Extract SourcePath::followSymlinks()

src/libfetchers/input-accessor.cc

@@ -61,6 +61,22 @@ SourcePath SourcePath::parent() const
     return {accessor, std::move(*p)};
 }
 
+SourcePath SourcePath::followSymlinks() const {
+    SourcePath path = *this;
+    unsigned int followCount = 0, maxFollow = 1024;
+
+    /* If `path' is a symlink, follow it.  This is so that relative
+       path references work. */
+    while (true) {
+        // Basic cycle/depth limit to avoid infinite loops.
+        if (++followCount >= maxFollow)
+            throw Error("too many symbolic links encountered while traversing the path '%s'", path);
+        if (path.lstat().type != InputAccessor::tSymlink) break;
+        path = {path.accessor, CanonPath(path.readLink(), path.path.parent().value_or(CanonPath::root))};
+    }
+    return path;
+}
+
 SourcePath SourcePath::resolveSymlinks() const
 {
     auto res = accessor->root();

src/libfetchers/input-accessor.hh

@@ -158,10 +158,19 @@ struct SourcePath
 
     /**
      * Resolve any symlinks in this `SourcePath` (including its
-     * parents). The result is a `SourcePath` in which no element is a
-     * symlink.
+     * parents).
+     *
+     * @return A `SourcePath` in which no element is a symlink.
      */
     SourcePath resolveSymlinks() const;
+
+    /**
+     * If this `SourcePath` is a symlink, resolve it, but do not resolve
+     * symlinks in its parent paths.
+     *
+     * @return A `SourcePath` in which the final element is not a symlink.
+     */
+    SourcePath followSymlinks() const;
 };
 
 std::ostream & operator << (std::ostream & str, const SourcePath & path);