Safer sh_posix_configure on Windows

sh/posix.bzl

@@ -250,16 +250,49 @@ Example:
     toolchains = [TOOLCHAIN_TYPE],
 )
 
+def _windows_detect_sh_dir(repository_ctx):
+    # Taken and adapted from @bazel_tools//tools/sh/sh_configure.bzl.
+    sh_path = repository_ctx.os.environ.get("BAZEL_SH")
+    if not sh_path:
+        sh_path = repository_ctx.which("bash.exe")
+        if sh_path:
+            # repository_ctx.which returns a path object, convert that to
+            # string so we can call string.startswith on it.
+            sh_path = str(sh_path)
+            # When the Windows Subsystem for Linux is installed there's a
+            # bash.exe under %WINDIR%\system32\bash.exe that launches Ubuntu
+            # Bash which cannot run native Windows programs so it's not what
+            # we want.
+            windir = repository_ctx.os.environ.get("WINDIR")
+            if windir and sh_path.startswith(windir):
+                sh_path = None
+
+    if sh_path != None:
+        sh_dir = str(repository_ctx.path(sh_path).dirname)
+
+    return sh_dir
+
 def _sh_posix_config_impl(repository_ctx):
     cpu = get_cpu_value(repository_ctx)
     env = repository_ctx.os.environ
+
+    windows_sh_dir = None
+    if cpu == "x64_windows":
+        windows_sh_dir = _windows_detect_sh_dir(repository_ctx)
+
     commands = {}
     for cmd in _commands:
         cmd_path = env.get("POSIX_%s" % cmd.upper(), None)
-        if cmd_path == None:
+        if cmd_path == None and cpu != "x64_windows":
             cmd_path = repository_ctx.which(cmd)
-        if cmd_path == None and cpu == "x64_windows":
-            cmd_path = repository_ctx.which(cmd + ".exe")
+        elif cmd_path == None and cpu == "x64_windows":
+            # Autodetection using `repository_ctx.which` is not safe on
+            # Windows, as it may turn up false friends. E.g. Windows has a
+            # `find.exe` which is unrelated to POSIX `find`. Instead we use
+            # tools next to `bash.exe`.
+            cmd_path = repository_ctx.path(windows_sh_dir + "/" + cmd + ".exe")
+            if not cmd_path.exists:
+                cmd_path = None
         commands[cmd] = cmd_path
     repository_ctx.file("BUILD.bazel", executable = False, content = """
 load("@rules_sh//sh:posix.bzl", "sh_posix_toolchain")