fix(spawn): always expand executable path on Windows (#2021)

This fixes some issues with the logic for expanding paths on Windows. If
a process is spawned with a custom `PATH` (either via the `env` or
`env_raw` arg) we still expand the path manually but with a temporarily
modified `vim.env.PATH`.

Fixes #2009.

Author: williamboman

lua/mason-core/spawn.lua

@@ -14,17 +14,30 @@ local spawn = {
 }
 
 ---@param cmd string
-local function exepath(cmd)
+---@param path? string
+local function exepath(cmd, path)
+    local function get_exepath(cmd)
+        if path then
+            local old_path = vim.env.PATH
+            vim.env.PATH = path
+            local expanded_cmd = vim.fn.exepath(cmd)
+            vim.env.PATH = old_path
+            return expanded_cmd
+        else
+            return vim.fn.exepath(cmd)
+        end
+    end
+
     if platform.is.win then
         -- On Windows, exepath() assumes the system is capable of executing "Unix-like" executables if the shell is a Unix
         -- shell. We temporarily override it to a Windows shell ("powershell") to avoid that behaviour.
         local old_shell = vim.o.shell
         vim.o.shell = "powershell"
-        local expanded_cmd = vim.fn.exepath(cmd)
+        local expanded_cmd = get_exepath(cmd)
         vim.o.shell = old_shell
         return expanded_cmd
     else
-        return vim.fn.exepath(cmd)
+        return get_exepath(cmd)
     end
 end
 
@@ -41,7 +54,7 @@ local function Failure(err, cmd)
     }))
 end
 
-local has_path = _.any(_.starts_with "PATH=")
+local get_path_from_env_list = _.compose(_.strip_prefix "PATH=", _.find_first(_.starts_with "PATH="))
 
 ---@class SpawnArgs
 ---@field with_paths string[]? Paths to add to the PATH environment variable.
@@ -80,9 +93,9 @@ setmetatable(spawn, {
 
             -- Find the executable path via vim.fn.exepath on Windows because libuv fails to resolve certain executables
             -- in PATH.
-            if platform.is.win and (spawn_args.env and has_path(spawn_args.env)) == nil then
+            if platform.is.win then
                 a.scheduler()
-                local expanded_cmd = exepath(canonical_cmd)
+                local expanded_cmd = exepath(canonical_cmd, spawn_args.env and get_path_from_env_list(spawn_args.env))
                 if expanded_cmd ~= "" then
                     cmd = expanded_cmd
                 end

tests/mason-core/spawn_spec.lua

@@ -148,6 +148,8 @@ describe("async spawn", function()
     end)
 
     describe("Windows", function()
+        -- Note: Tests assume they're executed in a Unix environment (e.g. uses Unix path separators in tests).
+
         before_each(function()
             platform.is.win = true
         end)
@@ -173,24 +175,43 @@ describe("async spawn", function()
             )
         end)
 
-        it("should not use exepath if env.PATH is set", function()
+        it("should use exepath if env.PATH is set", function()
             stub(process, "spawn", function(_, _, callback)
                 callback(true, 0, 0)
             end)
 
-            local result = a.run_blocking(spawn.bash, { "arg1", env = { PATH = "C:\\some\\path" } })
+            local result = a.run_blocking(spawn.bash, { "arg1", env = { PATH = "C:\\some\\path:" .. vim.env.PATH } })
             assert.is_true(result:is_success())
             assert.spy(process.spawn).was_called(1)
             assert.spy(process.spawn).was_called_with(
-                "bash",
+                vim.fn.exepath "bash",
+                match.tbl_containing {
+                    args = match.same { "arg1" },
+                    env = match.is_table(),
+                },
+                match.is_function()
+            )
+        end)
+
+        it("should use exepath if env_raw.PATH is set", function()
+            stub(process, "spawn", function(_, _, callback)
+                callback(true, 0, 0)
+            end)
+
+            local result = a.run_blocking(spawn.bash, { "arg1", env_raw = { "PATH=C:\\some\\path:" .. vim.env.PATH } })
+            assert.is_true(result:is_success())
+            assert.spy(process.spawn).was_called(1)
+            assert.spy(process.spawn).was_called_with(
+                vim.fn.exepath "bash",
                 match.tbl_containing {
                     args = match.same { "arg1" },
+                    env = match.is_table(),
                 },
                 match.is_function()
             )
         end)
 
-        it("should not use exepath if env_raw.PATH is set", function()
+        it("should default to provided cmd if exepath returns nothing", function()
             stub(process, "spawn", function(_, _, callback)
                 callback(true, 0, 0)
             end)
@@ -207,7 +228,7 @@ describe("async spawn", function()
             )
         end)
 
-        it("should not use exepath if with_paths is provided", function()
+        it("should use exepath if with_paths is provided", function()
             stub(process, "spawn", function(_, _, callback)
                 callback(true, 0, 0)
             end)
@@ -216,7 +237,7 @@ describe("async spawn", function()
             assert.is_true(result:is_success())
             assert.spy(process.spawn).was_called(1)
             assert.spy(process.spawn).was_called_with(
-                "bash",
+                vim.fn.exepath "bash",
                 match.tbl_containing {
                     args = match.same { "arg1" },
                 },