Add special casing for '--javac-jdk=system' as a temporary hack for p…

…antsbuild#12293.

This causes JavacBinary to pass the `--system-jvm` option to Coursier,
which selects whichever JDK Coursier happens to find already installed
on the system.  Long term, we shouldn't use this behavior in tests and
probably shouldn't even allow it at all since the inherent non-hermeticity
isn't properly captured in cache keys, but for now it allows testing
of Coursier-dependent backends in CI until pantsbuild#12293 is resolved properly.

src/python/pants/backend/java/compile/javac_binary.py

@@ -25,14 +25,24 @@ class JavacBinary:
 
 @rule
 async def setup_javac_binary(coursier: Coursier, javac: JavacSubsystem) -> JavacBinary:
-    javac_wrapper_script = textwrap.dedent(
-        f"""\
-        set -eux
-        javac_path="$({coursier.coursier.exe} java-home --jvm {javac.options.jdk})/bin/javac"
-        /bin/mkdir -p {JavacBinary.classfiles_relpath}
-        exec "${{javac_path}}" "$@"
-        """
-    )
+    if javac.options.jdk == "system":
+        javac_wrapper_script = textwrap.dedent(
+            f"""\
+            set -eux
+            javac_path="$({coursier.coursier.exe} java-home --system-jvm)/bin/javac"
+            /bin/mkdir -p {JavacBinary.classfiles_relpath}
+            exec "${{javac_path}}" "$@"
+            """
+        )
+    else:
+        javac_wrapper_script = textwrap.dedent(
+            f"""\
+            set -eux
+            javac_path="$({coursier.coursier.exe} java-home --jvm {javac.options.jdk})/bin/javac"
+            /bin/mkdir -p {JavacBinary.classfiles_relpath}
+            exec "${{javac_path}}" "$@"
+            """
+        )
     javac_wrapper_script_digest = await Get(
         Digest,
         CreateDigest(

src/python/pants/backend/java/compile/javac_subsystem.py

@@ -23,5 +23,8 @@ def register_options(cls, register):
             advanced=True,
             help="The JDK to use for invoking javac."
             " This string will be passed directly to Coursier's `--jvm` parameter."
-            " Run `cs java --available` to see a list of available JVM versions on your platform.",
+            " Run `cs java --available` to see a list of available JVM versions on your platform."
+            " If the string 'system' is passed, Coursier's `--system-jvm` option will be used"
+            " instead, but note that this can lead to inconsistent behavior since the JVM version"
+            " will be whatever happens to be found first on the system's PATH.",
         )

src/python/pants/backend/java/compile/javac_test.py

@@ -28,9 +28,6 @@
 from pants.jvm.util_rules import rules as util_rules
 from pants.testutil.rule_runner import QueryRule, RuleRunner
 
-# TODO(#12293): Stabilize network flakiness.
-pytestmark = pytest.mark.skip
-
 
 @pytest.fixture
 def rule_runner() -> RuleRunner:
@@ -47,6 +44,9 @@ def rule_runner() -> RuleRunner:
             QueryRule(CompiledClassfiles, (CompileJavaSourceRequest,)),
         ],
         target_types=[JvmDependencyLockfile, JavaLibrary],
+        # TODO(#12293): Remove this nonhermetic hack once Coursier JVM boostrapping flakiness
+        # is properly fixed in CI.
+        bootstrap_args=["--javac-jdk=system"],
     )