Fix non-determinism in bootstrap scalac actions when executing outsid…

…e of a sandbox

This fixes a bug where every bootstrap scalac action was writing class
files to the same directory: tmp/classes. Each action would run scalac
with the output directory set to tmp/classes and then create a jar from
all the class files in that directory.

If you're only running a single bootstrap scalac action, then this works
fine.

The bug happens when you run multiple bootstrap scalac actions because
you build up class files in the tmp/classes. This causes the jars you
produce to contain not only the current target's class files, but also
any class files from previous bootstrap scalac actions. Which specific
class files are in your jar depend on what actions have run up until
that point in time. This is not deterministic.

When executing with the sandboxed strategy this bug wasn't an issue
because the sandbox kept actions sufficiently isolated from each other.
Meaning you'd get only your action's classes in the output jar.

rules/private/phases/phase_bootstrap_compile.bzl

@@ -21,6 +21,8 @@ def phase_bootstrap_compile(ctx, g):
         transitive = [ctx.attr._jdk[java_common.JavaRuntimeInfo].files, g.classpaths.compile, g.classpaths.compiler],
     )
 
+    tmp = ctx.actions.declare_directory("{}/tmp/classes".format(ctx.label.name))
+
     compiler_classpath = ":".join([f.path for f in g.classpaths.compiler.to_list()])
     compile_classpath = ":".join([f.path for f in g.classpaths.compile.to_list()])
     srcs = " ".join([f.path for f in g.classpaths.srcs])
@@ -31,37 +33,39 @@ def phase_bootstrap_compile(ctx, g):
     if int(scala_configuration.version[0]) >= 3:
         main_class = "dotty.tools.dotc.Main"
 
-    ctx.actions.run_shell(
-        inputs = inputs,
-        tools = [ctx.executable._jar_creator],
-        outputs = [g.classpaths.jar],
-        command = _strip_margin(
-            """
+    command = _strip_margin(
+        """
             |set -eo pipefail
             |
-            |mkdir -p tmp/classes
-            |
             |{java} \\
             |  -cp {compiler_classpath} \\
             |  {main_class} \\
             |  -cp {compile_classpath} \\
-            |  -d tmp/classes \\
+            |  -d {tmp} \\
             |  {global_scalacopts} \\
             |  {scalacopts} \\
             |  {srcs}
             |
-            |{jar_creator} {output_jar} tmp/classes 2> /dev/null
+            |{jar_creator} {output_jar} {tmp} 2> /dev/null
             |""".format(
-                compiler_classpath = compiler_classpath,
-                compile_classpath = compile_classpath,
-                global_scalacopts = " ".join(scala_configuration.global_scalacopts),
-                java = ctx.attr._jdk[java_common.JavaRuntimeInfo].java_executable_exec_path,
-                jar_creator = ctx.executable._jar_creator.path,
-                main_class = main_class,
-                output_jar = g.classpaths.jar.path,
-                scalacopts = " ".join(ctx.attr.scalacopts),
-                srcs = srcs,
-            ),
+            compiler_classpath = compiler_classpath,
+            compile_classpath = compile_classpath,
+            global_scalacopts = " ".join(scala_configuration.global_scalacopts),
+            java = ctx.attr._jdk[java_common.JavaRuntimeInfo].java_executable_exec_path,
+            jar_creator = ctx.executable._jar_creator.path,
+            main_class = main_class,
+            output_jar = g.classpaths.jar.path,
+            scalacopts = " ".join(ctx.attr.scalacopts),
+            srcs = srcs,
+            tmp = tmp.path,
         ),
+    )
+
+    ctx.actions.run_shell(
+        inputs = inputs,
+        tools = [ctx.executable._jar_creator],
+        mnemonic = "BootstrapScalacompile",
+        outputs = [g.classpaths.jar, tmp],
+        command = command,
         execution_requirements = _resolve_execution_reqs(ctx, {}),
     )