command alias: Pull out a path parameter

Summary:
So that this can be controlled

Finally, this diff also makes `command_alias` public. I'm doing this because I'm potentially interested in re-using the code in two places:

 1. Toolchain definitions, specifically I'm about to write a `toolchain_command_alias` rule
 2. Ideally, we'd replace `cmd_script` from `prelude/utils.bzl` with this. Unfortunately, I'm still a bit worried about the remaining bugs in `command_alias`, so that's a future task

Reviewed By: dtolnay

Differential Revision: D74133692

fbshipit-source-id: 0425e67f15aeb7c3be457c771a0a54ab1148479d

Author: JakobDegen

prelude/command_alias.bzl

@@ -19,7 +19,14 @@ def command_alias_impl(ctx: AnalysisContext):
     else:
         base = _get_os_base(ctx, target_os.os)
 
-    output = _command_alias_impl(ctx, target_os, base, cmd_args(ctx.attrs.args), ctx.attrs.env)
+    output = command_alias(
+        ctx,
+        "__command_alias_trampoline",
+        target_os,
+        base,
+        cmd_args(ctx.attrs.args),
+        ctx.attrs.env,
+    )
 
     default_info = DefaultInfo(
         default_output = output.output.default_outputs[0],
@@ -77,16 +84,21 @@ CommandAliasOutput = record(
     maybe_directly_runnable = cmd_args | None,
 )
 
-def _command_alias_impl(
+def command_alias(
         ctx: AnalysisContext,
+        # The path at which to write the output to, without an extension - that will be added
+        path: str,
+        # The target where this script should be able to run (this may actually be your exec platform)
         target_os: OsLookup,
+        # Either the `RunInfo` to use, or in the case of a fat platform, the choice of `RunInfo`
+        # depending on `uname`
         base: RunInfo | dict[str, RunInfo],
         args: cmd_args,
         env: dict[str, ArgLike]) -> CommandAliasOutput:
     if target_os.script == ScriptLanguage("sh"):
-        trampoline, hidden = _command_alias_write_trampoline_unix(ctx, base, args, env)
+        trampoline, hidden = _command_alias_write_trampoline_unix(ctx, path + ".sh", base, args, env)
     elif target_os.script == ScriptLanguage("bat"):
-        trampoline, hidden = _command_alias_write_trampoline_windows(ctx, base, args, env)
+        trampoline, hidden = _command_alias_write_trampoline_windows(ctx, path + ".bat", base, args, env)
     else:
         fail("Unsupported script language: {}".format(target_os.script))
 
@@ -106,8 +118,7 @@ def _command_alias_impl(
 
 def _command_alias_write_trampoline_unix(
         ctx: AnalysisContext,
-        # Either the `RunInfo` to use, or in the case of a fat platform, the choice of `RunInfo`
-        # depending on `uname`
+        path: str,
         base: RunInfo | dict[str, RunInfo],
         args: cmd_args,
         env: dict[str, ArgLike]) -> (Artifact, cmd_args):
@@ -149,7 +160,7 @@ done
 
     trampoline_args.add('exec "${R_ARGS[@]}" "$@"')
 
-    trampoline = ctx.actions.declare_output("__command_alias_trampoline.sh")
+    trampoline = ctx.actions.declare_output(path)
     trampoline_args = cmd_args(
         trampoline_args,
         relative_to = (trampoline, 1),
@@ -166,6 +177,7 @@ done
 
 def _command_alias_write_trampoline_windows(
         ctx: AnalysisContext,
+        path: str,
         base: RunInfo,
         args: cmd_args,
         env: dict[str, ArgLike]) -> (Artifact, cmd_args):
@@ -192,7 +204,7 @@ def _command_alias_write_trampoline_windows(
 
     trampoline_args.add(cmd)
 
-    trampoline = ctx.actions.declare_output("__command_alias_trampoline.bat")
+    trampoline = ctx.actions.declare_output(path)
     trampoline_args = cmd_args(
         trampoline_args,
         relative_to = (trampoline, 1),