Obtain bootstrap class path from dedicated runtime toolchain type

This decouples the concepts of a Java runtime used to run on the target
platform from the Java runtime that provides the bootstrap class path
used during compilation.

The former needs constraints on the target platform, whereas the latter
should not have any constraints on the target platform to allow for
cross-compilation to target platforms for which a JDK runtime is not
available (e.g. Android).

Work towards #17085
Work towards #64
Split off from #18262

toolchains/BUILD

@@ -37,6 +37,17 @@ filegroup(
 # TODO: migrate away from using @bazel_tools//tools/jdk:runtime_toolchain_type ?
 # toolchain_type(name = "runtime_toolchain_type")
 
+# The Java runtime to extract the bootclasspath from that is then used to compile Java sources.
+#
+# As the bootclasspath is platform independent, toolchains of this type may have no constraints.
+# Purely as an optimization to prevent unnecessary fetches of remote runtimes for other
+# architectures, toolchains of this type may have constraints on the execution platform that match
+# those on the corresponding compilation toolchain.
+#
+# Toolchains of this type are only consumed internally by the bootclasspath rule and should not be
+# accessed from Starlark.
+toolchain_type(name = "bootstrap_runtime_toolchain_type")
+
 # Points to toolchain[":runtime_toolchain_type"] (was :legacy_current_java_runtime)
 java_runtime_alias(name = "current_java_runtime")
 
@@ -201,8 +212,7 @@ alias(
 bootclasspath(
     name = "platformclasspath",
     src = "DumpPlatformClassPath.java",
-    host_javabase = ":current_java_runtime",
-    target_javabase = ":current_java_runtime",
+    java_runtime_alias = ":current_java_runtime",
 )
 
 default_java_toolchain(

toolchains/default_java_toolchain.bzl

@@ -203,8 +203,10 @@ def java_runtime_files(name, srcs):
             tags = ["manual"],
         )
 
+_JAVA_BOOTSTRAP_RUNTIME_TOOLCHAIN_TYPE = Label("@bazel_tools//tools/jdk:bootstrap_runtime_toolchain_type")
+
 def _bootclasspath_impl(ctx):
-    host_javabase = ctx.attr.host_javabase[java_common.JavaRuntimeInfo]
+    exec_javabase = ctx.attr.java_runtime_alias[java_common.JavaRuntimeInfo]
 
     class_dir = ctx.actions.declare_directory("%s_classes" % ctx.label.name)
 
@@ -219,17 +221,15 @@ def _bootclasspath_impl(ctx):
     args.add(ctx.file.src)
 
     ctx.actions.run(
-        executable = "%s/bin/javac" % host_javabase.java_home,
+        executable = "%s/bin/javac" % exec_javabase.java_home,
         mnemonic = "JavaToolchainCompileClasses",
-        inputs = [ctx.file.src] + ctx.files.host_javabase,
+        inputs = [ctx.file.src] + ctx.files.java_runtime_alias,
         outputs = [class_dir],
         arguments = [args],
     )
 
     bootclasspath = ctx.outputs.output_jar
 
-    inputs = [class_dir] + ctx.files.host_javabase
-
     args = ctx.actions.args()
     args.add("-XX:+IgnoreUnrecognizedVMOptions")
     args.add("--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED")
@@ -239,16 +239,17 @@ def _bootclasspath_impl(ctx):
     args.add("DumpPlatformClassPath")
     args.add(bootclasspath)
 
+    any_javabase = ctx.toolchains[_JAVA_BOOTSTRAP_RUNTIME_TOOLCHAIN_TYPE].java_runtime
+    args.add(any_javabase.java_home)
+
     system_files = ("release", "modules", "jrt-fs.jar")
-    system = [f for f in ctx.files.target_javabase if f.basename in system_files]
+    system = [f for f in any_javabase.files.to_list() if f.basename in system_files]
     if len(system) != len(system_files):
         system = None
-    if ctx.attr.target_javabase:
-        inputs.extend(ctx.files.target_javabase)
-        args.add(ctx.attr.target_javabase[java_common.JavaRuntimeInfo].java_home)
 
+    inputs = depset([class_dir] + ctx.files.java_runtime_alias, transitive = [any_javabase.files])
     ctx.actions.run(
-        executable = str(host_javabase.java_executable_exec_path),
+        executable = str(exec_javabase.java_executable_exec_path),
         mnemonic = "JavaToolchainCompileBootClasspath",
         inputs = inputs,
         outputs = [bootclasspath],
@@ -266,7 +267,7 @@ def _bootclasspath_impl(ctx):
 _bootclasspath = rule(
     implementation = _bootclasspath_impl,
     attrs = {
-        "host_javabase": attr.label(
+        "java_runtime_alias": attr.label(
             cfg = "exec",
             providers = [java_common.JavaRuntimeInfo],
         ),
@@ -275,10 +276,8 @@ _bootclasspath = rule(
             cfg = "exec",
             allow_single_file = True,
         ),
-        "target_javabase": attr.label(
-            providers = [java_common.JavaRuntimeInfo],
-        ),
     },
+    toolchains = [_JAVA_BOOTSTRAP_RUNTIME_TOOLCHAIN_TYPE],
 )
 
 def bootclasspath(name, **kwargs):

toolchains/local_java_repository.bzl

@@ -101,6 +101,12 @@ def local_java_runtime(name, java_home, version, runtime_name = None, visibility
         toolchain_type = Label("@bazel_tools//tools/jdk:runtime_toolchain_type"),
         toolchain = runtime_name,
     )
+    native.toolchain(
+        name = "bootstrap_runtime_toolchain_definition",
+        target_settings = [":%s_settings_alias" % name],
+        toolchain_type = Label("//toolchains:bootstrap_runtime_toolchain_type"),
+        toolchain = runtime_name,
+    )
 
     if type(version) == type("") and version.isdigit() and int(version) > 8:
         for version in range(8, int(version) + 1):
@@ -245,6 +251,12 @@ toolchain(
    toolchain_type = "@bazel_tools//tools/jdk:runtime_toolchain_type",
    toolchain = ":jdk",
 )
+toolchain(
+   name = "bootstrap_runtime_toolchain_definition",
+   target_settings = [":localjdk_setting"],
+   toolchain_type = "@rules_java//toolchains:bootstrap_runtime_toolchain_type",
+   toolchain = ":jdk",
+)
 '''
 
 _local_java_repository_rule = repository_rule(

toolchains/remote_java_repository.bzl

@@ -79,6 +79,16 @@ toolchain(
     toolchain_type = "@bazel_tools//tools/jdk:runtime_toolchain_type",
     toolchain = "{toolchain}",
 )
+toolchain(
+    name = "bootstrap_runtime_toolchain",
+    # These constraints are not required for correctness, but prevent fetches of remote JDK for
+    # different architectures. As every Java compilation toolchain depends on a bootstrap runtime in
+    # the same configuration, this constraint will not result in toolchain resolution failures.
+    exec_compatible_with = {target_compatible_with},
+    target_settings = [":version_or_prefix_version_setting"],
+    toolchain_type = "@rules_java//toolchains:bootstrap_runtime_toolchain_type",
+    toolchain = "{toolchain}",
+)
 """.format(
             prefix = prefix,
             version = version,