct mem_pool *mem_pool, size_t block_alloc)
+{
+	struct mp_block *p;
+
+	/* Round up to a 'uintmax_t' alignment */
+	if (block_alloc & (sizeof(uintmax_t) - 1))
+		block_alloc += sizeof(uintmax_t) - (block_alloc & (sizeof(uintmax_t) - 1));
+
+	mem_pool->pool_alloc += block_alloc;
+
+	p = xmalloc(st_add(sizeof(struct mp_block), block_alloc));
+
+	p->next_block = mem_pool->mp_block;
+	p->next_free = (char *)p->space;
+	p->end = p->next_free + block_alloc;
+	mem_pool->mp_block = p;
+
+	return p;
+}
+
+void mem_pool_init(struct mem_pool **mem_pool, size_t block_alloc, size_t initial_size)
+{
+	if (!(*mem_pool))
+	{
+		if (block_alloc < MIN_ALLOC_GROWTH_SIZE)
+			block_alloc = MIN_ALLOC_GROWTH_SIZE;
+
+		*mem_pool = xmalloc(sizeof(struct mem_pool));
+		(*mem_pool)->pool_alloc = 0;
+		(*mem_pool)->mp_block = 0;
+		(*mem_pool)->block_alloc = block_alloc;
+
+		if (initial_size > 0)
+			mem_pool_alloc_block((*mem_pool), initial_size);
+	}
+}
+
+void mem_pool_discard(struct mem_pool *mem_pool)
+{
+	struct mp_block *block, *block_to_free;
+	for (block = mem_pool->mp_block; block;)
+	{
+		block_to_free = block;
+		block = block->next_block;
+		free(block_to_free);
+	}
+
+	free(mem_pool);
+}
+
+void *mem_pool_alloc(struct mem_pool *mem_pool, size_t len)
+{
+	struct mp_block *p;
+	void *r;
+
+	/* Round up to a 'uintmax_t' alignment */
+	if (len & (sizeof(uintmax_t) - 1))
+		len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
+
+	p = mem_pool->mp_block;
+
+	if (p &&
+	   (p->end - p->next_free < len)) {
+		for (p = p->next_block; p; p = p->next_block)
+			if (p->end - p->next_free >= len)
+				break;
+	}
+
+	if (!p) {
+		if (len >= ((mem_pool->block_alloc - sizeof(struct mp_block)) / 2)) {
+			p = mem_pool_alloc_block(mem_pool, len);
+		}
+		else
+			p = mem_pool_alloc_block(mem_pool, mem_pool->block_alloc);
+	}
+
+	r = p->next_free;
+	p->next_free += len;
+	return r;
+}
+
+void *mem_pool_calloc(struct mem_pool *mem_pool, size_t count, size_t size)
+{
+	size_t len = st_mult(count, size);
+	void *r = mem_pool_alloc(mem_pool, len);
+	memset(r, 0, len);
+	return r;
+}
+
+int mem_pool_contains(struct mem_pool *mem_pool, void *mem)
+{
+	struct mp_block *p;
+	for (p = mem_pool->mp_block; p; p = p->next_block)
+		if ((mem >= ((void *)p->space)) &&
+		    (mem < ((void *)p->end)))
+			return 1;
+
+	return 0;
+}
+
+void mem_pool_combine(struct mem_pool *dst, struct mem_pool *src)
+{
+	struct mp_block **tail = &dst->mp_block;
+	/* find pointer of dst's last block (if any) */
+	while (*tail)
+		tail = &(*tail)->next_block;
+
+	/* append the blocks from src to dst */
+	*tail = src->mp_block;
+
+	dst->pool_alloc += src->pool_alloc;
+	src->pool_alloc = 0;
+	src->mp_block = NULL;
+}
diff --git a/mem-pool.h b/mem-pool.h
new file mode 100644
index 0000000000..902ef8caf2
--- /dev/null
+++ b/mem-pool.h
@@ -0,0 +1,48 @@
+#ifndef MEM_POOL_H
+#define MEM_POOL_H
+
+struct mem_pool {
+	struct mp_block *mp_block;
+
+	/* The size of new blocks to grow the pool by. */
+	size_t block_alloc;
+
+	/* The total amount of memory allocated by the pool. */
+	size_t pool_alloc;
+};
+
+/*
+ * Initialize mem_pool with specified parameters for initial size and
+ * how much to grow when a larger memory block is required.
+ */
+void mem_pool_init(struct mem_pool **mem_pool, size_t alloc_growth_size, size_t initial_size);
+
+/*
+ * Move the memory associated with the 'src' pool to the 'dst' pool. The 'src'
+ * pool will be empty and not contain any memory. It still needs to be free'd
+ * with a call to `mem_pool_discard`.
+ */
+void mem_pool_combine(struct mem_pool *dst, struct mem_pool *src);
+
+/*
+ * Discard a memory pool and free all the memory it is responsible for.
+ */
+void mem_pool_discard(struct mem_pool *mem_pool);
+
+/*
+ * Alloc memory from the mem_pool.
+ */
+void *mem_pool_alloc(struct mem_pool *pool, size_t len);
+
+/*
+ * Allocate and zero memory from the memory pool.
+ */
+void *mem_pool_calloc(struct mem_pool *pool, size_t count, size_t size);
+
+/*
+ * Check if a memory pointed at by 'mem' is part of the range of
+ * memory managed by the specified mem_pool.
+ */
+int mem_pool_contains(struct mem_pool *mem_pool, void *mem);
+
+#endif
-- 
2.14.3