scripts: time-machine: Error when attempting to visit too old commits.

* doc/guix.texi (Invoking guix time-machine): Document limitation.
* guix/inferior.scm (cached-channel-instance): New VALIDATE-CHANNELS
argument.  Use it to validate channels when there are no cache hit.
* guix/scripts/time-machine.scm
(%options): Tag the given reference with 'tag-or-commit instead of 'commit.
(%oldest-possible-commit): New variable.
(guix-time-machine) <validate-guix-channel>: New nested procedure.  Pass it to
the 'cached-channel-instance' call.
* tests/guix-time-machine.sh: New test.
* Makefile.am (SH_TESTS): Register it.

Suggested-by: Simon Tournier <zimon.toutoune@gmail.com>
Reviewed-by: Ludovic Courtès <ludo@gnu.org>
Reviewed-by: Simon Tournier <zimon.toutoune@gmail.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 633d62bd98..e09083c1f8 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5070,6 +5070,23 @@ opens the door to security vulnerabilities.  @xref{Invoking guix pull,
 @option{--allow-downgrades}}.
 @end quotation
 
+Due to @command{guix time-machine} relying on the ``inferiors''
+mechanism (@pxref{Inferiors}), the oldest commit it can travel to is
+commit @samp{6298c3ff} (``v1.0.0''), dated May 1@sup{st}, 2019, which is
+the first release that included the inferiors mechanism.  An error is
+returned when attempting to navigate to older commits.
+
+@quotation Note
+Although it should technically be possible to travel to such an old
+commit, the ease to do so will largely depend on the availability of
+binary substitutes.  When traveling to a distant past, some packages may
+not easily build from source anymore.  One such example are old versions
+of Python 2 which had time bombs in its test suite, in the form of
+expiring SSL certificates.  This particular problem can be worked around
+by setting the hardware clock to a value in the past before attempting
+the build.
+@end quotation
+
 The general syntax is:
 
 @example