Merge pull request #9985 from alois31/symlink-resolution

Restore `builtins.pathExists` behavior on broken symlinks

(cherry picked from commit d53c8901ef7f2033855dd99063522e3d56a19dab)

===

note that this variant differs markedly from the source commit because
we haven't endured quite as much lazy trees.

Change-Id: I0facf282f21fe0db4134be5c65a8368c1b3a06fc

2 files changed, 40 insertions, 13 deletions

diff --git a/src/libutil/source-path.cc b/src/libutil/source-path.cc
index b6563ee90..3ccbca06b 100644
--- a/src/libutil/source-path.cc
+++ b/src/libutil/source-path.cc
@@ -56,7 +56,7 @@ InputAccessor::DirEntries SourcePath::readDirectory() const
     return res;
 }
 
-SourcePath SourcePath::resolveSymlinks() const
+SourcePath SourcePath::resolveSymlinks(SymlinkResolution mode) const
 {
     SourcePath res(CanonPath::root);
 
@@ -66,6 +66,8 @@ SourcePath SourcePath::resolveSymlinks() const
     for (auto & c : path)
         todo.push_back(std::string(c));
 
+    bool resolve_last = mode == SymlinkResolution::Full;
+
     while (!todo.empty()) {
         auto c = *todo.begin();
         todo.pop_front();
@@ -75,14 +77,16 @@ SourcePath SourcePath::resolveSymlinks() const
             res.path.pop();
         else {
             res.path.push(c);
-            if (auto st = res.maybeLstat(); st && st->type == InputAccessor::tSymlink) {
-                if (!linksAllowed--)
-                    throw Error("infinite symlink recursion in path '%s'", path);
-                auto target = res.readLink();
-                res.path.pop();
-                if (hasPrefix(target, "/"))
-                    res.path = CanonPath::root;
-                todo.splice(todo.begin(), tokenizeString<std::list<std::string>>(target, "/"));
+            if (resolve_last || !todo.empty()) {
+                if (auto st = res.maybeLstat(); st && st->type == InputAccessor::tSymlink) {
+                    if (!linksAllowed--)
+                        throw Error("infinite symlink recursion in path '%s'", path);
+                    auto target = res.readLink();
+                    res.path.pop();
+                    if (hasPrefix(target, "/"))
+                        res.path = CanonPath::root;
+                    todo.splice(todo.begin(), tokenizeString<std::list<std::string>>(target, "/"));
+                }
             }
         }
     }
diff --git a/src/libutil/source-path.hh b/src/libutil/source-path.hh
index 54698de77..03cc998e3 100644
--- a/src/libutil/source-path.hh
+++ b/src/libutil/source-path.hh
@@ -13,6 +13,26 @@
 namespace nix {
 
 /**
+ * Note there is a decent chance this type soon goes away because the problem is solved another way.
+ * See the discussion in https://github.com/NixOS/nix/pull/9985.
+ */
+enum class SymlinkResolution {
+    /**
+     * Resolve symlinks in the ancestors only.
+     *
+     * Only the last component of the result is possibly a symlink.
+     */
+    Ancestors,
+
+    /**
+     * Resolve symlinks fully, realpath(3)-style.
+     *
+     * No component of the result will be a symlink.
+     */
+    Full,
+};
+
+/**
  * An abstraction for accessing source files during
  * evaluation. Currently, it's just a wrapper around `CanonPath` that
  * accesses files in the regular filesystem, but in the future it will
@@ -121,11 +141,14 @@ struct SourcePath
     }
 
     /**
-     * Resolve any symlinks in this `SourcePath` (including its
-     * parents). The result is a `SourcePath` in which no element is a
-     * symlink.
+     * Resolve any symlinks in this `SourcePath` according to the
+     * given resolution mode.
+     *
+     * @param mode might only be a temporary solution for this. 
+     * See the discussion in https://github.com/NixOS/nix/pull/9985.
      */
-    SourcePath resolveSymlinks() const;
+    SourcePath resolveSymlinks(
+        SymlinkResolution mode = SymlinkResolution::Full) const;
 };
 
 std::ostream & operator << (std::ostream & str, const SourcePath & path);