fetchgit: Add rootDir argument (#427165)

6 files changed, 78 insertions, 23 deletions

diff --git a/doc/build-helpers/fetchers.chapter.md b/doc/build-helpers/fetchers.chapter.md
index 5ee1d78501ca..6a6ebe9013c2 100644
--- a/doc/build-helpers/fetchers.chapter.md
+++ b/doc/build-helpers/fetchers.chapter.md
@@ -829,6 +829,10 @@ Additionally, the following optional arguments can be given:
 
   See [git sparse-checkout](https://git-scm.com/docs/git-sparse-checkout) for more information.
 
+*`rootDir`* (String)
+
+: When not empty, copy only contents of the subdirectory of the repository to the result. Automatically sets `sparseCheckout` and `nonConeMode` to avoid checking out any extra pieces. Incompatible with `leaveDotGit`.
+
 Some additional parameters for niche use-cases can be found listed in the function parameters in the declaration of `fetchgit`: `pkgs/build-support/fetchgit/default.nix`.
 Future parameters additions might also happen without immediately being documented here.
 
diff --git a/doc/release-notes/rl-2511.section.md b/doc/release-notes/rl-2511.section.md
index 68f68cb4e64f..6cd94f483531 100644
--- a/doc/release-notes/rl-2511.section.md
+++ b/doc/release-notes/rl-2511.section.md
@@ -103,6 +103,7 @@
   * `$debug/lib/debug/.build-id/48/3bd7f7229bdb06462222e1e353e4f37e15c293.sourceoverlay` is a symlink to a directory with the same structure as the expanded `$sourceRoot` but containing only a copy of files which were patched during the build
   * `$debug/lib/debug/.build-id/48/3bd7f7229bdb06462222e1e353e4f37e15c293.debug` is the file containing debug symbols (like before).
 
+- `fetchgit`: Add `rootDir` argument to limit the resulting source to one subdirectory of the whole Git repository. Corresponding `--root-dir` option added to `nix-prefetch-git`.
 
 ## Nixpkgs Library {#sec-nixpkgs-release-25.11-lib}
 
diff --git a/pkgs/build-support/fetchgit/builder.sh b/pkgs/build-support/fetchgit/builder.sh
index 819b0127a852..393d0a57d163 100644
--- a/pkgs/build-support/fetchgit/builder.sh
+++ b/pkgs/build-support/fetchgit/builder.sh
@@ -16,6 +16,7 @@ $SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" --name "$name"
   ${fetchTags:+--fetch-tags} \
   ${sparseCheckout:+--sparse-checkout "$sparseCheckout"} \
   ${nonConeMode:+--non-cone-mode} \
-  ${branchName:+--branch-name "$branchName"}
+  ${branchName:+--branch-name "$branchName"} \
+  ${rootDir:+--root-dir "$rootDir"}
 
 runHook postFetch
diff --git a/pkgs/build-support/fetchgit/default.nix b/pkgs/build-support/fetchgit/default.nix
index d364c8ebf708..b2f5f15a309d 100644
--- a/pkgs/build-support/fetchgit/default.nix
+++ b/pkgs/build-support/fetchgit/default.nix
@@ -8,12 +8,16 @@
 
 let
   urlToName =
-    url: rev:
+    {
+      url,
+      rev,
+      append,
+    }:
     let
       shortRev = lib.sources.shortRev rev;
       appendShort = lib.optionalString ((builtins.match "[a-f0-9]*" rev) != null) "-${shortRev}";
     in
-    "${lib.sources.urlToName url}${appendShort}";
+    "${lib.sources.urlToName url}${if append == "" then appendShort else append}";
 in
 
 lib.makeOverridable (
@@ -24,15 +28,20 @@ lib.makeOverridable (
       url,
       tag ? null,
       rev ? null,
-      name ? urlToName url (lib.revOrTag rev tag),
+      name ? urlToName {
+        inherit url;
+        rev = lib.revOrTag rev tag;
+        # when rootDir is specified, avoid invalidating the result when rev changes
+        append = if rootDir != "" then "-${lib.strings.sanitizeDerivationName rootDir}" else "";
+      },
       leaveDotGit ? deepClone || fetchTags,
       outputHash ? lib.fakeHash,
       outputHashAlgo ? null,
       fetchSubmodules ? true,
       deepClone ? false,
       branchName ? null,
-      sparseCheckout ? [ ],
-      nonConeMode ? false,
+      sparseCheckout ? lib.optional (rootDir != "") rootDir,
+      nonConeMode ? rootDir != "",
       nativeBuildInputs ? [ ],
       # Shell code executed before the file has been fetched.  This, in
       # particular, can do things like set NIX_PREFETCH_GIT_CHECKOUT_HOOK to
@@ -53,6 +62,8 @@ lib.makeOverridable (
       allowedRequisites ? null,
       # fetch all tags after tree (useful for git describe)
       fetchTags ? false,
+      # make this subdirectory the root of the result
+      rootDir ? "",
     }:
 
     /*
@@ -80,6 +91,7 @@ lib.makeOverridable (
 
     assert nonConeMode -> (sparseCheckout != [ ]);
     assert fetchTags -> leaveDotGit;
+    assert rootDir != "" -> !leaveDotGit;
 
     let
       revWithTag =
@@ -135,6 +147,7 @@ lib.makeOverridable (
           preFetch
           postFetch
           fetchTags
+          rootDir
           ;
         rev = revWithTag;
 
diff --git a/pkgs/build-support/fetchgit/nix-prefetch-git b/pkgs/build-support/fetchgit/nix-prefetch-git
index 259138641ea9..7f3616af92fc 100755
--- a/pkgs/build-support/fetchgit/nix-prefetch-git
+++ b/pkgs/build-support/fetchgit/nix-prefetch-git
@@ -59,6 +59,7 @@ Options:
       --fetch-submodules Fetch submodules.
       --fetch-tags    Fetch all tags (useful for git describe).
       --builder       Clone as fetchgit does, but url, rev, and out option are mandatory.
+      --root-dir dir  Directory in the repository that will be copied to the output instead of the full repository.
       --quiet         Only print the final json summary.
 "
     exit 1
@@ -90,6 +91,7 @@ for arg; do
             --fetch-submodules) fetchSubmodules=true;;
             --fetch-tags) fetchTags=true;;
             --builder) builder=true;;
+            --root-dir) argfun=set_rootDir;;
             -h|--help) usage; exit;;
             *)
                 : $((++argi))
@@ -155,10 +157,16 @@ url_to_name(){
     local base
     base=$(basename "$url" .git | cut -d: -f2)
 
-    if [[ $ref =~ ^[a-z0-9]+$ ]]; then
-        echo "$base-${ref:0:7}"
+    if test -n "$rootDir"; then
+        # Sanitize by removing leading dots and replacing all invalid character sequences with dashes.
+        # See sanitizeDerivationName in ../../../lib/strings.nix for reference.
+        echo "$base-$(sed -E 's/^\.+//;s/[^[:alnum:]+._?=-]+/-/g' <<< $rootDir)"
     else
-        echo "$base"
+        if [[ $ref =~ ^[a-z0-9]+$ ]]; then
+            echo "$base-${ref:0:7}"
+        else
+            echo "$base"
+        fi
     fi
 }
 
@@ -353,6 +361,23 @@ clone_user_rev() {
     fi
 }
 
+clone_user_rev_to_tmpfile(){
+    local url="$1"
+    local rev="${2:-HEAD}"
+
+    # nix>=2.20 rejects adding symlinked paths to the store, so use realpath
+    # to resolve to a physical path. https://github.com/NixOS/nix/issues/11941
+    tmpPath="$(realpath "$(mktemp -d --tmpdir git-checkout-tmp-XXXXXXXX)")"
+    exit_handlers+=(remove_tmpPath)
+
+    tmpOut="$tmpPath/out/$storePathName"
+    tmpClone="$tmpPath/clone"
+    mkdir -p "$tmpPath/out" "$tmpClone"
+
+    # Perform the checkout.
+    clone_user_rev "$tmpClone" "$url" "$rev"
+}
+
 exit_handlers=()
 
 run_exit_handlers() {
@@ -418,7 +443,8 @@ print_results() {
   "fetchSubmodules": $([[ -n "$fetchSubmodules" ]] && echo true || echo false),
   "deepClone": $([[ -n "$deepClone" ]] && echo true || echo false),
   "fetchTags": $([[ -n "$fetchTags" ]] && echo true || echo false),
-  "leaveDotGit": $([[ -n "$leaveDotGit" ]] && echo true || echo false)
+  "leaveDotGit": $([[ -n "$leaveDotGit" ]] && echo true || echo false),
+  "rootDir": "$(json_escape "$rootDir")"
 }
 EOF
     fi
@@ -456,8 +482,13 @@ export GIT_CONFIG_NOSYSTEM=1
 
 if test -n "$builder"; then
     test -n "$out" -a -n "$url" -a -n "$rev" || usage
-    mkdir -p "$out"
-    clone_user_rev "$out" "$url" "$rev"
+    if test -n "$rootDir"; then
+        clone_user_rev_to_tmpfile "$url" "$rev"
+        mv "$tmpClone/$rootDir" "$out"
+    else
+        mkdir -p "$out"
+        clone_user_rev "$out" "$url" "$rev"
+    fi
 else
     if test -z "$hashType"; then
         hashType=sha256
@@ -476,22 +507,19 @@ else
     # If we don't know the hash or a path with that hash doesn't exist,
     # download the file and add it to the store.
     if test -z "$finalPath"; then
-        # nix>=2.20 rejects adding symlinked paths to the store, so use realpath
-        # to resolve to a physical path. https://github.com/NixOS/nix/issues/11941
-        tmpPath="$(realpath "$(mktemp -d --tmpdir git-checkout-tmp-XXXXXXXX)")"
-        exit_handlers+=(remove_tmpPath)
+        clone_user_rev_to_tmpfile "$url" "$rev"
 
-        tmpFile="$tmpPath/$storePathName"
-        mkdir -p "$tmpFile"
-
-        # Perform the checkout.
-        clone_user_rev "$tmpFile" "$url" "$rev"
+        if test -z "$rootDir"; then
+            mv "$tmpClone" "$tmpOut"
+        else
+            mv "$tmpClone/$rootDir" "$tmpOut"
+        fi
 
         # Compute the hash.
-        hash=$(nix-hash --type $hashType --base32 "$tmpFile")
+        hash=$(nix-hash --type $hashType --base32 "$tmpOut")
 
         # Add the downloaded file to the Nix store.
-        finalPath=$(nix-store --add-fixed --recursive "$hashType" "$tmpFile")
+        finalPath=$(nix-store --add-fixed --recursive "$hashType" "$tmpOut")
 
         if test -n "$expHash" -a "$expHash" != "$hash"; then
             echo "hash mismatch for URL \`$url'. Got \`$hash'; expected \`$expHash'." >&2
diff --git a/pkgs/build-support/fetchgit/tests.nix b/pkgs/build-support/fetchgit/tests.nix
index fd6aebc335f7..e3fdd5fb8672 100644
--- a/pkgs/build-support/fetchgit/tests.nix
+++ b/pkgs/build-support/fetchgit/tests.nix
@@ -97,4 +97,12 @@
       rm -rf .git
     '';
   };
+
+  rootDir = testers.invalidateFetcherByDrvHash fetchgit {
+    name = "fetchgit-with-rootdir";
+    url = "https://github.com/NixOS/nix";
+    rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+    rootDir = "misc/systemd";
+    sha256 = "sha256-UhxHk4SrXYq7ZDMtXLig5SigpbITrVgkpFTmryuvpcM=";
+  };
 }