1 files changed, 176 insertions, 0 deletions
diff --git a/nix/utils/default.nix b/nix/utils/default.nix
new file mode 100644
index 0000000000..a29f346519
--- /dev/null
+++ b/nix/utils/default.nix
@@ -0,0 +1,176 @@
+{ depot, lib, ... }:
+
+let
+  /* Get the basename of a store path without
+     the leading hash.
+
+     Type: (path | drv | string) -> string
+
+     Example:
+       storePathName ./foo.c
+       => "foo.c"
+
+       storePathName (writeText "foo.c" "int main() { return 0; }")
+       => "foo.c"
+
+       storePathName "${hello}/bin/hello"
+       => "hello"
+  */
+  storePathName = p:
+    if lib.isDerivation p
+    then p.name
+    else if builtins.isPath p
+    then builtins.baseNameOf p
+    else if builtins.isString p || (builtins.isAttrs p && (p ? outPath || p ? __toString))
+    then
+      let
+        strPath = toString p;
+        # strip leading storeDir and trailing slashes
+        noStoreDir = lib.removeSuffix "/"
+          (lib.removePrefix "${builtins.storeDir}/" strPath);
+        # a basename of a child of a store path isn't really
+        # referring to a store path, so removing the string
+        # context is safe (e. g. "hello" for "${hello}/bin/hello").
+        basename = builtins.unsafeDiscardStringContext
+          (builtins.baseNameOf strPath);
+      in
+      # If p is a direct child of storeDir, we need to remove
+        # the leading hash as well to make sure that:
+        # `storePathName drv == storePathName (toString drv)`.
+      if noStoreDir == basename
+      then builtins.substring 33 (-1) basename
+      else basename
+    else builtins.throw "Don't know how to get (base)name of "
+      + lib.generators.toPretty { } p;
+
+  /* Retrieves the drvPath attribute from a given derivation, but ensures that
+     the resulting string only depends on the `.drv` file in the nix store and
+     not on its realised outputs as well.
+
+     Type: drv -> string
+  */
+  onlyDrvPath = drv:
+    let
+      inherit (drv) drvPath;
+      unsafeDrvPath = builtins.unsafeDiscardStringContext drvPath;
+    in
+    builtins.appendContext unsafeDrvPath {
+      ${unsafeDrvPath} = { path = true; };
+    };
+
+  /* Query the type of a path exposing the same information as would be by
+     `builtins.readDir`, but for a single, specific target path.
+
+     The information is returned as a tagged value, i. e. an attribute set with
+     exactly one attribute where the type of the path is encoded in the name
+     of the single attribute. The allowed tags and values are as follows:
+
+     * `regular`: is a regular file, always `true` if returned
+     * `directory`: is a directory, always `true` if returned
+     * `missing`: path does not exist, always `true` if returned
+     * `symlink`: path is a symlink, always `true` if returned
+
+     Type: path(-like) -> tag
+
+     `tag` refers to the attribute set format of `//nix/tag`.
+
+     Example:
+       pathType ./foo.c
+       => { regular = true; }
+
+       pathType /home/lukas
+       => { directory = true; }
+
+       pathType ./result
+       => { symlink = true; }
+
+       pathType ./link-to-file
+       => { symlink = true; }
+
+       pathType /does/not/exist
+       => { missing = true; }
+
+       # Check if a path exists
+       !(pathType /file ? missing)
+
+       # Check if a path is a directory or a symlink to a directory
+       # A handy shorthand for this is provided as `realPathIsDirectory`.
+       pathType /path ? directory || (pathType /path).symlink or null == "directory"
+
+       # Match on the result using //nix/tag
+       nix.tag.match (nix.utils.pathType ./result) {
+         symlink = _: "symlink";
+         directory  = _: "directory";
+         regular = _: "regular";
+         missing = _: "path does not exist";
+       }
+       => "symlink"
+
+       # Query path type
+       nix.tag.tagName (pathType /path)
+  */
+  pathType = path:
+    let
+      # baseNameOf is very annoyed if we proceed with string context.
+      # We need to call toString to prevent unsafeDiscardStringContext
+      # from importing a path into store which messes with base- and
+      # dirname of course.
+      path' = builtins.unsafeDiscardStringContext (toString path);
+      # To read the containing directory we absolutely need
+      # to keep the string context, otherwise a derivation
+      # would not be realized before our check (at eval time)
+      containingDir = builtins.readDir (builtins.dirOf path);
+      # Construct tag to use for the value
+      thisPathType = containingDir.${builtins.baseNameOf path'} or "missing";
+      # Trick to check if the symlink target exists and is a directory:
+      # if we append a "/." to the string version of the path, Nix won't
+      # canocalize it (which would strip any "/." in the path), so if
+      # path' + "/." exists, we know that the symlink points to an existing
+      # directory. If not, either the target doesn't exist or is a regular file.
+      # TODO(sterni): is there a way to check reliably if the symlink target exists?
+      isSymlinkDir = builtins.pathExists (path' + "/.");
+    in
+    {
+      ${thisPathType} = true;
+    };
+
+  pathType' = path:
+    let
+      p = pathType path;
+    in
+    if p ? missing
+    then builtins.throw "${lib.generators.toPretty {} path} does not exist"
+    else p;
+
+  /* Check whether the given path is a directory.
+     Throws if the path in question doesn't exist.
+
+     Type: path(-like) -> bool
+  */
+  isDirectory = path: pathType' path ? directory;
+
+  /* Check whether the given path is a regular file.
+     Throws if the path in question doesn't exist.
+
+     Type: path(-like) -> bool
+  */
+  isRegularFile = path: pathType' path ? regular;
+
+  /* Check whether the given path is a symbolic link.
+     Throws if the path in question doesn't exist.
+
+     Type: path(-like) -> bool
+  */
+  isSymlink = path: pathType' path ? symlink;
+
+in
+{
+  inherit
+    storePathName
+    onlyDrvPath
+    pathType
+    isDirectory
+    isRegularFile
+    isSymlink
+    ;
+}