bug#29255: "Profile contains conflicting entries" could be more helpful

[Ludovic Courtès]

Hi,

Ricardo Wurmus <address@hidden> skribis:

> In this case it is not entirely clear that the existing python-requests
> package in the profile is “old”.  The version looks the same and the
> hash is opaque.
>
> Would it be possible to record something about the Guix version that was
> used to install a package?  Then we could say:
>
>   An older variant of python-requests is installed in this profile
>   (propagated from package “foo-bar”) and conflicts with a newer variant
>   (propagated from package “python-twine”).

When the version numbers are the same, we cannot tell whether a variant
is “older”, we can just tell that it’s different.  Also, I find it
useful to see the propagation stack as is currently the case.

With the patch below, I get:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix package -p foo -i address@hidden python
The following packages will be installed:
   python       2.7.13  
/gnu/store/vysfxizaddh1q8s5qjgbdkzxx0585dzi-python-2.7.13
   python       3.5.3   /gnu/store/m4rdgmvdqcxs2zhv42idnz1s1w391i8j-python-3.5.3

guix package: error: profile contains conflicting entries for python:out
guix package: error:   first entry: address@hidden 
/gnu/store/vysfxizaddh1q8s5qjgbdkzxx0585dzi-python-2.7.13
guix package: error:   second entry: address@hidden 
/gnu/store/m4rdgmvdqcxs2zhv42idnz1s1w391i8j-python-3.5.3
hint: You cannot have two different versions or variants of `python' in the 
same profile.
--8<---------------cut here---------------end--------------->8---

and:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix package -i guile-cairo -p foo --no-grafts
The following package will be installed:
   guile-cairo  1.4.1   
/gnu/store/dsdbp9sqla6zz2skljlcr5zfjyzvargf-guile-cairo-1.4.1

guix package: error: profile contains conflicting entries for cairo:out
guix package: error:   first entry: address@hidden 
/gnu/store/c4vl4hw5jccg0b23sfvs0kdnfdbxdlgm-cairo-1.14.10
guix package: error:    ... propagated from address@hidden
guix package: error:   second entry: address@hidden 
/gnu/store/nwxv9s2q8pi0m6gn6fyidpj8442dwp6f-cairo-1.14.10
guix package: error:    ... propagated from address@hidden
hint: Try upgrading both `guile-cairo' and `cairomm', or remove one of them 
from the profile.
--8<---------------cut here---------------end--------------->8---

How does that sound?

We could further refine the hint to suggest using ‘guix package -m’,
though I’m not sure if it’d be a useful hint (it’s a useful
recommendation, but not necessarily good as a “fix hint.”)

Thoughts?

Thanks,
Ludo’.

diff --git a/guix/ui.scm b/guix/ui.scm
index 13cbe3a0f..660f6ea5c 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -502,6 +502,25 @@ interpreted."
           (x
            (leave (G_ "unknown unit: ~a~%") unit)))))))
 
+(define (display-collision-resolution-hint collision)
+  (define (top-most-entry entry)
+    (let loop ((entry entry))
+      (match (force (manifest-entry-parent entry))
+        (#f entry)
+        (parent (loop parent)))))
+
+  (let* ((first  (profile-collision-error-entry collision))
+         (second (profile-collision-error-conflict collision))
+         (name1  (manifest-entry-name (top-most-entry first)))
+         (name2  (manifest-entry-name (top-most-entry second))))
+    (if (string=? name1 name2)
+        (display-hint (format #f (G_ "You cannot have two different versions
+or variants of @code{~a} in the same profile.")
+                              name1))
+        (display-hint (format #f (G_ "Try upgrading both @code{~a} and 
@code{~a},
+or remove one of them from the profile.")
+                              name1 name2)))))
+
 (define (call-with-error-handling thunk)
   "Call THUNK within a user-friendly error handler."
   (define (port-filename* port)
@@ -570,6 +589,7 @@ interpreted."
                              (manifest-entry-output* conflict)
                              (manifest-entry-item conflict))
                (report-parent-entries conflict)
+               (display-collision-resolution-hint c)
                (exit 1)))
             ((nar-error? c)
              (let ((file (nar-error-file c))