From: Brian Candler <b.candler@...>
Date: 2009-10-27T18:27:10+09:00
Subject: Re: file open failure test

Derek Smith wrote:
> unless File.exists?( "#{RUNLOG}" )
>     FileUtils.touch( "#{RUNLOG}" ) \
>     || warn("Touch file failed for /var/log/prod_db_bkup.log!")
> end

1. Why do you want to touch the file? You're going to write to it later 
anyway. Opening the file in mode "a" will create it if necessary.

2. If you do touch the file, why would you want to continue if it fails? 
If it does fail, you're almost bound to fail when you write to it 
anyway.

> def log_mtd(arg)
>     File.open( "#{RUNLOG}", "a" ) do |file|
>         file.puts(arg)
>     end
> end

Looks OK - as pointed out, File.open(RUNLOG,"a") is sufficient.

> def db_bkup(arg)
>     Dir.chdir( "#{DB_DIR}" ) \
>     || die_mtd("Change_Dir_Failed_To_#{DB_DIR}_Dump_Will_Fail!")

Dir.chdir will raise an exception if it fails, not return false/nil.

So you could do something like this:

  def db_bkup(arg)
    begin
      Dir.chdir(DBDIR)
    rescue
      die_mtd "Change_Dir_Failed_To_#{DB_DIR}_Dump_Will_Fail!"
    end

If you end up doing this a lot, you can write a helper function:

  def failex(msg)
    begin
      yield
    rescue
      die_mtd msg
    end
  end

  ...

  failex "Change_Dir_Failed_To_#{DB_DIR}_Dump_Will_Fail!" do
    Dir.chdir(DBDIR)
  end

(Note: 'rescue' by itself captures StandardError and subclasses. If you 
want to catch absolutely all exceptions, use 'rescue Exception'. However 
this will also catch things like programming errors)

But to be honest, this is not regular Ruby practice. You'd normally just 
write your script without error handling, and let the program fail. It 
will usually give a fairly obvious exception message itself. This is 
easy to try out in irb:

>> Dir.chdir("/wibble")
Errno::ENOENT: No such file or directory - /wibble
  from (irb):3:in `chdir'
  from (irb):3

So if your program gave that message, it would be fairly clear where the 
problem lies.

>         %x(echo ".dump"|sqlite3 development.sqlite3 |gzip -c > 
> "#{DB_BKUP.+(arg)}") \
>         || die_mtd("SQL_Dump_Cmd_Failed.+(d.to_s).+(t.hour.to_s)")

%x{...} forks an new process to run the command, and will not raise an 
exception if that program 'fails'. However you can check $? which is the 
status of the most recently terminated child.

>> %x{flurble}
(irb):4: command not found: flurble
=> ""
>> $?
=> #<Process::Status: pid=7755,exited(127)>
>> $?.methods - Object.methods
=> ["success?", "exitstatus", "&", ">>", "stopped?", "stopsig", "to_i", 
"coredump?", "to_int", "signaled?", "termsig", "pid", "exited?"]
>> $?.success?
=> false

So you would write your code as:

    res = %x{...whatever...}
    unless $?.success?
      die_mtd "SQL_Dump_Cmd_Failed with status #{$?.exitstatus}"
    end

Note that the stdout from the command is captured in 'res'. If you are 
not going to make use of this, it's probably better to do

    system "...whatever..."

which will allow the child's stdout to go to your ruby script's stdout.

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