Add defenses against pre-crash files to BufFileOpenShared().

Crash restarts currently don't clean up temporary files, as a debugging aid.
If a left-over file happens to have the same name as a segment file we're
trying to create, we'll just truncate and reuse it, but there is a problem:
BufFileOpenShared() determines how many segment files exist by trying to open
.0, .1, .2, ... until it finds no more files.  It might be confused by a junk
file that has the next segment number.  To defend against that, make sure we
always create a gap after the end file by unlinking the following name if it
exists.  Also make it an error to try to open a BufFile that doesn't exist
(has no segment 0), so as not to encourage the development of client code
that depends on an interface that we can't reliably provide.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm%3D2jhCbC_GFQJaaDhWxLB4EXtT3vVd5czuRNaqF5CWSTog%40mail.gmail.com

diff --git a/src/backend/storage/file/buffile.c b/src/backend/storage/file/buffile.c
index fa9940da9b3a6b71eca475b9b90b575a56c5d5ed..c6b210d1379d3544efc31f275d1c7d5b1803393d 100644 (file)
--- a/src/backend/storage/file/buffile.c
+++ b/src/backend/storage/file/buffile.c
@@ -211,6 +211,16 @@ MakeNewSharedSegment(BufFile *buffile, int segment)
    char        name[MAXPGPATH];
    File        file;
 
+   /*
+    * It is possible that there are files left over from before a crash
+    * restart with the same name.  In order for BufFileOpenShared()
+    * not to get confused about how many segments there are, we'll unlink
+    * the next segment number if it already exists.
+    */
+   SharedSegmentName(name, buffile->name, segment + 1);
+   SharedFileSetDelete(buffile->fileset, name, true);
+
+   /* Create the new segment. */
    SharedSegmentName(name, buffile->name, segment);
    file = SharedFileSetCreate(buffile->fileset, name);
 
@@ -303,7 +313,9 @@ BufFileOpenShared(SharedFileSet *fileset, const char *name)
     * name.
     */
    if (nfiles == 0)
-       return NULL;
+       ereport(ERROR,
+               (errcode_for_file_access(),
+                errmsg("could not open BufFile \"%s\"", name)));
 
    file->numFiles = nfiles;
    file->files = files;