From 1cc2c2f13d7a548759a55f710fd0222da14c5403 Mon Sep 17 00:00:00 2001
From: Silvan Mosberger <silvan.mosberger@tweag.io>
Date: Tue, 7 Nov 2023 01:04:13 +0100
Subject: [PATCH] lib.fileset.maybeMissing: init

---
 lib/fileset/README.md    |  5 -----
 lib/fileset/default.nix  | 31 +++++++++++++++++++++++++++++++
 lib/fileset/internal.nix |  3 ++-
 lib/fileset/tests.sh     | 37 ++++++++++++++++++++++++++++++++++++-
 4 files changed, 69 insertions(+), 7 deletions(-)

diff --git a/lib/fileset/README.md b/lib/fileset/README.md
index 14b6877a9065..f44df6fffeca 100644
--- a/lib/fileset/README.md
+++ b/lib/fileset/README.md
@@ -252,8 +252,3 @@ The `fileFilter` function takes a path, and not a file set, as its second argume
     - (+) That can change depending on which files are included, so if it's used for `fileFilter`
       it would change the `subpath`/`components` value depending on which files are included.
 - (+) If necessary, this restriction can be relaxed later, the opposite wouldn't be possible
-
-## To update in the future
-
-Here's a list of places in the library that need to be updated in the future:
-- If/Once a function exists that can optionally include a path depending on whether it exists, the error message for the path not existing in `_coerce` should mention the new function
diff --git a/lib/fileset/default.nix b/lib/fileset/default.nix
index 2cb361ec9ba1..9ccbf0ed7ce7 100644
--- a/lib/fileset/default.nix
+++ b/lib/fileset/default.nix
@@ -11,6 +11,10 @@
   Basics:
   - [Implicit coercion from paths to file sets](#sec-fileset-path-coercion)
 
+  - [`lib.fileset.maybeMissing`](#function-library-lib.fileset.maybeMissing):
+
+    Create a file set from a path that may be missing.
+
   - [`lib.fileset.trace`](#function-library-lib.fileset.trace)/[`lib.fileset.traceVal`](#function-library-lib.fileset.trace):
 
     Pretty-print file sets for debugging.
@@ -105,6 +109,7 @@ let
     _difference
     _mirrorStorePath
     _fetchGitSubmodulesMinver
+    _emptyWithoutBase
     ;
 
   inherit (builtins)
@@ -148,6 +153,32 @@ let
 
 in {
 
+  /*
+    Create a file set from a path that may or may not exist:
+    - If the path does exist, the path is [coerced to a file set](#sec-fileset-path-coercion).
+    - If the path does not exist, a file set containing no files is returned.
+
+    Type:
+      maybeMissing :: Path -> FileSet
+
+    Example:
+      # All files in the current directory, but excluding main.o if it exists
+      difference ./. (maybeMissing ./main.o)
+  */
+  maybeMissing =
+    path:
+    if ! isPath path then
+      if isStringLike path then
+        throw ''
+          lib.fileset.maybeMissing: Argument ("${toString path}") is a string-like value, but it should be a path instead.''
+      else
+        throw ''
+          lib.fileset.maybeMissing: Argument is of type ${typeOf path}, but it should be a path instead.''
+    else if ! pathExists path then
+      _emptyWithoutBase
+    else
+      _singleton path;
+
   /*
     Incrementally evaluate and trace a file set in a pretty way.
     This function is only intended for debugging purposes.
diff --git a/lib/fileset/internal.nix b/lib/fileset/internal.nix
index 23d7b847204c..2fddf0d02285 100644
--- a/lib/fileset/internal.nix
+++ b/lib/fileset/internal.nix
@@ -181,7 +181,8 @@ rec {
           ${context} is of type ${typeOf value}, but it should be a file set or a path instead.''
     else if ! pathExists value then
       throw ''
-        ${context} (${toString value}) is a path that does not exist.''
+        ${context} (${toString value}) is a path that does not exist.
+            To create a file set from a path that may not exist, use `lib.fileset.maybeMissing`.''
     else
       _singleton value;
 
diff --git a/lib/fileset/tests.sh b/lib/fileset/tests.sh
index 06f92f297d88..81376bc451b1 100755
--- a/lib/fileset/tests.sh
+++ b/lib/fileset/tests.sh
@@ -413,7 +413,8 @@ expectFailure 'toSource { root = ./.; fileset = cleanSourceWith { src = ./.; };
 \s*Note that this only works for sources created from paths.'
 
 # Path coercion errors for non-existent paths
-expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` \('"$work"'/a\) is a path that does not exist.'
+expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` \('"$work"'/a\) is a path that does not exist.
+\s*To create a file set from a path that may not exist, use `lib.fileset.maybeMissing`.'
 
 # File sets cannot be evaluated directly
 expectFailure 'union ./. ./.' 'lib.fileset: Directly evaluating a file set is not supported.
@@ -1450,6 +1451,40 @@ checkGitTracked
 
 rm -rf -- *
 
+## lib.fileset.maybeMissing
+
+# Argument must be a path
+expectFailure 'maybeMissing "someString"' 'lib.fileset.maybeMissing: Argument \("someString"\) is a string-like value, but it should be a path instead.'
+expectFailure 'maybeMissing null' 'lib.fileset.maybeMissing: Argument is of type null, but it should be a path instead.'
+
+tree=(
+)
+checkFileset 'maybeMissing ./a'
+checkFileset 'maybeMissing ./b'
+checkFileset 'maybeMissing ./b/c'
+
+# Works on single files
+tree=(
+    [a]=1
+    [b/c]=0
+    [b/d]=0
+)
+checkFileset 'maybeMissing ./a'
+tree=(
+    [a]=0
+    [b/c]=1
+    [b/d]=0
+)
+checkFileset 'maybeMissing ./b/c'
+
+# Works on directories
+tree=(
+    [a]=0
+    [b/c]=1
+    [b/d]=1
+)
+checkFileset 'maybeMissing ./b'
+
 # TODO: Once we have combinators and a property testing library, derive property tests from https://en.wikipedia.org/wiki/Algebra_of_sets
 
 echo >&2 tests ok