populate: follow branches and tags of git sources

A ref that consists of 4 to 40 hex digits is a commit, which populate
fetches only if the work tree lacks it, unless fetchAlways is set.  Any
other ref is a branch or tag, which populate fetches on every run,
resetting the work tree to the commit fetched.  Resetting to the name
itself resolves to a local branch, which fetching doesn't move.

4 files changed, 69 insertions, 18 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d120e39..78721ea 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changes
 
+## Unreleased
+
+- `git` sources with a branch or tag as `ref` follow it: populate fetches
+  the branch or tag on every run, and resets the work tree to what it
+  fetched.
+
 ## 1.31.0
 
 - New source types `flake` and `flake-input` deliver a flake, or one input
diff --git a/README.md b/README.md
index d88f4e1..2784a9d 100644
--- a/README.md
+++ b/README.md
@@ -560,7 +560,11 @@ Supported attributes:
   URL of the Git repository that should be fetched.
 
 * `ref` -
-  Branch / tag / commit that should be fetched.
+  Branch / tag / commit that should be fetched.  A branch or tag gets fetched
+  on every run, so the work tree follows it.  A commit, given by its hash or a
+  prefix thereof of at least four hex digits, only gets fetched if the work
+  tree lacks it, unless `fetchAlways` is set.  So a branch or tag whose name
+  consists of hex digits only counts as a commit.
 
 * `clean.exclude` -
   List of patterns that should be excluded from Git cleaning.
diff --git a/pkgs/populate/default.nix b/pkgs/populate/default.nix
index a5daee8..1787b65 100644
--- a/pkgs/populate/default.nix
+++ b/pkgs/populate/default.nix
@@ -1209,25 +1209,32 @@ let
       git remote set-url origin ${quote source.url}
     fi
 
-    # TODO resolve git_ref to commit hash
-    hash=${quote source.ref}
-
-    if ! test "$(git log --format=%H -1)" = "$hash"; then
-      ${if source.fetchAlways then /* sh */ ''
-        ${if source.shallow then /* sh */ ''
-          git fetch --depth=1 origin "$hash"
-        '' else /* sh */ ''
-          git fetch origin
-        ''}
+    # A commit, given by its hash or a prefix thereof, gets fetched only if
+    # the work tree lacks it, unless fetchAlways is set.  A branch or tag
+    # gets fetched on every run, so the work tree follows it: resetting to
+    # its name would reset to a local branch, which fetching doesn't move.
+    ref=${quote source.ref}
+    fetch() {
+      ${if source.shallow then /* sh */ ''
+        git fetch --depth=1 origin "$1"
       '' else /* sh */ ''
-        if ! git log -1 "$hash" >/dev/null 2>&1; then
-          ${if source.shallow then /* sh */ ''
-            git fetch --depth=1 origin "$hash"
-          '' else /* sh */ ''
-            git fetch origin
-          ''}
-        fi
+        git fetch origin "$1"
       ''}
+    }
+    if printf %s "$ref" | grep -Eqx '[0-9a-f]{4,40}'; then
+      hash=$ref
+      if ! test "$(git log --format=%H -1)" = "$hash"; then
+        ${if source.fetchAlways then /* sh */ ''
+          ${if source.shallow then ''fetch "$hash"'' else "git fetch origin"}
+        '' else /* sh */ ''
+          if ! git log -1 "$hash" >/dev/null 2>&1; then
+            ${if source.shallow then ''fetch "$hash"'' else "git fetch origin"}
+          fi
+        ''}
+      fi
+    else
+      fetch "$ref"
+      hash=$(git rev-parse --verify 'FETCH_HEAD^{commit}')
     fi
     # Also if HEAD is there already: rewriting a lock file in place, e.g.,
     # changes the work tree, and stages the lock file if Nix created it.
diff --git a/tests/scenarios.nix b/tests/scenarios.nix
index 4e869dc..e618fcd 100644
--- a/tests/scenarios.nix
+++ b/tests/scenarios.nix
@@ -595,6 +595,40 @@ assert !(krops.isLocalTarget (targets.remote "/"));
       }
     ];
   })) targetKinds
+  # Branches and tags follow the server: a branch that moves between two
+  # deploys, and a tag, with full and shallow clones.
+  ++ concatMap (targetKind: map (shallow: scenario {
+    name = "git-${targetKind}-shallow${if shallow then "1" else "0"}-refs";
+    inherit targetKind;
+    prepareDeployer = ''
+      git -C /fixtures/git/gitsrc.git update-ref refs/heads/moving ${gitFirst}
+      git -C /fixtures/git/gitsrc.git update-ref refs/tags/first ${gitFirst}
+    '';
+    phases = [
+      {
+        sources.x.git = { url = gitUrl; ref = "moving"; inherit shallow; };
+        check = ''
+          expect_eq HEAD "$(git -C "$T"/x rev-parse HEAD)" ${gitFirst}
+        '';
+        mutateDeployer = ''
+          git -C /fixtures/git/gitsrc.git update-ref refs/heads/moving ${gitHead}
+        '';
+      }
+      {
+        sources.x.git = { url = gitUrl; ref = "moving"; inherit shallow; };
+        check = ''
+          expect_eq HEAD "$(git -C "$T"/x rev-parse HEAD)" ${gitHead}
+        '';
+      }
+      {
+        sources.x.git = { url = gitUrl; ref = "first"; inherit shallow; };
+        check = ''
+          expect_eq HEAD "$(git -C "$T"/x rev-parse HEAD)" ${gitFirst}
+        '';
+      }
+    ];
+  }) [ false true ]) targetKinds
+
   # A git source stops drawing inputs: its lock file, which Nix created and
   # staged, goes, so the work tree is as the commit has it.
   ++ map (targetKind: scenario {