Hello, This patch introduces two optional target-specific variables, 'valid-for' and 'valid-max', that allow controlling the freshness and expiration of existing targets based on their modification time.
Motivation
----------
GNU make treats an existing target with no prerequisites as
permanently up to date. In practice, many build workflows generate
cache or artifact files that should be reused while fresh but
periodically regenerated even without explicit dependencies. Today
this: requires introducing artificial phony prerequisites or external
timestamp logic.
The new variables provide a declarative mechanism for this:
valid-for = DURATION
If the target exists and its age is less than or equal to the
specified duration, make treats it as up to date and skips
dependency checking.
valid-max = DURATION
If the target exists and its age exceeds the specified duration,
make forces a rebuild even if the target has no prerequisites.
Durations are specified as either an integer number of seconds or a
sequence of <number><unit> segments without spaces, where unit is one
of 's', 'm', 'h', 'd', or 'w' (e.g. "300", "5m", "1d5h").
When both variables are present, 'valid-for' takes precedence for
fresh targets, while 'valid-max' forces rebuild of expired targets.
Implementation
--------------
The logic is implemented in update_file_1() via a small helper that
parses and evaluates the TTL variables. Invalid values are ignored
and reported under --debug=v. Successful use of the TTL is reported
under --debug=b.
This approach preserves existing make semantics while enabling
expiration of leaf targets and optional dependency skipping for
recent artifacts.
-------
Example 1: Periodic reloading of remote resources that change very
frequently.
cache.json: private valid-max=1d
cache.json:
curl -o $@ https://example/api
The file will be regenerated once per day even without prerequisites.This
can be useful when a test job is running every few minutes, but loading of
remote resources is time consuming.
Other use cases include periodically refreshing generated version
stamps, expiring downloaded resources in reproducible builds, and
limiting reuse of expensive intermediate artifacts in CI or code
generation workflows.
---
Patch attached. Comments welcome.
Best regards,
Yair Lenga
0001-Add-support-valid-for-and-valid-max.patch
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