From 242b8904682cec326d059e3d11355ca2315c869c Mon Sep 17 00:00:00 2001
From: Amul Sul <sulamul@gmail.com>
Date: Sat, 21 Mar 2026 20:57:23 +0530
Subject: [PATCH v3 3/3] pg_waldump: Handle archive exhaustion in
 init_archive_reader().

When read_archive_file() returns 0, the archive may have already
buffered a complete WAL file into the hash table before exhausting
the input.  Instead of immediately reporting an error, search the
hash table for an entry containing at least sizeof(XLogLongPageHeader)
bytes.  Report a specific error if a WAL entry exists but is too
short (truncated/corrupt), or a generic error if no WAL was found
at all.

Also tighten the loop condition to check for sizeof(XLogLongPageHeader)
rather than XLOG_BLCKSZ, since only the long page header is needed
at this stage.
---
 src/bin/pg_waldump/archive_waldump.c | 55 ++++++++++++++++++++++++++--
 1 file changed, 51 insertions(+), 4 deletions(-)

diff --git a/src/bin/pg_waldump/archive_waldump.c b/src/bin/pg_waldump/archive_waldump.c
index ee292b6dc8d..943c843e05b 100644
--- a/src/bin/pg_waldump/archive_waldump.c
+++ b/src/bin/pg_waldump/archive_waldump.c
@@ -176,13 +176,60 @@ init_archive_reader(XLogDumpPrivate *privateInfo,
 	 * the first WAL segment in the archive so we can extract the WAL segment
 	 * size from the long page header.
 	 */
-	while (entry == NULL || entry->buf->len < XLOG_BLCKSZ)
+	while (entry == NULL || entry->read_len < sizeof(XLogLongPageHeader))
 	{
 		if (read_archive_file(privateInfo, XLOG_BLCKSZ) == 0)
-			pg_fatal("could not find WAL in archive \"%s\"",
-					 privateInfo->archive_name);
+		{
+			ArchivedWAL_iterator iter;
+			ArchivedWALFile *e = NULL;
+			ArchivedWALFile *short_entry = NULL;
 
-		entry = privateInfo->cur_file;
+			entry = NULL;
+
+			/*
+			 * read_archive_file() returned 0, meaning the archive is
+			 * exhausted.  However, a sufficiently compressed archive may have
+			 * already read a complete WAL file and inserted it into the hash
+			 * table before returning.  Search the hash table for any entry
+			 * that already has enough buffered data to contain the long page
+			 * header; if none is found, the archive contains no usable WAL.
+			 */
+			ArchivedWAL_start_iterate(privateInfo->archive_wal_htab, &iter);
+			while ((e = ArchivedWAL_iterate(privateInfo->archive_wal_htab,
+											&iter)) != NULL)
+			{
+				if (e->read_len >= sizeof(XLogLongPageHeader))
+				{
+					entry = e;
+					break;
+				}
+				/* Remember a short entry in case we need to report it */
+				short_entry = e;
+			}
+
+			if (entry == NULL)
+			{
+				/*
+				 * A WAL file was found in the hash table but it does not
+				 * contain enough data to read the long page header,
+				 * indicating a truncated or corrupt WAL segment.
+				 */
+				if (short_entry != NULL)
+					pg_fatal("could not read file \"%s\" from \"%s\" archive: read %d of %d",
+							 short_entry->fname, privateInfo->archive_name,
+							 short_entry->read_len,
+							 (int) sizeof(XLogLongPageHeader));
+
+				/*
+				 * The hash table contains no WAL entries at all, meaning the
+				 * archive holds no WAL data.
+				 */
+				pg_fatal("could not find WAL in archive \"%s\"",
+						 privateInfo->archive_name);
+			}
+		}
+		else
+			entry = privateInfo->cur_file;
 	}
 
 	/* Extract the WAL segment size from the long page header */
-- 
2.47.1

