nixos/version: fix case where .git is a symlink

Before c9214c394b248e1f26e45dbe1be2bd82363af3a6 and
9d396d2e426b9a765de0b8999aa214f1259633e6 if .git is symlink the version
would gracefully default to no git revision. With those changes an
exception is thrown instead.

This introduces a new function `pathIsGitRepo` that checks if
`commitIdFromGitRepo` fails without error so we don't have to
reimplement this logic again and can fail gracefully.

3 files changed, 3 insertions, 2 deletions

diff --git a/lib/default.nix b/lib/default.nix
index 5abafe1b2ac..d2e9f0e8086 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -101,7 +101,7 @@ let
     inherit (sources) pathType pathIsDirectory cleanSourceFilter
       cleanSource sourceByRegex sourceFilesBySuffices
       commitIdFromGitRepo cleanSourceWith pathHasContext
-      canCleanSource pathIsRegularFile;
+      canCleanSource pathIsRegularFile pathIsGitRepo;
     inherit (modules) evalModules unifyModuleSyntax
       applyIfFunction mergeModules
       mergeModules' mergeOptionDecls evalOptionValue mergeDefinitions
diff --git a/lib/sources.nix b/lib/sources.nix
index a5765c0fda5..05519c3e392 100644
--- a/lib/sources.nix
+++ b/lib/sources.nix
@@ -105,6 +105,7 @@ rec {
       in type == "directory" || lib.any (ext: lib.hasSuffix ext base) exts;
     in cleanSourceWith { inherit filter; src = path; };
 
+  pathIsGitRepo = path: (builtins.tryEval (commitIdFromGitRepo path)).success;
 
   # Get the commit id of a git repo
   # Example: commitIdFromGitRepo <nixpkgs/.git>
diff --git a/lib/trivial.nix b/lib/trivial.nix
index 940ec1a3d59..a281cd70fb0 100644
--- a/lib/trivial.nix
+++ b/lib/trivial.nix
@@ -191,7 +191,7 @@ rec {
     let
       revisionFile = "${toString ./..}/.git-revision";
       gitRepo      = "${toString ./..}/.git";
-    in if builtins.pathExists gitRepo
+    in if lib.pathIsGitRepo gitRepo
        then lib.commitIdFromGitRepo gitRepo
        else if lib.pathExists revisionFile then lib.fileContents revisionFile
        else default;