Clean up assorted messiness around AllocateDir() usage.

This patch fixes a couple of low-probability bugs that could lead to
reporting an irrelevant errno value (and hence possibly a wrong SQLSTATE)
concerning directory-open or file-open failures.  It also fixes places
where we took shortcuts in reporting such errors, either by using elog
instead of ereport or by using ereport but forgetting to specify an
errcode.  And it eliminates a lot of just plain redundant error-handling
code.

In service of all this, export fd.c's formerly-static function
ReadDirExtended, so that external callers can make use of the coding
pattern

	dir = AllocateDir(path);
	while ((de = ReadDirExtended(dir, path, LOG)) != NULL)

if they'd like to treat directory-open failures as mere LOG conditions
rather than errors.  Also fix FreeDir to be a no-op if we reach it
with dir == NULL, as such a coding pattern would cause.

Then, remove code at many call sites that was throwing an error or log
message for AllocateDir failure, as ReadDir or ReadDirExtended can handle
that job just fine.  Aside from being a net code savings, this gets rid of
a lot of not-quite-up-to-snuff reports, as mentioned above.  (In some
places these changes result in replacing a custom error message such as
"could not open tablespace directory" with more generic wording "could not
open directory", but it was agreed that the custom wording buys little as
long as we report the directory name.)  In some other call sites where we
can't just remove code, change the error reports to be fully
project-style-compliant.

Also reorder code in restoreTwoPhaseData that was acquiring a lock
between AllocateDir and ReadDir; in the unlikely but surely not
impossible case that LWLockAcquire changes errno, AllocateDir failures
would be misreported.  There is no great value in opening the directory
before acquiring TwoPhaseStateLock, so just do it in the other order.

Also fix CheckXLogRemoved to guarantee that it preserves errno,
as quite a number of call sites are implicitly assuming.  (Again,
it's unlikely but I think not impossible that errno could change
during a SpinLockAcquire.  If so, this function was broken for its
own purposes as well as breaking callers.)

And change a few places that were using not-per-project-style messages,
such as "could not read directory" when "could not open directory" is
more correct.

Back-patch the exporting of ReadDirExtended, in case we have occasion
to back-patch some fix that makes use of it; it's not needed right now
but surely making it global is pretty harmless.  Also back-patch the
restoreTwoPhaseData and CheckXLogRemoved fixes.  The rest of this is
essentially cosmetic and need not get back-patched.

Michael Paquier, with a bit of additional work by me

Discussion: https://postgr.es/m/CAB7nPqRpOCxjiirHmebEFhXVTK7V5Jvw4bz82p7Oimtsm3TyZA@mail.gmail.com

1 files changed, 12 insertions, 19 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index fba201f6599..a11406c741c 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3764,10 +3764,16 @@ PreallocXlogFiles(XLogRecPtr endptr)
  * existed while the server has been running, as this function always
  * succeeds if no WAL segments have been removed since startup.
  * 'tli' is only used in the error message.
+ *
+ * Note: this function guarantees to keep errno unchanged on return.
+ * This supports callers that use this to possibly deliver a better
+ * error message about a missing file, while still being able to throw
+ * a normal file-access error afterwards, if this does return.
  */
 void
 CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
 {
+	int			save_errno = errno;
 	XLogSegNo	lastRemovedSegNo;
 
 	SpinLockAcquire(&XLogCtl->info_lck);
@@ -3779,11 +3785,13 @@ CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
 		char		filename[MAXFNAMELEN];
 
 		XLogFileName(filename, tli, segno, wal_segment_size);
+		errno = save_errno;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("requested WAL segment %s has already been removed",
 						filename)));
 	}
+	errno = save_errno;
 }
 
 /*
@@ -3837,13 +3845,6 @@ RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr PriorRedoPtr, XLogRecPtr endptr)
 	struct dirent *xlde;
 	char		lastoff[MAXFNAMELEN];
 
-	xldir = AllocateDir(XLOGDIR);
-	if (xldir == NULL)
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not open write-ahead log directory \"%s\": %m",
-						XLOGDIR)));
-
 	/*
 	 * Construct a filename of the last segment to be kept. The timeline ID
 	 * doesn't matter, we ignore that in the comparison. (During recovery,
@@ -3854,6 +3855,8 @@ RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr PriorRedoPtr, XLogRecPtr endptr)
 	elog(DEBUG2, "attempting to remove WAL segments older than log file %s",
 		 lastoff);
 
+	xldir = AllocateDir(XLOGDIR);
+
 	while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
 	{
 		/* Ignore files that are not XLOG segments */
@@ -3912,13 +3915,6 @@ RemoveNonParentXlogFiles(XLogRecPtr switchpoint, TimeLineID newTLI)
 
 	XLByteToPrevSeg(switchpoint, endLogSegNo, wal_segment_size);
 
-	xldir = AllocateDir(XLOGDIR);
-	if (xldir == NULL)
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not open write-ahead log directory \"%s\": %m",
-						XLOGDIR)));
-
 	/*
 	 * Construct a filename of the last segment to be kept.
 	 */
@@ -3927,6 +3923,8 @@ RemoveNonParentXlogFiles(XLogRecPtr switchpoint, TimeLineID newTLI)
 	elog(DEBUG2, "attempting to remove WAL segments newer than log file %s",
 		 switchseg);
 
+	xldir = AllocateDir(XLOGDIR);
+
 	while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
 	{
 		/* Ignore files that are not XLOG segments */
@@ -4108,11 +4106,6 @@ CleanupBackupHistory(void)
 	char		path[MAXPGPATH + sizeof(XLOGDIR)];
 
 	xldir = AllocateDir(XLOGDIR);
-	if (xldir == NULL)
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not open write-ahead log directory \"%s\": %m",
-						XLOGDIR)));
 
 	while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
 	{