rules: make ACTION expression support "cwd"

2 files changed, 21 insertions, 2 deletions

diff --git a/doc/concepts/rules.md b/doc/concepts/rules.md
index ba901dca..a9db5705 100644
--- a/doc/concepts/rules.md
+++ b/doc/concepts/rules.md
@@ -178,12 +178,19 @@ following arguments.
    non-empty list of strings. The 0'th element of that list will
    also be the program to be executed.
 
+ - `"cwd"` The directory inside the action root to change to before
+   executing the command. The directory has to be given as a string
+   decribing a non-upwards relative path. This field is optional
+   and defaults to `""`.
+
  - `"env"` The environment in which the command should be executed,
    given as a map of strings to strings.
 
  - `"outs"` and `"out_dirs"` Two list of strings naming the files
    and directories, respectively, the command is expected to
-   create. It is an error if the command fails to create the
+   create. Those paths are interpreted relative to the action root
+   (not relative to `"cwd"`).
+   It is an error if the command fails to create the
    promised output files. These two lists have to be disjoint, but
    an entry of `"outs"` may well name a location inside one of the
    `"out_dirs"`.
diff --git a/src/buildtool/build_engine/target_map/target_map.cpp b/src/buildtool/build_engine/target_map/target_map.cpp
index 75bf6607..14d897b9 100644
--- a/src/buildtool/build_engine/target_map/target_map.cpp
+++ b/src/buildtool/build_engine/target_map/target_map.cpp
@@ -593,6 +593,18 @@ void withDependencies(
                   }
                   cmd.emplace_back(arg->String());
               }
+              auto cwd_exp =
+                  eval(expr->Get("cwd", Expression::kEmptyString), env);
+              if (not cwd_exp->IsString()) {
+                  throw Evaluator::EvaluationError{
+                      fmt::format("cwd has to be a string, but found {}",
+                                  cwd_exp->ToString())};
+              }
+              if (not PathIsNonUpwards(cwd_exp->String())) {
+                  throw Evaluator::EvaluationError{fmt::format(
+                      "cwd has to be a non-upwards relative path, but found {}",
+                      cwd_exp->ToString())};
+              }
               auto env_exp = eval(expr->Get("env", empty_map_exp), env);
               if (not env_exp->IsMap()) {
                   throw Evaluator::EvaluationError{
@@ -697,7 +709,7 @@ void withDependencies(
                   outputs,
                   output_dirs,
                   std::move(cmd),
-                  "",
+                  cwd_exp->String(),
                   env_exp,
                   may_fail,
                   no_cache,