Merge pull request #5362 from nbp/update-channels

Add a script to add git branches for each channel.

To create / update references to remote / local channels, you have to run `./maintainers/scripts/update-channel-branches.sh` while you are at the top-level of nixpkgs work directory.  To make this convenient for Nixpkgs / NixOS developer, one can run the following command:

```
$ git config --add alias.fetch-channels '!sh -c "$(git rev-parse --show-cdup)maintainers/scripts/update-channel-branches.sh"'
```

Which will register the alias fetch-channels such that the script can used from sub-directory of nixpkgs by running `git fetch-channels`.

maintainers/scripts/update-channel-branches.sh

@@ -0,0 +1,111 @@
+#!/bin/sh
+
+: ${NIXOS_CHANNELS:=https://nixos.org/channels/}
+: ${CHANNELS_NAMESPACE:=refs/heads/channels/}
+
+# List all channels which are currently in the repository which we would
+# have to remove if they are not found again.
+deadChannels=$(git for-each-ref --format="%(refname)" $CHANNELS_NAMESPACE)
+
+function updateRef() {
+    local channelName=$1
+    local newRev=$2
+
+    # if the inputs are not valid, then we do not update any branch.
+    test -z "$newRev" -o -z "$channelName" && return;
+
+    # Update the local refs/heads/channels/* branches to be in-sync with the
+    # channel references.
+    local branch=$CHANNELS_NAMESPACE$channelName
+    oldRev=$(git rev-parse --short $branch 2>/dev/null || true)
+    if test "$oldRev" != "$newRev"; then
+        if git update-ref $branch $newRev 2>/dev/null; then
+            if test -z "$oldRev"; then
+                echo " * [new branch]      $newRev           -> ${branch#refs/heads/}"
+            else
+                echo "                     $oldRev..$newRev  -> ${branch#refs/heads/}"
+            fi
+        else
+            if test -z "$oldRev"; then
+                echo " * [missing rev]     $newRev           -> ${branch#refs/heads/}"
+            else
+                echo "   [missing rev]     $oldRev..$newRev  -> ${branch#refs/heads/}"
+            fi
+        fi
+    fi
+
+    # Filter out the current channel from the list of dead channels.
+    deadChannels=$(grep -v $CHANNELS_NAMESPACE$channelName <<EOF
+$deadChannels
+EOF
+)
+}
+
+# Find the name of all channels which are listed in the directory.
+echo "Fetching channels from $NIXOS_CHANNELS:"
+for channelName in : $(curl -s $NIXOS_CHANNELS | sed -n '/folder/ { s,.*href=",,; s,/".*,,; p }'); do
+    test "$channelName" = : && continue;
+
+    # Do not follow redirections, such that we can extract the
+    # short-changeset from the name of the directory where we are
+    # redirected to.
+    sha1=$(curl -sI $NIXOS_CHANNELS$channelName | sed -n '/Location/ { s,.*\.\([a-f0-9]*\)[ \r]*$,\1,; p; }')
+
+    updateRef "remotes/$channelName" "$sha1"
+done
+
+echo "Fetching channels from nixos-version:"
+if currentSystem=$(nixos-version 2>/dev/null); then
+    # If the system is entirely build from a custom nixpkgs version,
+    # then the version is not annotated in git version. This sed
+    # expression is basically matching that the expressions end with
+    # ".<sha1> (Name)" to extract the sha1.
+    sha1=$(echo $currentSystem | sed -n 's,^.*\.\([a-f0-9]*\) *(.*)$,\1,; T skip; p; :skip;')
+
+    updateRef current-system "$sha1"
+fi
+
+echo "Fetching channels from ~/.nix-defexpr:"
+for revFile in : $(find -L ~/.nix-defexpr/ -maxdepth 4 -name svn-revision); do
+    test "$revFile" = : && continue;
+
+    # Deconstruct a path such as, into:
+    #
+    #   /home/luke/.nix-defexpr/channels_root/nixos/nixpkgs/svn-revision
+    #     channelName = root/nixos
+    #
+    #   /home/luke/.nix-defexpr/channels/nixpkgs/svn-revision
+    #     channelName = nixpkgs
+    #
+    user=${revFile#*.nix-defexpr/channels}
+    repo=${user#*/}
+    repo=${repo%%/*}
+    user=${user%%/*}
+    user=${user#_}
+    test -z "$user" && user=$USER
+    channelName="$user${user:+/}$repo"
+
+    sha1=$(cat $revFile | sed -n 's,^.*\.\([a-f0-9]*\)$,\1,; T skip; p; :skip;')
+
+    updateRef "$channelName" "$sha1"
+done
+
+# Suggest to remove channel branches which are no longer found by this
+# script. This is to handle the cases where a local/remote channel
+# disappear. We should not attempt to remove manually any branches, as they
+# might be user branches.
+if test -n "$deadChannels"; then
+
+    echo "
+Some old channel branches are still in your repository, if you
+want to remove them, run the following command(s):
+"
+
+    while read branch; do
+        echo "    git update-ref -d $branch"
+    done <<EOF
+$deadChannels
+EOF
+
+    echo
+fi

nixos/doc/manual/development/sources.xml

@@ -40,20 +40,22 @@ rebuild everything from source. So you may want to create a local
 branch based on your current NixOS version:
 
 <screen>
-$ nixos-version
-14.04.273.ea1952b (Baboon)
-
-$ git checkout -b local ea1952b
+$ <replaceable>/my/sources</replaceable>/nixpkgs/maintainers/scripts/update-channel-branches.sh
+Fetching channels from https://nixos.org/channels:
+ * [new branch]      cbe467e           -> channels/remotes/nixos-unstable
+Fetching channels from nixos-version:
+ * [new branch]      9ff4738           -> channels/current-system
+Fetching channels from ~/.nix-defexpr:
+ * [new branch]      0d4acad           -> channels/root/nixos
+$ git checkout -b local channels/current-system
 </screen>
 
 Or, to base your local branch on the latest version available in the
 NixOS channel:
 
 <screen>
-$ curl -sI https://nixos.org/channels/nixos-unstable/ | grep Location
-Location: https://releases.nixos.org/nixos/unstable/nixos-14.10pre43986.acaf4a6/
-
-$ git checkout -b local acaf4a6
+$ <replaceable>/my/sources</replaceable>/nixpkgs/maintainers/scripts/update-channel-branches.sh
+$ git checkout -b local channels/remotes/nixos-unstable
 </screen>
 
 You can then use <command>git rebase</command> to sync your local