Re: Makefile idiom for referencing URL sources

[Johnson, Richard]

On Dec 21, 2021, at 3:54 AM, Mark Piffer 
<mark.piffer@gmail.com<mailto:mark.piffer@gmail.com>> wrote:

As no one else asks this: how does make check the timestamps of the remote 
resource? What I am seeing  here (and I may be wrong) is checking against a 
local file timestamp, like always. To my understanding it is not possible for 
vanilla make to check anything else but stat-able files - how did you achieve 
this?

Creation of local files derived from remote sources was tested.  Alas, my 
project is not mature enough to test updating against a volatile remote sources.

The theory is that a stat-able goal's timestamp is conditionally updated by the 
recipe.   The result is that the goal's stat-time can be significantly older 
than the recipe's execution time.
So we have to ask whether further processing is based upon the recipe execution 
time or the goal's actual stat-time?  I've always assumed the actual stat-time 
would be used when available.  But I could be wrong on that detail.

If the goal's stat-time is used, checking the tracking files can be be forced 
with the updated rules below.
If recipe execution time is used, two passes are required.  One to update the 
local timestamps and one to perform the build.

REMOTE:

.http/%: REMOTE
        #...establish timestamp of the source
mkdir -p $$(dirname $@)
tstamp="$$(curl -sI $(subst .http,http:,$@) 2>/dev/null | sed -n 
's/Last-Modified: //p')" && \
touch -d "$${tstamp}" $@