cmake -E env: Add --modify flag

When `cmake -E env` is given the `--modify` flag, try to parse the
following argument as an `ENVIRONMENT_MODIFICATION` operation and apply
it to the environment.

This generalizes `--unset=`:

1.  When implementing `ENVIRONMENT_MODIFICATION` features for other CMake
    commands, the `MYVAR=OP:VALUE` strings do not need to be translated
    to OP-specific flags.
2.  This provides a natural and consistent extension point to introduce
    new operations without introducing very many flags.
3.  Users need to learn only one syntax to access the same functionality.

There is one difference between the behavior here as compared to CTest's
interpretation of the `ENVIRONMENT_MODIFICATION` test property.
The `MYVAR=reset:` command when run in `cmake -E env` will reset `MYVAR`
to whatever its value was when `cmake -E env` launched, rather than try
to checkpoint after plain `MYVAR=VALUE` options.  This makes `MYVAR=VALUE`
and `--modify MYVAR=set:VALUE` semantically equivalent.

1 files changed, 35 insertions, 3 deletions

diff --git a/Source/cmcmd.cxx b/Source/cmcmd.cxx
index b1f1bcf..00c9bda 100644
--- a/Source/cmcmd.cxx
+++ b/Source/cmcmd.cxx
@@ -791,6 +791,10 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string> const& args,
     }
 
     if (args[1] == "env") {
+#ifndef CMAKE_BOOTSTRAP
+      cmSystemTools::EnvDiff env;
+#endif
+
       auto ai = args.cbegin() + 2;
       auto ae = args.cend();
       for (; ai != ae; ++ai) {
@@ -803,16 +807,40 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string> const& args,
         }
         if (cmHasLiteralPrefix(a, "--unset=")) {
           // Unset environment variable.
+#ifdef CMAKE_BOOTSTRAP
           cmSystemTools::UnPutEnv(a.substr(8));
+#else
+          env.UnPutEnv(a.substr(8));
+#endif
+        } else if (a == "--modify") {
+#ifdef CMAKE_BOOTSTRAP
+          std::cerr
+            << "cmake -E env: --modify not available during bootstrapping\n";
+          return 1;
+#else
+          if (++ai == ae) {
+            std::cerr << "cmake -E env: --modify missing a parameter\n";
+            return 1;
+          }
+          std::string const& op = *ai;
+          if (!env.ParseOperation(op)) {
+            std::cerr << "cmake -E env: invalid parameter to --modify: " << op
+                      << '\n';
+            return 1;
+          }
+#endif
         } else if (!a.empty() && a[0] == '-') {
           // Environment variable and command names cannot start in '-',
           // so this must be an unknown option.
-          std::cerr << "cmake -E env: unknown option '" << a << '\''
-                    << std::endl;
+          std::cerr << "cmake -E env: unknown option '" << a << "'\n";
           return 1;
         } else if (a.find('=') != std::string::npos) {
           // Set environment variable.
+#ifdef CMAKE_BOOTSTRAP
           cmSystemTools::PutEnv(a);
+#else
+          env.PutEnv(a);
+#endif
         } else {
           // This is the beginning of the command.
           break;
@@ -820,10 +848,14 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string> const& args,
       }
 
       if (ai == ae) {
-        std::cerr << "cmake -E env: no command given" << std::endl;
+        std::cerr << "cmake -E env: no command given\n";
         return 1;
       }
 
+#ifndef CMAKE_BOOTSTRAP
+      env.ApplyToCurrentEnv();
+#endif
+
       // Execute command from remaining arguments.
       std::vector<std::string> cmd(ai, ae);
       int retval;