Database with 64K filegroups takes unexpectedly long to startup by xor88

SQL Server burns 100% of a single CPU core for a *very* long time when starting up a database with 64K filegroups and files. It does almost no IO, it is just burning CPU. This is unexpected because startup should probably be IO bound.

Repro:

1. Create a database with 64K filegroups and one file each (on an SSD drive)
2. Kill SQL Server
3. Start it
4. Observe high CPU for a long time, almost no IO and the database being inaccessible due to X-locks.

Script:

--setup
CREATE DATABASE FgTest ON PRIMARY
( NAME = N'FgTest_Data', FILENAME = N'C:\FgTest.mdf', SIZE = 16MB, MAXSIZE = UNLIMITED, FILEGROWTH = 16MB)
LOG ON
( NAME = N'FgTest_Log', FILENAME = N'C:\FgTest.ldf', SIZE = 16MB, MAXSIZE = UNLIMITED, FILEGROWTH = 16MB)
GO
ALTER DATABASE FgTest SET RECOVERY SIMPLE
GO
USE FgTest
GO


--test limits
DECLARE @i INT = 0
WHILE @i < 100
BEGIN

    DECLARE @nextID INT = (SELECT MAX(file_id) FROM sys.database_files) + 2
    DECLARE @filegroupName NVARCHAR(MAX) = 'FG_' + CONVERT(NVARCHAR(MAX), @nextID)
    DECLARE @fileName NVARCHAR(MAX) = 'F_' + CONVERT(NVARCHAR(MAX), @nextID)
    DECLARE @sql NVARCHAR(MAX) = '
        ALTER DATABASE FgTest ADD FILEGROUP ' + @filegroupName + '
        ALTER DATABASE FgTest ADD FILE (NAME = N''' + @fileName + ''', FILENAME = N''C:\FgTest\FgTest_' + @fileName + '.ndf'', SIZE = 512KB) TO FILEGROUP ' + @filegroupName + '
    '
    EXEC sp_executesql @sql

    IF @i % 10 = 0
        PRINT 'Added filegroup ' + @filegroupName + ', file ' + @filegroupName

    SET @i = @i + 1

END



This is an availability problem because the server is unavailable for a really long time.