Check if there is enough memory for HNSW index (#4602)

## Problem

HNSW index is created in memory.
Try to prevent OOM by checking of available RAM.


## Summary of changes

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/hnsw/hnsw.c

@@ -122,6 +122,43 @@ hnsw_populate(HierarchicalNSW* hnsw, Relation indexRel, Relation heapRel)
 						   true, true, hnsw_build_callback, (void *) hnsw, NULL);
 }
 
+#ifdef __APPLE__
+
+#include <sys/types.h>
+#include <sys/sysctl.h>
+
+static void
+hnsw_check_available_memory(Size requested)
+{
+	size_t total;
+	if (sysctlbyname("hw.memsize", NULL, &total, NULL, 0) < 0)
+		elog(ERROR, "Failed to get amount of RAM: %m");
+
+	if ((Size)NBuffers*BLCKSZ + requested >= total)
+		elog(ERROR, "HNSW index requeries %ld bytes while only %ld are available",
+			requested, total - (Size)NBuffers*BLCKSZ);
+}
+
+#else
+
+#include <sys/sysinfo.h>
+
+static void
+hnsw_check_available_memory(Size requested)
+{
+	struct sysinfo si;
+	Size total;
+	if (sysinfo(&si) < 0)
+		elog(ERROR, "Failed to get amount of RAM: %n");
+
+	total = si.totalram*si.mem_unit;
+	if ((Size)NBuffers*BLCKSZ + requested >= total)
+		elog(ERROR, "HNSW index requeries %ld bytes while only %ld are available",
+			requested, total - (Size)NBuffers*BLCKSZ);
+}
+
+#endif
+
 static HierarchicalNSW*
 hnsw_get_index(Relation indexRel, Relation heapRel)
 {
@@ -156,6 +193,8 @@ hnsw_get_index(Relation indexRel, Relation heapRel)
 		size_data_per_element = size_links_level0 + data_size + sizeof(label_t);
 		shmem_size =  hnsw_sizeof() + maxelements * size_data_per_element;
 
+		hnsw_check_available_memory(shmem_size);
+
 		/* first try to attach to existed index */
 		if (!dsm_impl_op(DSM_OP_ATTACH, handle, 0, &impl_private,
 						 &mapped_address, &mapped_size, DEBUG1))

pgxn/hnsw/hnsw.control

@@ -1,4 +1,4 @@
-comment = 'hNsw index'
+comment = 'hnsw index'
 default_version = '0.1.0'
 module_pathname = '$libdir/hnsw'
 relocatable = true