fix(compute): validate prewarm_local_cache() input (#12648)

## Problem
```
postgres=> select neon.prewarm_local_cache('\xfcfcfcfc01000000ffffffff070000000000000000000000000000000000000000000000000000000000000000000000000000ff', 1);
WARNING:  terminating connection because of crash of another server process
DETAIL:  The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
HINT:  In a moment you should be able to reconnect to the database and repeat your command.
FATAL:  server conn crashed?
```

The function takes a bytea argument and casts it to a C struct, without
validating the contents.

## Summary of changes

Added validation for number of pages to be prefetched and for the chunks
as well.

pgxn/neon/file_cache.c

@@ -49,6 +49,7 @@
 #include "neon.h"
 #include "neon_lwlsncache.h"
 #include "neon_perf_counters.h"
+#include "neon_utils.h"
 #include "pagestore_client.h"
 #include "communicator.h"
 
@@ -673,8 +674,19 @@ lfc_get_state(size_t max_entries)
 			{
 				if (GET_STATE(entry, j) != UNAVAILABLE)
 				{
-					BITMAP_SET(bitmap, i*lfc_blocks_per_chunk + j);
+					/* Validate the buffer tag before including it */
-					n_pages += 1;
+					BufferTag test_tag = entry->key;
+					test_tag.blockNum += j;
+
+					if (BufferTagIsValid(&test_tag))
+					{
+						BITMAP_SET(bitmap, i*lfc_blocks_per_chunk + j);
+						n_pages += 1;
+					}
+					else
+					{
+						elog(ERROR, "LFC: Skipping invalid buffer tag during cache state capture: blockNum=%u", test_tag.blockNum);
+					}
 				}
 			}
 			if (++i == n_entries)
@@ -683,7 +695,7 @@ lfc_get_state(size_t max_entries)
 		Assert(i == n_entries);
 		fcs->n_pages = n_pages;
 		Assert(pg_popcount((char*)bitmap, ((n_entries << lfc_chunk_size_log) + 7)/8) == n_pages);
-		elog(LOG, "LFC: save state of %d chunks %d pages", (int)n_entries, (int)n_pages);
+		elog(LOG, "LFC: save state of %d chunks %d pages (validated)", (int)n_entries, (int)n_pages);
 	}
 
 	LWLockRelease(lfc_lock);
@@ -702,6 +714,7 @@ lfc_prewarm(FileCacheState* fcs, uint32 n_workers)
 	size_t n_entries;
 	size_t prewarm_batch = Min(lfc_prewarm_batch, readahead_buffer_size);
 	size_t fcs_size;
+	uint32_t max_prefetch_pages;
 	dsm_segment *seg;
 	BackgroundWorkerHandle* bgw_handle[MAX_PREWARM_WORKERS];
 
@@ -746,6 +759,11 @@ lfc_prewarm(FileCacheState* fcs, uint32 n_workers)
 	n_entries = Min(fcs->n_chunks, lfc_prewarm_limit);
 	Assert(n_entries != 0);
 
+	max_prefetch_pages = n_entries << fcs_chunk_size_log;
+	if (fcs->n_pages > max_prefetch_pages) {
+		elog(ERROR, "LFC: Number of pages in file cache state (%d) is more than the limit (%d)", fcs->n_pages, max_prefetch_pages);
+	}
+
 	LWLockAcquire(lfc_lock, LW_EXCLUSIVE);
 
 	/* Do not prewarm more entries than LFC limit */
@@ -898,6 +916,11 @@ lfc_prewarm_main(Datum main_arg)
 				{
 					tag = fcs->chunks[snd_idx >> fcs_chunk_size_log];
 					tag.blockNum += snd_idx & ((1 << fcs_chunk_size_log) - 1);
+
+					if (!BufferTagIsValid(&tag)) {
+						elog(ERROR, "LFC: Invalid buffer tag: %u", tag.blockNum);
+					}
+
 					if (!lfc_cache_contains(BufTagGetNRelFileInfo(tag), tag.forkNum, tag.blockNum))
 					{
 						(void)communicator_prefetch_register_bufferv(tag, NULL, 1, NULL);

pgxn/neon/neon_utils.c

@@ -183,3 +183,22 @@ alloc_curl_handle(void)
 }
 
 #endif
+
+/*
+ * Check if a BufferTag is valid by verifying all its fields are not invalid.
+ */
+bool
+BufferTagIsValid(const BufferTag *tag)
+{
+	#if PG_MAJORVERSION_NUM >= 16
+	return (tag->spcOid != InvalidOid) &&
+		(tag->relNumber != InvalidRelFileNumber) &&
+		(tag->forkNum != InvalidForkNumber) &&
+		(tag->blockNum != InvalidBlockNumber);
+	#else
+	return (tag->rnode.spcNode != InvalidOid) &&
+		(tag->rnode.relNode != InvalidOid) &&
+		(tag->forkNum != InvalidForkNumber) &&
+		(tag->blockNum != InvalidBlockNumber);
+	#endif
+}

pgxn/neon/neon_utils.h

@@ -2,6 +2,7 @@
 #define __NEON_UTILS_H__
 
 #include "lib/stringinfo.h"
+#include "storage/buf_internals.h"
 
 #ifndef WALPROPOSER_LIB
 #include <curl/curl.h>
@@ -16,6 +17,9 @@ void		pq_sendint32_le(StringInfo buf, uint32 i);
 void		pq_sendint64_le(StringInfo buf, uint64 i);
 void        disable_core_dump(void);
 
+/* Buffer tag validation function */
+bool		BufferTagIsValid(const BufferTag *tag);
+
 #ifndef WALPROPOSER_LIB
 
 CURL *		alloc_curl_handle(void);