v2-0005-fixup-Use-more-efficient-zero-copy-API.patch

From 71d9875fca9d011a899cd1cfb477a979a3cacd44 Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Sun, 5 Apr 2026 03:07:19 +1200
Subject: [PATCH v2 5/6] fixup: Use more efficient zero-copy API?

We can pass a pointer to data in libarchive's internal buffer directly
to the next streamer, avoiding one copy.  To do this we also have to
expand any sparse regions ourselves.

XXX not sure it's worth the complexity for non-performance critical
code?
---
 src/fe_utils/astreamer_libarchive.c | 62 ++++++++++++++++++++++++-----
 1 file changed, 52 insertions(+), 10 deletions(-)

diff --git a/src/fe_utils/astreamer_libarchive.c b/src/fe_utils/astreamer_libarchive.c
index 967eca84abe..2ac0c565cc4 100644
--- a/src/fe_utils/astreamer_libarchive.c
+++ b/src/fe_utils/astreamer_libarchive.c
@@ -29,7 +29,8 @@ typedef struct astreamer_libarchive_reader
 	struct archive *archive;
 	bool		end_of_file;
 	bool		end_of_archive;
-	char		data[ASTREAMER_LIBARCHIVE_READER_BUFFER_SIZE];
+	pgoff_t		offset;
+	char		zeroes[8192];
 } astreamer_libarchive_reader;
 
 static bool astreamer_libarchive_reader_pull_content(astreamer *streamer);
@@ -117,11 +118,33 @@ astreamer_libarchive_reader_fill_member(astreamer_member *member,
 	}
 }
 
+/* Emit zeroes up to offset. */
+static void
+astreamer_libarchive_reader_expand_sparse(astreamer_libarchive_reader *mystreamer,
+										  pgoff_t offset)
+{
+	size_t		size;
+
+	while (mystreamer->offset < offset)
+	{
+		size = offset - mystreamer->offset;
+		size = Min(size, sizeof(mystreamer->zeroes));
+		astreamer_content(mystreamer->base.bbs_next,
+						  &mystreamer->member,
+						  mystreamer->zeroes,
+						  size,
+						  ASTREAMER_MEMBER_CONTENTS);
+		mystreamer->offset += size;
+	}
+}
+
 static bool
 astreamer_libarchive_reader_pull_content(astreamer *streamer)
 {
 	astreamer_libarchive_reader *mystreamer;
-	ssize_t		size;
+	const void *data;
+	size_t		size;
+	pgoff_t		offset;
 
 	mystreamer = (astreamer_libarchive_reader *) streamer;
 
@@ -148,6 +171,7 @@ astreamer_libarchive_reader_pull_content(astreamer *streamer)
 				case ARCHIVE_OK:
 					/* Send file header, then fall through to send one chunk. */
 					mystreamer->end_of_file = false;
+					mystreamer->offset = 0;
 					astreamer_libarchive_reader_fill_member(&mystreamer->member,
 															entry);
 					astreamer_content(mystreamer->base.bbs_next,
@@ -171,12 +195,19 @@ astreamer_libarchive_reader_pull_content(astreamer *streamer)
 			}
 		}
 
-		/* Stream a chunk of data, or discover end of file. */
+		/*
+		 * Stream a chunk of data, or discover end of file.
+		 *
+		 * It would be a bit simpler to use archive_read_data(), but this
+		 * interface removes the need for copying to an output buffer.  In
+		 * exchange for that, we have to deal with expanding (rare) sparse
+		 * file zeroes.
+		 */
 		Assert(!mystreamer->end_of_file);
-		size = archive_read_data(mystreamer->archive,
-								 mystreamer->data,
-								 sizeof(mystreamer->data));
-		switch (size)
+		switch (archive_read_data_block(mystreamer->archive,
+										&data,
+										&size,
+										&offset))
 		{
 			case ARCHIVE_RETRY:
 				continue;
@@ -187,11 +218,20 @@ astreamer_libarchive_reader_pull_content(astreamer *streamer)
 			case ARCHIVE_WARN:
 				pg_log_warning("libarchive: %s",
 							   archive_error_string(mystreamer->archive));
-				continue;
+				break;
+			case ARCHIVE_EOF:
+				size = 0;
+				break;
+			case ARCHIVE_OK:
+				break;
 			default:
+				pg_fatal("unexpected result from archive_read_next_data_block()");
 				break;
 		}
 
+		/* Expand any intervening sparse region. */
+		astreamer_libarchive_reader_expand_sparse(mystreamer, offset);
+
 		if (size == 0)
 		{
 			/* Send trailer, and go around to start another file. */
@@ -204,12 +244,14 @@ astreamer_libarchive_reader_pull_content(astreamer *streamer)
 			continue;
 		}
 
-		/* Stream large chunk and return. */
+		/* Stream large chunk directly from libarchive's buffer and return. */
+		Assert(mystreamer->offset == offset);
 		astreamer_content(mystreamer->base.bbs_next,
 						  &mystreamer->member,
-						  mystreamer->data,
+						  data,
 						  size,
 						  ASTREAMER_MEMBER_CONTENTS);
+		mystreamer->offset += size;
 		return true;
 	}
 	return false;
-- 
2.53.0