cmSystemTools: Add EnvDiff class to hold ENVIRONMENT_MODIFICATION logic

Prepare to re-use this logic when enhancing `cmake -E env`.

1 files changed, 121 insertions, 0 deletions

diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 672cdc7..c35c1e7 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -16,6 +16,7 @@
 
 #include <cm/optional>
 #include <cmext/algorithm>
+#include <cmext/string_view>
 
 #include <cm3p/uv.h>
 
@@ -1563,6 +1564,126 @@ void cmSystemTools::AppendEnv(std::vector<std::string> const& env)
   }
 }
 
+bool cmSystemTools::EnvDiff::ParseOperation(const std::string& envmod)
+{
+  char path_sep = GetSystemPathlistSeparator();
+
+  auto apply_diff = [this](const std::string& name,
+                           std::function<void(std::string&)> const& apply) {
+    cm::optional<std::string> old_value = diff[name];
+    std::string output;
+    if (old_value) {
+      output = *old_value;
+    } else {
+      // This only works because the environment is actually modified when
+      // processing the ENVIRONMENT property in CTest and cmake -E env
+      // (`AppendEnv`). If either one ever just creates an environment block
+      // directly, that block will need to be queried for the subprocess'
+      // value instead.
+      const char* curval = cmSystemTools::GetEnv(name);
+      if (curval) {
+        output = curval;
+      }
+    }
+    apply(output);
+    diff[name] = output;
+  };
+
+  // Split on `=`
+  auto const eq_loc = envmod.find_first_of('=');
+  if (eq_loc == std::string::npos) {
+    cmSystemTools::Error(cmStrCat(
+      "Error: Missing `=` after the variable name in: ", envmod, '\n'));
+    return false;
+  }
+
+  auto const name = envmod.substr(0, eq_loc);
+
+  // Split value on `:`
+  auto const op_value_start = eq_loc + 1;
+  auto const colon_loc = envmod.find_first_of(':', op_value_start);
+  if (colon_loc == std::string::npos) {
+    cmSystemTools::Error(
+      cmStrCat("Error: Missing `:` after the operation in: ", envmod, '\n'));
+    return false;
+  }
+  auto const op = envmod.substr(op_value_start, colon_loc - op_value_start);
+
+  auto const value_start = colon_loc + 1;
+  auto const value = envmod.substr(value_start);
+
+  // Determine what to do with the operation.
+  if (op == "reset"_s) {
+    auto entry = diff.find(name);
+    if (entry != diff.end()) {
+      diff.erase(entry);
+    }
+  } else if (op == "set"_s) {
+    diff[name] = value;
+  } else if (op == "unset"_s) {
+    diff[name] = {};
+  } else if (op == "string_append"_s) {
+    apply_diff(name, [&value](std::string& output) { output += value; });
+  } else if (op == "string_prepend"_s) {
+    apply_diff(name,
+               [&value](std::string& output) { output.insert(0, value); });
+  } else if (op == "path_list_append"_s) {
+    apply_diff(name, [&value, path_sep](std::string& output) {
+      if (!output.empty()) {
+        output += path_sep;
+      }
+      output += value;
+    });
+  } else if (op == "path_list_prepend"_s) {
+    apply_diff(name, [&value, path_sep](std::string& output) {
+      if (!output.empty()) {
+        output.insert(output.begin(), path_sep);
+      }
+      output.insert(0, value);
+    });
+  } else if (op == "cmake_list_append"_s) {
+    apply_diff(name, [&value](std::string& output) {
+      if (!output.empty()) {
+        output += ';';
+      }
+      output += value;
+    });
+  } else if (op == "cmake_list_prepend"_s) {
+    apply_diff(name, [&value](std::string& output) {
+      if (!output.empty()) {
+        output.insert(output.begin(), ';');
+      }
+      output.insert(0, value);
+    });
+  } else {
+    cmSystemTools::Error(cmStrCat(
+      "Error: Unrecognized environment manipulation argument: ", op, '\n'));
+    return false;
+  }
+
+  return true;
+}
+
+void cmSystemTools::EnvDiff::ApplyToCurrentEnv(std::ostringstream* measurement)
+{
+  for (auto const& env_apply : diff) {
+    if (env_apply.second) {
+      auto const env_update =
+        cmStrCat(env_apply.first, '=', *env_apply.second);
+      cmSystemTools::PutEnv(env_update);
+      if (measurement) {
+        *measurement << env_update << std::endl;
+      }
+    } else {
+      cmSystemTools::UnsetEnv(env_apply.first.c_str());
+      if (measurement) {
+        // Signify that this variable is being actively unset
+        *measurement << '#' << env_apply.first << "=\n";
+      }
+    }
+  }
+}
+
 cmSystemTools::SaveRestoreEnvironment::SaveRestoreEnvironment()
 {
   this->Env = cmSystemTools::GetEnvironmentVariables();