[tor-bugs] #31922 [Core Tor/Tor]: Recommend pkg-config when systemd fails

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#31922: Recommend pkg-config when systemd fails
-----------------------------+------------------------------------
 Reporter:  teor             |          Owner:  teor
     Type:  defect           |         Status:  needs_review
 Priority:  Medium           |      Milestone:  Tor: 0.4.2.x-final
Component:  Core Tor/Tor     |        Version:  Tor: 0.3.1.1-alpha
 Severity:  Normal           |     Resolution:
 Keywords:  042-should, doc  |  Actual Points:  0.3
Parent ID:  #28993           |         Points:  0.3
 Reviewer:                   |        Sponsor:
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Changes (by teor):

 * keywords:  042-should, regression, doc => 042-should, doc
 * status:  assigned => needs_review
 * version:  Tor: 0.4.0.1-alpha => Tor: 0.3.1.1-alpha
 * points:   => 0.3
 * actualpoints:   => 0.3


Comment:

 Replying to [comment:4 nickm]:
 > I suggest that we procede with caution if there is a stability risk.
 This is a regression, but it is not a regression in 0.4.2 but rather in (I
 think) some earlier version.  As such, I believe it might be better to
 target the fix for 0.4.3 if it looks the least bit tricky: autoconf and
 linker changes have a way of seeming to work for everybody until a while
 after the release is actually out.
 >
 > Of course, if the fix is simple and obvious, we can go ahead and take it
 now.

 This PR improves the configure log messages when:
 * pkg-config is not installed, or
 * any of the libraries we check using pkg-config is not present.

 It's a log-only change, it does not change the behaviour of configure:
 * 0.4.2: https://github.com/torproject/tor/pull/1390

 The root cause of this issue is that we replace the PKG_CHECK_MODULES
 error message with a variable assignment. So PKG_CHECK_MODULES doesn't
 exit on error (which is sometimes what we want).

 It also doesn't log a message describing how to install or configure pkg-
 config, or the replacement env vars that the user can set. But some users
 would like this information.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/31922#comment:7>
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