Merge pull request #99: Add loal development override option

README.md

@@ -376,6 +376,15 @@ Parameter example:
 $ npins add gitlab my-org my-private-repo --token H_BRqzV3NcaPvXcYs2Xf
 ```
 
+### Using local sources during development
+
+While npins allows you to pin dependencies in reproducible fashion, it is often desirable to allow fast impure iterations during development.
+Npins supports local overrides for this.
+If your `sources.json` contains a source named `abc`, you can e.g. develop from `/abc` by exposing the environment variable `NPINS_OVERRIDE_abc=/abc`.
+Please note, that only alphanumerical characters and _ are allow characters in overriden sources.
+All other characters are converted to _.
+Also check, that you are building impure, if you are wondering, why these overrides are maybe not becoming active.
+
 ## Contributing
 
 Contributions to this project are welcome in the form of GitHub Issues or PRs. Please consider the following before creating PRs:

README.md.in

@@ -224,6 +224,15 @@ Parameter example:
 $ npins add gitlab my-org my-private-repo --token H_BRqzV3NcaPvXcYs2Xf
 ```
 
+### Using local sources during development
+
+While npins allows you to pin dependencies in reproducible fashion, it is often desirable to allow fast impure iterations during development.
+Npins supports local overrides for this.
+If your `sources.json` contains a source named `abc`, you can e.g. develop from `/abc` by exposing the environment variable `NPINS_OVERRIDE_abc=/abc`.
+Please note, that only alphanumerical characters and _ are allow characters in overriden sources.
+All other characters are converted to _.
+Also check, that you are building impure, if you are wondering, why these overrides are maybe not becoming active.
+
 ## Contributing
 
 Contributions to this project are welcome in the form of GitHub Issues or PRs. Please consider the following before creating PRs:

src/default.nix

@@ -3,8 +3,42 @@ let
   data = builtins.fromJSON (builtins.readFile ./sources.json);
   version = data.version;
 
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/lists.nix#L295
+  range =
+    first: last: if first > last then [ ] else builtins.genList (n: first + n) (last - first + 1);
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L257
+  stringToCharacters = s: map (p: builtins.substring p 1 s) (range 0 (builtins.stringLength s - 1));
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L269
+  stringAsChars = f: s: concatStrings (map f (stringToCharacters s));
+  concatMapStrings = f: list: concatStrings (map f list);
+  concatStrings = builtins.concatStringsSep "";
+
+  # If the environment variable NPINS_OVERRIDE_${name} is set, then use
+  # the path directly as opposed to the fetched source.
+  # (Taken from Niv for compatibility)
+  mayOverride =
+    name: path:
+    let
+      envVarName = "NPINS_OVERRIDE_${saneName}";
+      saneName = stringAsChars (c: if (builtins.match "[a-zA-Z0-9]" c) == null then "_" else c) name;
+      ersatz = builtins.getEnv envVarName;
+    in
+    if ersatz == "" then
+      path
+    else
+      # this turns the string into an actual Nix path (for both absolute and
+      # relative paths)
+      builtins.trace "Overriding path of \"${name}\" with \"${ersatz}\" due to set \"${envVarName}\"" (
+        if builtins.substring 0 1 ersatz == "/" then
+          /. + ersatz
+        else
+          /. + builtins.getEnv "PWD" + "/${ersatz}"
+      );
+
   mkSource =
-    spec:
+    name: spec:
     assert spec ? type;
     let
       path =
@@ -19,7 +53,7 @@ let
         else
           builtins.throw "Unknown source type ${spec.type}";
     in
-    spec // { outPath = path; };
+    spec // { outPath = mayOverride name path; };
 
   mkGitSource =
     {
@@ -84,6 +118,6 @@ let
     };
 in
 if version == 4 then
-  builtins.mapAttrs (_: mkSource) data.pins
+  builtins.mapAttrs mkSource data.pins
 else
   throw "Unsupported format version ${toString version} in sources.json. Try running `npins upgrade`"

test.nix

@@ -20,6 +20,13 @@ let
       if [[ "$a" = "$b" ]]; then echo " OK"; else echo " FAIL"; exit 1; fi
     }
 
+    function neq() {
+      local a=$1
+      local b=$2
+      printf '[[ "%s" != "%s" ]]' "$a" "$b"
+      if [[ "$a" != "$b" ]]; then echo " OK"; else echo " FAIL"; exit 1; fi
+    }
+
     function resolveGitCommit() {
       local repo=$1
       local commitish=''${2:-main}
@@ -28,7 +35,8 @@ let
   '';
 
   inherit (pkgs) lib;
-  # Generate a git repository hat can be served via HTTP.
+
+  # Generate a git repository that can be served via HTTP.
   #
   # By default the repository will contain an empty `test.txt`
   # file. For all defined tags the name of the tag is written to that
@@ -555,4 +563,20 @@ in
           '';
         };
     };
+
+  gitDependencyOverride = mkGitTest rec {
+    name = "git-dependency-override";
+    repositories."foo" = gitRepo;
+    commands = ''
+      npins init --bare
+      npins add git http://localhost:8000/foo -b test-branch
+      npins show
+
+      OUTPATH=$(NPINS_OVERRIDE_foo=/foo_overriden nix-instantiate --eval npins -A foo.outPath --impure)
+      eq "$OUTPATH" "/foo_overriden"
+
+      OUTPATH=$(nix-instantiate --eval npins -A foo.outPath)
+      neq "$OUTPATH" "/foo_overriden"
+    '';
+  };
 }