RE: GNU Make bug report: broken dry-run functionality with sub-make invocations

[Ambrus Sumegi]

Thanks a lot for your suggestions, Martin and Paul. I understand the reasoning 
behind why Make cannot improve this behavior, and the conditional execution of 
tee that you both proposed looks like a concise and elegant solution to my 
problem. My only remaining concern is about the man page, which currently has 
this rather vague description of the switch in question:

        -n, --just-print, --dry-run, --recon
                Print the commands that would be executed, but do not execute 
them (except in certain circumstances).

Perhaps the "(except in certain circumstances)" could be expanded to something 
like "If the line contains a call to $(MAKE), the entire line will still be 
executed, with the -n option passed to the sub-make instance. Be prepared for 
side effects of output redirection."

-----Original Message-----
From: Paul Smith <psmith@gnu.org>
Sent: 17 March 2022 20:08
To: Ambrus Sumegi <Ambrus.Sumegi@arm.com>
Cc: bug-make@gnu.org
Subject: Re: GNU Make bug report: broken dry-run functionality with sub-make 
invocations

On Thu, 2022-03-17 at 18:27 +0000, Martin Dorey wrote:
> That coped with -nj --no-print-directory on the one version of Make
> that I tested it with, but I don't know how portable that would prove.

Modern versions of make guarantee a canonical format of MAKEFLAGS such that you 
can parse them relatively easily, and with confidence.

The details are in the manual I believe but the short is:

 * All options that have "short" variants with no arguments (for
   example --dry-run -> -n) are converted to the short variants and put
   into the first word, with no leading "-"
 * All other options are added after that.
 * Short options have a single dash prefix and if they have an argument
   it's attached to the option
 * Long options have a double-dash prefix (obviously) and if they have
   an argument it's attached to the option with an "="
 * Options that have both short and long forms, prefer the short form.

So, for -n, you can use:

$(findstring n,$(word 1,$(MAKEFLAGS)))

and it will expand to "n" if dry run is enabled (regardless of which form the 
option was given in), or empty if not.

So, the OP could use something like this:

DRYRUN = $(findstring n,$(word 1,$(MAKEFLAGS)))

 main_target: create_logdirs
 $(MAKE) external_target $(if $(DRYRUN),,| tee logs/external_task.log)

There are other alternatives. For example, you could add "+" as a prefix to the 
recipe in the create_logdirs so that the directories are created even when "-n" 
is given.


But ultimately Martin's comment is correct: this is not a bug in make and 
there's no possible way that make could do anything "better" than what it does. 
At some level, especially if you're writing recursive makefile environments, 
your recipes have to be written to be resilient to the possibility that make 
was invoked with "-n".
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