From d5706ad7b70a03c53d06abff019d4cc56342ab09 Mon Sep 17 00:00:00 2001
From: Thomas Munro <tmunro@postgresql.org>
Date: Tue, 1 Dec 2020 13:46:27 +1300
Subject: [PATCH] Free disk space for dropped relations on commit.

When committing a transaction that dropped a relation, we previously
truncated only the first segment file to free up disk space (the one
that won't be unlinked until the next checkpoint).

Truncate higher numbered segments too, even though we unlink them on
commit.  This frees the disk space immediately, even if other backends
have open file descriptors and might take a long time to get around to
handling shared invalidation events and closing them.  Also extend the
same behavior to the first segment, in recovery.

Back-patch to all supported releases.

Bug: #16663
Reported-by: Denis Patron <denis.patron@previnet.it>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>
Reviewed-by: Neil Chen <carpenter.nail.cz@gmail.com>
Reviewed-by: David Zhang <david.zhang@highgo.ca>
Discussion: https://postgr.es/m/16663-fe97ccf9932fc800%40postgresql.org
---
 src/backend/storage/smgr/md.c | 89 +++++++++++++++++++++++++----------
 1 file changed, 65 insertions(+), 24 deletions(-)

diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 200cc7f657a..424dfef6dd1 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -406,6 +406,41 @@ mdunlink(RelFileNodeBackend rnode, ForkNumber forkNum, bool isRedo)
 		mdunlinkfork(rnode, forkNum, isRedo);
 }
 
+/*
+ * Truncate a file to release disk space.
+ */
+static int
+do_truncate(const char *path)
+{
+	int			save_errno;
+	int			ret;
+	int			fd;
+
+	/* truncate(2) would be easier here, but Windows hasn't got it */
+	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
+	if (fd >= 0)
+	{
+		ret = ftruncate(fd, 0);
+		save_errno = errno;
+		CloseTransientFile(fd);
+		errno = save_errno;
+	}
+	else
+		ret = -1;
+
+	/* Log a warning here to avoid repetition in callers. */
+	if (ret < 0 && errno != ENOENT)
+	{
+		save_errno = errno;
+		ereport(WARNING,
+				(errcode_for_file_access(),
+				 errmsg("could not truncate file \"%s\": %m", path)));
+		errno = save_errno;
+	}
+
+	return ret;
+}
+
 static void
 mdunlinkfork(RelFileNodeBackend rnode, ForkNumber forkNum, bool isRedo)
 {
@@ -419,33 +454,28 @@ mdunlinkfork(RelFileNodeBackend rnode, ForkNumber forkNum, bool isRedo)
 	 */
 	if (isRedo || forkNum != MAIN_FORKNUM || RelFileNodeBackendIsTemp(rnode))
 	{
-		ret = unlink(path);
-		if (ret < 0 && errno != ENOENT)
-			ereport(WARNING,
-					(errcode_for_file_access(),
-					 errmsg("could not remove file \"%s\": %m", path)));
+		if (!RelFileNodeBackendIsTemp(rnode))
+		{
+			/* Prevent other backends' fds from holding on to the disk space */
+			ret = do_truncate(path);
+		}
+		else
+			ret = 0;
+
+		/* Next unlink the file, unless it was already found to be missing */
+		if (ret == 0 || errno != ENOENT)
+		{
+			ret = unlink(path);
+			if (ret < 0 && errno != ENOENT)
+				ereport(WARNING,
+						(errcode_for_file_access(),
+						 errmsg("could not remove file \"%s\": %m", path)));
+		}
 	}
 	else
 	{
-		/* truncate(2) would be easier here, but Windows hasn't got it */
-		int			fd;
-
-		fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
-		if (fd >= 0)
-		{
-			int			save_errno;
-
-			ret = ftruncate(fd, 0);
-			save_errno = errno;
-			CloseTransientFile(fd);
-			errno = save_errno;
-		}
-		else
-			ret = -1;
-		if (ret < 0 && errno != ENOENT)
-			ereport(WARNING,
-					(errcode_for_file_access(),
-					 errmsg("could not truncate file \"%s\": %m", path)));
+		/* Prevent other backends' fds from holding on to the disk space */
+		ret = do_truncate(path);
 
 		/* Register request to unlink first segment later */
 		register_unlink(rnode);
@@ -466,6 +496,17 @@ mdunlinkfork(RelFileNodeBackend rnode, ForkNumber forkNum, bool isRedo)
 		for (segno = 1;; segno++)
 		{
 			sprintf(segpath, "%s.%u", path, segno);
+
+			if (!RelFileNodeBackendIsTemp(rnode))
+			{
+				/*
+				 * Prevent other backends' fds from holding on to the disk
+				 * space.
+				 */
+				if (do_truncate(segpath) < 0 && errno == ENOENT)
+					break;
+			}
+
 			if (unlink(segpath) < 0)
 			{
 				/* ENOENT is expected after the last segment... */