Libguestfs - Oct 2015 - [PATCH v2] rpm: Choose providers better (RHBZ#1266918).

This is v2 of the 4/4 patch from the original series.

Changes:

 - memoize the function this time

 - check packages are installed using rpm_package_of_string

However I didn't combine the two case together, because the code is a
bit simpler with them separate.

Rich.

In the referenced bug, a customer had installed a web browser called
'palemoon'.  The RPM of this web browser provides and requires various
core libraries, such as:

  Provides: libnss3.so()(64bit) # normally provided by 'nss'
  Requires: libxul.so()(64bit)  # normally provided by 'firefox'

Our previous algorithm -- inherited from the days when we used to run
'rpm' commands -- takes every provider of a particular requirement and
adds it to a big list, so if any other package requires
'libnss3.so()(64bit)', then both 'nss' and 'palemoon'
are added to the
package list.

Yum used to handle this differently - it used to only pick the package
with the shortest name.  Later on, before yum was retired, it had a
more complex decision algorithm described here:
http://yum.baseurl.org/wiki/CompareProviders

This change makes supermin use the shortest name algorithm, so in the
case above, it always picks 'nss' over 'palemoon'.

There is a second possible problem which is not fixed by the current
patch set: If a package both provides and requires the same
dependency, we should ignore that dependency.  For example, 'palemoon'
both Provides and Requires 'libxul.so()(64bit)', so if
'palemoon' is
pulled in, then 'firefox' would not be an additional requirement.
Because we store all the packages in a big list, we lose track of
where a dependency originates from, so it is not easy to implement
this second change.
---
 src/rpm.ml | 64 ++++++++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 48 insertions(+), 16 deletions(-)

diff --git a/src/rpm.ml b/src/rpm.ml
index 4d31472..1db47b6 100644
--- a/src/rpm.ml
+++ b/src/rpm.ml
@@ -210,8 +210,51 @@ let rpm_package_name pkg  let
rpm_get_package_database_mtime ()    (lstat
"/var/lib/rpm/Packages").st_mtime
 
-(* Memo of resolved provides. *)
-let rpm_providers = Hashtbl.create 13
+(* Return the best provider of a particular RPM requirement.
+ *
+ * There may be multiple, or no providers.  In case there are multiple,
+ * choose the one with the shortest name (as yum used to).
+ *
+ * We could do better here: http://yum.baseurl.org/wiki/CompareProviders
+ *)
+let provider +  (* Memo of resolved provides. *)
+  let rpm_providers = Hashtbl.create 13 in
+  fun req ->
+    try Hashtbl.find rpm_providers req
+    with Not_found ->
+      let ret +        try
+          (* 'providers' here is an array of just package names. *)
+          let providers = rpm_pkg_whatprovides (get_rpm ()) req in
+          let providers = Array.to_list providers in
+          (* --whatprovides will return duplicate identical packages, so: *)
+          let providers = sort_uniq providers in
+          (* Only packages which are actually installed: *)
+          let providers +            List.filter (fun name ->
rpm_package_of_string name <> None)
+                        providers in
+
+          match providers with
+          | [] -> None
+          | [name] ->
+             Some name
+          | names ->
+             if !settings.debug >= 2 then
+               printf "supermin: rpm: multiple providers: requirement %s:
providers: %s\n"
+                      req (String.concat " " names);
+             let cmp name1 name2 +               let len1 = String.length name1
and len2 = String.length name2 in
+               if len1 <> len2 then compare len1 len2
+               else compare name1 name2 in
+             let names = List.sort cmp names in
+             let best_name = List.hd names in
+             if !settings.debug >= 2 then
+               printf "supermin: rpm: multiple providers: picked
%s\n" best_name;
+             Some best_name
+        with Not_found -> None in
+      Hashtbl.add rpm_providers req ret;
+      ret
 
 let rpm_get_all_requires pkgs    let get pkg @@ -226,20 +269,9 @@ let
rpm_get_all_requires pkgs                rpm_pkg_requires (get_rpm ())
(rpm_package_to_string pkg) in
     let pkgs' = Array.fold_left (
       fun set x ->
-        try
-          let provides -            try Hashtbl.find rpm_providers x
-            with Not_found ->
-              let p = rpm_pkg_whatprovides (get_rpm ()) x in
-              Hashtbl.add rpm_providers x p;
-              p in
-          Array.fold_left (
-            fun newset p ->
-              match rpm_package_of_string p with
-                | None -> newset
-                | Some x -> StringSet.add p newset
-          ) set provides
-        with Not_found -> set
+        match provider x with
+        | None -> set
+        | Some p -> StringSet.add p set
     ) StringSet.empty reqs in
     pkgs'
   in
-- 
2.5.0