Don't allocate large buffer on the stack in pg_verifybackup

Per complaint from Andres Freund. Follow his suggestion to allocate the
buffer once in the calling routine instead.

Also make a tiny indentation improvement.

Discussion: https://postgr.es/m/20240411190147.a3yries632olfcgg@awork3.anarazel.de

Author: adunstan

src/bin/pg_verifybackup/pg_verifybackup.c

@@ -144,7 +144,8 @@ static void verify_control_file(const char *controlpath,
 static void report_extra_backup_files(verifier_context *context);
 static void verify_backup_checksums(verifier_context *context);
 static void verify_file_checksum(verifier_context *context,
-								 manifest_file *m, char *fullpath);
+								 manifest_file *m, char *fullpath,
+								 uint8 *buffer);
 static void parse_required_wal(verifier_context *context,
 							   char *pg_waldump_path,
 							   char *wal_directory);
@@ -480,8 +481,8 @@ parse_manifest_file(char *manifest_path)
 							 (long long int) statbuf.st_size);
 			}
 			bytes_left -= rc;
-			json_parse_manifest_incremental_chunk(
-												  inc_state, buffer, rc, bytes_left == 0);
+			json_parse_manifest_incremental_chunk(inc_state, buffer, rc,
+												  bytes_left == 0);
 		}
 
 		/* Release the incremental state memory */
@@ -812,9 +813,12 @@ verify_backup_checksums(verifier_context *context)
 	manifest_data *manifest = context->manifest;
 	manifest_files_iterator it;
 	manifest_file *m;
+	uint8	   *buffer;
 
 	progress_report(false);
 
+	buffer = pg_malloc(READ_CHUNK_SIZE * sizeof(uint8));
+
 	manifest_files_start_iterate(manifest->files, &it);
 	while ((m = manifest_files_iterate(manifest->files, &it)) != NULL)
 	{
@@ -828,13 +832,15 @@ verify_backup_checksums(verifier_context *context)
 								m->pathname);
 
 			/* Do the actual checksum verification. */
-			verify_file_checksum(context, m, fullpath);
+			verify_file_checksum(context, m, fullpath, buffer);
 
 			/* Avoid leaking memory. */
 			pfree(fullpath);
 		}
 	}
 
+	pfree(buffer);
+
 	progress_report(true);
 }
 
@@ -843,14 +849,13 @@ verify_backup_checksums(verifier_context *context)
  */
 static void
 verify_file_checksum(verifier_context *context, manifest_file *m,
-					 char *fullpath)
+					 char *fullpath, uint8 *buffer)
 {
 	pg_checksum_context checksum_ctx;
 	char	   *relpath = m->pathname;
 	int			fd;
 	int			rc;
 	size_t		bytes_read = 0;
-	uint8		buffer[READ_CHUNK_SIZE];
 	uint8		checksumbuf[PG_CHECKSUM_MAX_LENGTH];
 	int			checksumlen;