munin-asyncd doesn't start on Solaris

[rorth]

Describe the bug
A clear and concise description of what the bug is.

When building/installing munin 2.0.73 on current Solaris 11.4, munin-asyncd wouldn't start.

To Reproduce
Steps to reproduce the behavior:

Start munin-asyncd as user munin.

Expected behavior
munin-asyncd should just start running.

Screenshots & Logs
Instead, it fails with

Cannot lock tied file '/var/opt/munin/spool/SPOOL-META' - Bad file number at /opt/munin/lib/perl5/Munin/Common/SyncDictFile.pm line 183.

Running it under truss (the Solaris syscall tracer), one sees

0339:  fcntl(6, F_SETLKW, 0xFFFFFFCC447A1BA8)          Err#9 EBADF
10339:          typ=F_WRLCK  whence=SEEK_SET start=0     len=0     sys=0  pid=1

According to the fcntl(2) manpage, the EBADF error happens under those condititions:

       EBADF

           The fildes argument is not a valid open file descriptor.

           The cmd argument is F_SETLK, F_SETLK64,  F_SETLKW,  or  F_SETLKW64,
           the type of lock, l_type, is a shared lock (F_RDLCK), and fildes is
           not  a  valid file descriptor open for reading; or the type of lock
           l_type is an exclusive lock (F_WRLCK) and fildes  is  not  a  valid
           file descriptor open for writing.

and indeed truss reveals that the file has been opened read-only:

10339:  openat(AT_FDCWD, "/var/opt/munin/spool/SPOOL-META", O_RDONLY|O_CLOEXEC) = 6

Desktop (please complete the following information):

• Solaris 11.4 SRU 67
• Munin 2.0.73

Additional context
I've managed to work around/fix the problem with the following patchlet to lib/perl5/Munin/Common/SyncDictFile.pm:

--- SyncDictFile.pm.dist        2024-04-03 16:31:18.000000000 +0200
+++ SyncDictFile.pm     2024-04-14 22:18:04.243103145 +0200
@@ -31,7 +31,7 @@
        
 
        use IO::File;
-       my $fh = _lock_write($self->{filename}, "r");
+       my $fh = _lock_write($self->{filename}, "w");
        my $fh_tmp = _lock_write($self->{filename} . ".tmp");
 
        # Read the whole file, writing it to $fh_tmp

It allowed munin-asyncd to start and it's collecting data for the configured plugins just fine.