weather: Report unauthorized substitute servers.

The goal is to make it easier to diagnose substitute
misconfiguration (where we’re passing a substitute URL whose
corresponding key is not authorized).

Suggested by Emmanuel Agullo.

* guix/scripts/weather.scm (check-narinfo-authorization): New procedure.
(report-server-coverage): Use it.
* doc/guix.texi (Invoking guix weather): Document it.
(Getting Substitutes from Other Servers): Add “Troubleshooting” frame.

Change-Id: I0a049c39eefb10d6a06634c8b16aa86902769791

1 files changed, 20 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 7fd94f562f..4c0f6b54e5 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -4059,6 +4059,7 @@ guix-daemon}).  It can also be disabled temporarily by passing the
 
 @node Getting Substitutes from Other Servers
 @subsection Getting Substitutes from Other Servers
+@c Note: This section name appears in a hint printed by 'guix weather'.
 
 @cindex substitute servers, adding more
 Guix can look up and fetch substitutes from several servers.  This is
@@ -4158,6 +4159,21 @@ can list as many substitute servers as you like, with the caveat that
 substitute lookup can be slowed down if too many servers need to be
 contacted.
 
+@quotation Troubleshooting
+To diagnose problems, you can run @command{guix weather}.  For example,
+running:
+
+@example
+guix weather coreutils
+@end example
+
+@noindent
+not only tells you which of the currently-configured servers has
+substitutes for the @code{coreutils} package, it also reports whether
+one of these servers is unauthorized.  @xref{Invoking guix weather}, for
+more information.
+@end quotation
+
 Note that there are also situations where one may want to add the URL of
 a substitute server @emph{without} authorizing its key.
 @xref{Substitute Authentication}, to understand this fine point.
@@ -16499,7 +16515,10 @@ up building packages by yourself (@pxref{Substitutes}).  The
 specified servers so you can have an idea of whether you'll be grumpy
 today.  It can sometimes be useful info as a user, but it is primarily
 useful to people running @command{guix publish} (@pxref{Invoking guix
-publish}).
+publish}).  Sometimes substitutes @emph{are} available but they are not
+authorized on your system; @command{guix weather} reports it so you can
+authorize them if you want (@pxref{Getting Substitutes from Other
+Servers}).
 
 @cindex statistics, for substitutes
 @cindex availability of substitutes