Fix issues in pgarch's new directory-scanning logic.

The arch_filenames[] array elements were one byte too small, so that
a maximum-length filename would get corrupted if another entry
were made after it.  (Noted by Thomas Munro, fix by Nathan Bossart.)

Move these arrays into a palloc'd struct, so that we aren't wasting
a few kilobytes of static data in each non-archiver process.

Add a binaryheap_reset() call to make it plain that we start the
directory scan with an empty heap.  I don't think there's any live
bug of that sort, but it seems fragile, and this is very cheap
insurance.

Cleanup for commit beb4e9ba1, so no back-patch needed.

Discussion: https://postgr.es/m/CA+hUKGLHAjHuKuwtzsW7uMJF4BVPcQRL-UMZG_HM-g0y7yLkUg@mail.gmail.com

src/backend/postmaster/pgarch.c

@@ -111,11 +111,20 @@ static PgArchData *PgArch = NULL;
  * completes, the file names are stored in ascending order of priority in
  * arch_files.  pgarch_readyXlog() returns files from arch_files until it
  * is empty, at which point another directory scan must be performed.
+ *
+ * We only need this data in the archiver process, so make it a palloc'd
+ * struct rather than a bunch of static arrays.
  */
-static binaryheap *arch_heap = NULL;
-static char arch_filenames[NUM_FILES_PER_DIRECTORY_SCAN][MAX_XFN_CHARS];
-static char *arch_files[NUM_FILES_PER_DIRECTORY_SCAN];
-static int arch_files_size = 0;
+struct arch_files_state
+{
+	binaryheap *arch_heap;
+	int			arch_files_size;	/* number of live entries in arch_files[] */
+	char	   *arch_files[NUM_FILES_PER_DIRECTORY_SCAN];
+	/* buffers underlying heap, and later arch_files[], entries: */
+	char		arch_filenames[NUM_FILES_PER_DIRECTORY_SCAN][MAX_XFN_CHARS + 1];
+};
+
+static struct arch_files_state *arch_files = NULL;
 
 /*
  * Flags set by interrupt handlers for later service in the main loop.
@@ -231,9 +240,13 @@ PgArchiverMain(void)
 	 */
 	PgArch->pgprocno = MyProc->pgprocno;
 
+	/* Create workspace for pgarch_readyXlog() */
+	arch_files = palloc(sizeof(struct arch_files_state));
+	arch_files->arch_files_size = 0;
+
 	/* Initialize our max-heap for prioritizing files to archive. */
-	arch_heap = binaryheap_allocate(NUM_FILES_PER_DIRECTORY_SCAN,
-									ready_file_comparator, NULL);
+	arch_files->arch_heap = binaryheap_allocate(NUM_FILES_PER_DIRECTORY_SCAN,
+												ready_file_comparator, NULL);
 
 	pgarch_MainLoop();
 
@@ -363,7 +376,7 @@ pgarch_ArchiverCopyLoop(void)
 	char		xlog[MAX_XFN_CHARS + 1];
 
 	/* force directory scan in the first call to pgarch_readyXlog() */
-	arch_files_size = 0;
+	arch_files->arch_files_size = 0;
 
 	/*
 	 * loop through all xlogs with archive_status of .ready and archive
@@ -658,7 +671,7 @@ pgarch_readyXlog(char *xlog)
 	SpinLockRelease(&PgArch->arch_lck);
 
 	if (force_dir_scan)
-		arch_files_size = 0;
+		arch_files->arch_files_size = 0;
 
 	/*
 	 * If we still have stored file names from the previous directory scan,
@@ -666,14 +679,14 @@ pgarch_readyXlog(char *xlog)
 	 * is still present, as the archive_command for a previous file may
 	 * have already marked it done.
 	 */
-	while (arch_files_size > 0)
+	while (arch_files->arch_files_size > 0)
 	{
 		struct stat	st;
 		char		status_file[MAXPGPATH];
 		char	   *arch_file;
 
-		arch_files_size--;
-		arch_file = arch_files[arch_files_size];
+		arch_files->arch_files_size--;
+		arch_file = arch_files->arch_files[arch_files->arch_files_size];
 		StatusFilePath(status_file, arch_file, ".ready");
 
 		if (stat(status_file, &st) == 0)
@@ -687,6 +700,9 @@ pgarch_readyXlog(char *xlog)
 					 errmsg("could not stat file \"%s\": %m", status_file)));
 	}
 
+	/* arch_heap is probably empty, but let's make sure */
+	binaryheap_reset(arch_files->arch_heap);
+
 	/*
 	 * Open the archive status directory and read through the list of files
 	 * with the .ready suffix, looking for the earliest files.
@@ -720,53 +736,53 @@ pgarch_readyXlog(char *xlog)
 		/*
 		 * Store the file in our max-heap if it has a high enough priority.
 		 */
-		if (arch_heap->bh_size < NUM_FILES_PER_DIRECTORY_SCAN)
+		if (arch_files->arch_heap->bh_size < NUM_FILES_PER_DIRECTORY_SCAN)
 		{
 			/* If the heap isn't full yet, quickly add it. */
-			arch_file = arch_filenames[arch_heap->bh_size];
+			arch_file = arch_files->arch_filenames[arch_files->arch_heap->bh_size];
 			strcpy(arch_file, basename);
-			binaryheap_add_unordered(arch_heap, CStringGetDatum(arch_file));
+			binaryheap_add_unordered(arch_files->arch_heap, CStringGetDatum(arch_file));
 
 			/* If we just filled the heap, make it a valid one. */
-			if (arch_heap->bh_size == NUM_FILES_PER_DIRECTORY_SCAN)
-				binaryheap_build(arch_heap);
+			if (arch_files->arch_heap->bh_size == NUM_FILES_PER_DIRECTORY_SCAN)
+				binaryheap_build(arch_files->arch_heap);
 		}
-		else if (ready_file_comparator(binaryheap_first(arch_heap),
+		else if (ready_file_comparator(binaryheap_first(arch_files->arch_heap),
 									   CStringGetDatum(basename), NULL) > 0)
 		{
 			/*
 			 * Remove the lowest priority file and add the current one to
 			 * the heap.
 			 */
-			arch_file = DatumGetCString(binaryheap_remove_first(arch_heap));
+			arch_file = DatumGetCString(binaryheap_remove_first(arch_files->arch_heap));
 			strcpy(arch_file, basename);
-			binaryheap_add(arch_heap, CStringGetDatum(arch_file));
+			binaryheap_add(arch_files->arch_heap, CStringGetDatum(arch_file));
 		}
 	}
 	FreeDir(rldir);
 
 	/* If no files were found, simply return. */
-	if (arch_heap->bh_size == 0)
+	if (arch_files->arch_heap->bh_size == 0)
 		return false;
 
 	/*
 	 * If we didn't fill the heap, we didn't make it a valid one.  Do that
 	 * now.
 	 */
-	if (arch_heap->bh_size < NUM_FILES_PER_DIRECTORY_SCAN)
-		binaryheap_build(arch_heap);
+	if (arch_files->arch_heap->bh_size < NUM_FILES_PER_DIRECTORY_SCAN)
+		binaryheap_build(arch_files->arch_heap);
 
 	/*
 	 * Fill arch_files array with the files to archive in ascending order
 	 * of priority.
 	 */
-	arch_files_size = arch_heap->bh_size;
-	for (int i = 0; i < arch_files_size; i++)
-		arch_files[i] = DatumGetCString(binaryheap_remove_first(arch_heap));
+	arch_files->arch_files_size = arch_files->arch_heap->bh_size;
+	for (int i = 0; i < arch_files->arch_files_size; i++)
+		arch_files->arch_files[i] = DatumGetCString(binaryheap_remove_first(arch_files->arch_heap));
 
 	/* Return the highest priority file. */
-	arch_files_size--;
-	strcpy(xlog, arch_files[arch_files_size]);
+	arch_files->arch_files_size--;
+	strcpy(xlog, arch_files->arch_files[arch_files->arch_files_size]);
 
 	return true;
 }