fetchpatch: add `relative`

Allows restricting patches to a specific subdirectory, à la
`git diff --relative=subdir`.

This cannot be done (cleanly) currently because the `includes` logic
happens *after* `stripLen` is applied, so we can't match on `subdir/*`.

This change adds a `relative` argument that makes this possible by
filtering files before doing any processing, and setting `stripLen` and
`extraPrefix` accordingly.

3 files changed, 49 insertions, 17 deletions

diff --git a/doc/contributing/coding-conventions.chapter.md b/doc/contributing/coding-conventions.chapter.md
index cfe8582e514..dac6d828ac0 100644
--- a/doc/contributing/coding-conventions.chapter.md
+++ b/doc/contributing/coding-conventions.chapter.md
@@ -540,10 +540,11 @@ If you do need to do create this sort of patch file, one way to do so is with gi
 
 If a patch is available online but does not cleanly apply, it can be modified in some fixed ways by using additional optional arguments for `fetchpatch`:
 
+- `relative`: Similar to using `git-diff`'s `--relative` flag, only keep changes inside the specified directory, making paths relative to it.
 - `stripLen`: Remove the first `stripLen` components of pathnames in the patch.
 - `extraPrefix`: Prefix pathnames by this string.
-- `excludes`: Exclude files matching this pattern.
-- `includes`: Include only files matching this pattern.
+- `excludes`: Exclude files matching these patterns (applies after the above arguments).
+- `includes`: Include only files matching these patterns (applies after the above arguments).
 - `revert`: Revert the patch.
 
 Note that because the checksum is computed after applying these effects, using or modifying these arguments will have no effect unless the `sha256` argument is changed as well.
diff --git a/pkgs/build-support/fetchpatch/default.nix b/pkgs/build-support/fetchpatch/default.nix
index 6e25b2d6ecc..d46162c97ff 100644
--- a/pkgs/build-support/fetchpatch/default.nix
+++ b/pkgs/build-support/fetchpatch/default.nix
@@ -9,7 +9,8 @@ let
   # 0.3.4 would change hashes: https://github.com/NixOS/nixpkgs/issues/25154
   patchutils = buildPackages.patchutils_0_3_3;
 in
-{ stripLen ? 0
+{ relative ? null
+, stripLen ? 0
 , extraPrefix ? null
 , excludes ? []
 , includes ? []
@@ -17,7 +18,18 @@ in
 , postFetch ? ""
 , ...
 }@args:
-
+let
+  args' = if relative != null then {
+    stripLen = 1 + lib.length (lib.splitString "/" relative) + stripLen;
+    extraPrefix = if extraPrefix != null then extraPrefix else "";
+  } else {
+    inherit stripLen extraPrefix;
+  };
+in let
+  inherit (args') stripLen extraPrefix;
+in
+lib.throwIfNot (excludes == [] || includes == [])
+  "fetchpatch: cannot use excludes and includes simultaneously"
 fetchurl ({
   postFetch = ''
     tmpfile="$TMPDIR/patch"
@@ -27,17 +39,19 @@ fetchurl ({
       exit 1
     fi
 
-    "${patchutils}/bin/lsdiff" "$out" \
-      | sort -u | sed -e 's/[*?]/\\&/g' \
-      | xargs -I{} \
-        "${patchutils}/bin/filterdiff" \
-        --include={} \
-        --strip=${toString stripLen} \
-        ${lib.optionalString (extraPrefix != null) ''
-           --addoldprefix=a/${extraPrefix} \
-           --addnewprefix=b/${extraPrefix} \
-        ''} \
-        --clean "$out" > "$tmpfile"
+    "${patchutils}/bin/lsdiff" \
+      ${lib.optionalString (relative != null) "-p1 -i ${lib.escapeShellArg relative}/'*'"} \
+      "$out" \
+    | sort -u | sed -e 's/[*?]/\\&/g' \
+    | xargs -I{} \
+      "${patchutils}/bin/filterdiff" \
+      --include={} \
+      --strip=${toString stripLen} \
+      ${lib.optionalString (extraPrefix != null) ''
+          --addoldprefix=a/${lib.escapeShellArg extraPrefix} \
+          --addnewprefix=b/${lib.escapeShellArg extraPrefix} \
+      ''} \
+      --clean "$out" > "$tmpfile"
 
     if [ ! -s "$tmpfile" ]; then
       echo "error: Normalized patch '$tmpfile' is empty (while the fetched file was not)!" 1>&2
@@ -64,5 +78,6 @@ fetchurl ({
     ${patchutils}/bin/interdiff "$out" /dev/null > "$tmpfile"
     mv "$tmpfile" "$out"
   '' + postFetch;
-  meta.broken = excludes != [] && includes != [];
-} // builtins.removeAttrs args ["stripLen" "extraPrefix" "excludes" "includes" "revert" "postFetch"])
+} // builtins.removeAttrs args [
+  "relative" "stripLen" "extraPrefix" "excludes" "includes" "revert" "postFetch"
+])
diff --git a/pkgs/build-support/fetchpatch/tests.nix b/pkgs/build-support/fetchpatch/tests.nix
index 4240b325d65..ff2b81bf3a1 100644
--- a/pkgs/build-support/fetchpatch/tests.nix
+++ b/pkgs/build-support/fetchpatch/tests.nix
@@ -5,4 +5,20 @@
     url = "https://github.com/facebook/zstd/pull/2724/commits/e1f85dbca3a0ed5ef06c8396912a0914db8dea6a.patch";
     sha256 = "sha256-PuYAqnJWAE+L9bsroOnnBGJhERW8LHrGSLtIEkKU9vg=";
   };
+
+  relative = invalidateFetcherByDrvHash fetchpatch {
+    url = "https://github.com/boostorg/math/commit/7d482f6ebc356e6ec455ccb5f51a23971bf6ce5b.patch";
+    relative = "include";
+    sha256 = "sha256-KlmIbixcds6GyKYt1fx5BxDIrU7msrgDdYo9Va/KJR4=";
+  };
+
+  full = invalidateFetcherByDrvHash fetchpatch {
+    url = "https://github.com/boostorg/math/commit/7d482f6ebc356e6ec455ccb5f51a23971bf6ce5b.patch";
+    relative = "test";
+    stripLen = 1;
+    extraPrefix = "foo/bar/";
+    excludes = [ "foo/bar/bernoulli_no_atomic_mp.cpp" ];
+    revert = true;
+    sha256 = "sha256-+UKmEbr2rIAweCav/hR/7d4ZrYV84ht/domTrHtm8sM=";
+  };
 }