pty: windows: allow loading a newer conpty dll

This commit allows loading the console functions from `conpty.dll`
instead of `kernel32.dll` which means that we can update and
track newer features than have been deployed to Windows.

In practical terms this means that we can now unlock mouse input
reporting in eg: VIM running under WSL.
refs: microsoft/terminal#376

We're jumping the gun on this issue, which is tracking making
a proper supportable way to deploy this sort of update:
refs: microsoft/terminal#1130

For now it seems easier just for us to bundle our own copy of
these bits.

This includes a speculative change to include those in our
Windows downloads also.

The binaries were built from
microsoft/terminal@4f8acb4

assets/windows/conhost/README.md

@@ -0,0 +1,22 @@
+# Console Host
+
+This directory contains a copy of built artifacts from the Microsoft
+Terminal project which is provided by Microsoft under the terms
+of the MIT license.
+
+Why are they here?  At the time of writing, the conpty implementation
+that ships with windows is lacking support for mouse reporting but
+that support is available in the opensource project so it is desirable
+to point to that so that we can enable mouse reporting in wezterm.
+
+It looks like we'll eventually be able to drop this once Windows
+and/or the build for the terminal project make some more progress.
+
+https://github.com/microsoft/terminal/issues/1130
+
+These assets were built by opening the solution in visual studio 2019 and
+building the `Host.EXE` and `winconpty.DLL` projects.
+
+It's possible that you'll need to download this runtime support package
+from MS in order for this to work:
+https://www.microsoft.com/en-us/download/details.aspx?id=53175

build.rs

@@ -18,6 +18,23 @@ fn main() {
     println!("cargo:rustc-env=WEZTERM_CI_TAG={}", ci_tag);
     println!("cargo:rustc-env=MACOSX_DEPLOYMENT_TARGET=10.9");
 
+    #[cfg(windows)]
+    {
+        use std::path::Path;
+        let profile = std::env::var("PROFILE").unwrap();
+        let exe_output_dir = Path::new("target").join(profile);
+        let exe_src_dir = Path::new("assets/windows/conhost");
+
+        for name in &["conpty.dll", "OpenConsole.exe"] {
+            let dest_name = exe_output_dir.join(name);
+            let src_name = exe_src_dir.join(name);
+
+            if !dest_name.exists() {
+                std::fs::copy(src_name, dest_name).unwrap();
+            }
+        }
+    }
+
     #[cfg(windows)]
     embed_resource::compile("assets/windows/resource.rc");
 }

ci/deploy.sh

@@ -39,7 +39,11 @@ case $OSTYPE in
     fi
     rm -rf $zipdir $zipname
     mkdir $zipdir
-    cp $TARGET_DIR/release/wezterm.exe $TARGET_DIR/release/wezterm.pdb $zipdir
+    cp $TARGET_DIR/release/wezterm.exe \
+      $TARGET_DIR/release/wezterm.pdb \
+      assets/windows/conhost/conpty.dll \
+      assets/windows/conhost/OpenConsole.exe \
+      $zipdir
     cp -r assets/colors $zipdir/
     7z a -tzip $zipname $zipdir
     ;;

pty/src/win/psuedocon.rs

@@ -36,10 +36,25 @@ shared_library!(ConPtyFuncs,
     pub fn ClosePseudoConsole(hpc: HPCON),
 );
 
-lazy_static! {
-    static ref CONPTY: ConPtyFuncs = ConPtyFuncs::open(Path::new("kernel32.dll")).expect(
-        "this system does not support conpty.  Windows 10 October 2018 or newer is required"
+fn load_conpty() -> ConPtyFuncs {
+    // If the kernel doesn't export these functions then their system is
+    // too old and we cannot run.
+    let kernel = ConPtyFuncs::open(Path::new("kernel32.dll")).expect(
+        "this system does not support conpty.  Windows 10 October 2018 or newer is required",
     );
+
+    // We prefer to use a sideloaded conpty.dll and openconsole.exe host deployed
+    // alongside the application.  We check for this after checking for kernel
+    // support so that we don't try to proceed and do something crazy.
+    if let Ok(sideloaded) = ConPtyFuncs::open(Path::new("conpty.dll")) {
+        sideloaded
+    } else {
+        kernel
+    }
+}
+
+lazy_static! {
+    static ref CONPTY: ConPtyFuncs = load_conpty();
 }
 
 pub struct PsuedoCon {