Fix checkpointer shared memory allocation

Use Min(NBuffers, MAX_CHECKPOINT_REQUESTS) instead of NBuffers in
CheckpointerShmemSize() to match the actual array size limit set in
CheckpointerShmemInit().  This prevents wasting shared memory when
NBuffers > MAX_CHECKPOINT_REQUESTS.  Also, fix the comment.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1439188.1754506714%40sss.pgh.pa.us
Author: Xuneng Zhou <xunengzhou@gmail.com>
Co-authored-by: Alexander Korotkov <aekorotkov@gmail.com>

diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
index 17a068b27b82673d88bccfeb7bc8bd05f8a97e96..afaa580ae4d643c3e61faed1fc601f93594cfe7a 100644 (file)
--- a/src/backend/postmaster/checkpointer.c
+++ b/src/backend/postmaster/checkpointer.c
@@ -876,11 +876,14 @@ CheckpointerShmemSize(void)
    Size        size;
 
    /*
-    * Currently, the size of the requests[] array is arbitrarily set equal to
-    * NBuffers.  This may prove too large or small ...
+    * The size of the requests[] array is arbitrarily set equal to NBuffers.
+    * But there is a cap of MAX_CHECKPOINT_REQUESTS to prevent accumulating
+    * too many checkpoint requests in the ring buffer.
     */
    size = offsetof(CheckpointerShmemStruct, requests);
-   size = add_size(size, mul_size(NBuffers, sizeof(CheckpointerRequest)));
+   size = add_size(size, mul_size(Min(NBuffers,
+                                      MAX_CHECKPOINT_REQUESTS),
+                                  sizeof(CheckpointerRequest)));
 
    return size;
 }