unstableGitUpdater: Allow using stable versions

It is important to keep the version as parsed by `builtins.parseDrvName`
monotonic for packages that often switch between stable and unstable versions,
for nix-env to be able to update them reliably.

Let’s optionally use the version format described by RFC 107:
https://github.com/NixOS/rfcs/pull/107
The downside is that it requires fetching
more commits to be able to determine the latest tag.

pkgs/common-updater/unstable-updater.nix

@@ -10,6 +10,8 @@
 # commit.
 { url ? null # The git url, if empty it will be set to src.url
 , branch ? null
+, stableVersion ? false # Use version format according to RFC 107 (i.e. LAST_TAG+date=YYYY-MM-DD)
+, tagPrefix ? "" # strip this prefix from a tag name when using stable version
 }:
 
 let
@@ -18,6 +20,8 @@ let
 
     url=""
     branch=""
+    use_stable_version=""
+    tag_prefix=""
 
     while (( $# > 0 )); do
         flag="$1"
@@ -29,6 +33,12 @@ let
           --branch=*)
             branch="''${flag#*=}"
             ;;
+          --use-stable-version)
+            use_stable_version=1
+            ;;
+          --tag-prefix=*)
+            tag_prefix="''${flag#*=}"
+            ;;
           *)
             echo "$0: unknown option ‘''${flag}’"
             exit 1
@@ -60,13 +70,34 @@ let
     pushd "$tmpdir"
     commit_date="$(${git}/bin/git show -s --pretty='format:%cs')"
     commit_sha="$(${git}/bin/git show -s --pretty='format:%H')"
+    if [[ -z "$use_stable_version" ]]; then
+        new_version="unstable-$commit_date"
+    else
+        depth=100
+        while (( $depth < 10000 )); do
+            last_tag="$(${git}/bin/git describe --tags --abbrev=0 2> /dev/null || true)"
+            if [[ -n "$last_tag" ]]; then
+                break
+            fi
+            ${git}/bin/git fetch --depth="$depth" --tags
+            depth=$(( $depth * 2 ))
+        done
+        if [[ -z "$last_tag" ]]; then
+            echo "Cound not found a tag within last 10000 commits" > /dev/stderr
+            exit 1
+        fi
+        if [[ -n "$tag_prefix" ]]; then
+          last_tag="''${last_tag#$tag_prefix}"
+        fi
+        new_version="$last_tag+date=$commit_date"
+    fi
     popd
-    ${coreutils}/bin/rm -rf "$tmpdir"
+    # ${coreutils}/bin/rm -rf "$tmpdir"
 
     # update the nix expression
     ${common-updater-scripts}/bin/update-source-version \
         "$UPDATE_NIX_ATTR_PATH" \
-        "unstable-$commit_date" \
+        "$new_version" \
         --rev="$commit_sha"
   '';
 
@@ -75,5 +106,7 @@ in [
   "--url=${builtins.toString url}"
 ] ++ lib.optionals (branch != null) [
   "--branch=${branch}"
+] ++ lib.optionals stableVersion [
+  "--use-stable-version"
+  "--tag-prefix=${tagPrefix}"
 ]
-