nix-prefetch-git: add --no-add-path argument

Adds a new argument to `nix-prefetch-git`: `--no-add-path`. It disables
adding the path to the store. It is useful when working with a read-only
store.

doc/release-notes/rl-2511.section.md

@@ -229,6 +229,8 @@
 
 - `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`.
 
+- `nix-prefetch-git`: Added a `--no-add-path` argument to disable adding the path to the store; this is useful when working with a [read-only store](https://nix.dev/manual/nix/2.28/command-ref/new-cli/nix3-help-stores#store-experimental-local-overlay-store-read-only).
+
 - `sftpman` has been updated to version 2, a rewrite in Rust which is mostly backward compatible but does include some changes to the CLI.
   For more information, [check the project's README](https://github.com/spantaleev/sftpman-rs#is-sftpman-v2-compatible-with-sftpman-v1).
 

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.
+      --no-add-path   Do not actually add the contents of the git repo to the store.
       --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.
 "
@@ -91,6 +92,7 @@ for arg; do
             --fetch-submodules) fetchSubmodules=true;;
             --fetch-tags) fetchTags=true;;
             --builder) builder=true;;
+            --no-add-path) noAddPath=true;;
             --root-dir) argfun=set_rootDir;;
             -h|--help) usage; exit;;
             *)
@@ -519,7 +521,14 @@ else
         hash=$(nix-hash --type $hashType --base32 "$tmpOut")
 
         # Add the downloaded file to the Nix store.
-        finalPath=$(nix-store --add-fixed --recursive "$hashType" "$tmpOut")
+
+        if test -z "$noAddPath"; then
+            finalPath=$(nix-store --add-fixed --recursive "$hashType" "$tmpOut") \
+                || { printf "maybe try again with \`nix-prefetch-git --no-add-path <...>' ?\n" >&2; exit 1; }
+        else
+            printf "the path for \`%s' has NOT been added to the store\n" "$url" >&2
+            finalPath=$(nix-store --print-fixed-path --recursive "$hashType" "$hash" "$storePathName")
+        fi
 
         if test -n "$expHash" -a "$expHash" != "$hash"; then
             echo "hash mismatch for URL \`$url'. Got \`$hash'; expected \`$expHash'." >&2

pkgs/build-support/fetchgit/tests.nix

@@ -1,4 +1,13 @@
-{ testers, fetchgit, ... }:
+{
+  runCommand,
+  testers,
+  fetchgit,
+  nix-prefetch-git,
+  jq,
+  cacert,
+  nix,
+  ...
+}:
 {
   simple = testers.invalidateFetcherByDrvHash fetchgit {
     name = "simple-nix-source";
@@ -105,4 +114,54 @@
     rootDir = "misc/systemd";
     sha256 = "sha256-UhxHk4SrXYq7ZDMtXLig5SigpbITrVgkpFTmryuvpcM=";
   };
+
+  prefetch-git-no-add-path =
+    testers.invalidateFetcherByDrvHash
+      (
+        {
+          name,
+          url,
+          rev,
+          hash,
+          ...
+        }:
+        runCommand name
+          {
+            buildInputs = [
+              nix-prefetch-git
+              nix
+              cacert
+              jq
+            ];
+            outputHashMode = "recursive";
+            outputHashAlgo = null;
+            outputHash = hash;
+            inherit url rev;
+          }
+          ''
+            store_root="$(mktemp -d)"
+            prefetch() { NIX_REMOTE="local?root=$store_root" nix-prefetch-git $@ "$url" --rev "$rev" | jq -r .path; }
+            path="$(prefetch --no-add-path)"
+            if test -e "$store_root/$path"; then
+              echo "$path exists in $NIX_REMOTE when it shouldn't" >&2
+              exit 1
+            fi
+            path_added="$(prefetch)"
+            if ! test -e "$store_root/$path"; then
+              echo "$path_added doesn't exist in NIX_REMOTE when it should" >&2
+              exit 1
+            fi
+            if test "$path" != "$path_added"; then
+              echo "Paths are different with and without --no-add-path: $path != $path_added" >&2
+              exit 1
+            fi
+            cp -r "$store_root/$path_added" "$out"
+          ''
+      )
+      {
+        name = "nix-prefetch-git-no-add-path";
+        url = "https://github.com/NixOS/nix";
+        rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+        hash = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
+      };
 }