stop swallowing warnings from Pex by default (pantsbuild#20480)

Pants has traditionally run Pex with --no-emit-warnings which as the
name implies... doesn't emit warnings. This hides pertinent information
like "the pex file you created won't actually work" from the user and
sends them off on a debugging hunt.

* Create an `emit_warnings` option, defaulting to True like the
underlying tool..
 * A little logic for pipeing Pex's stderr to Pants' logging.
* Fix the logic for old per target `emit_warnings` that didn't quite
work.

ref pantsbuild#19957

src/python/pants/backend/python/goals/lockfile.py

@@ -18,7 +18,8 @@
 from pants.backend.python.util_rules.interpreter_constraints import InterpreterConstraints
 from pants.backend.python.util_rules.lockfile_diff import _generate_python_lockfile_diff
 from pants.backend.python.util_rules.lockfile_metadata import PythonLockfileMetadata
-from pants.backend.python.util_rules.pex_cli import PexCliProcess
+from pants.backend.python.util_rules.pex_cli import PexCliProcess, maybe_log_pex_stderr
+from pants.backend.python.util_rules.pex_environment import PexSubsystem
 from pants.backend.python.util_rules.pex_requirements import (
     PexRequirements,
     ResolvePexConfig,
@@ -103,6 +104,7 @@ async def generate_lockfile(
     req: GeneratePythonLockfile,
     generate_lockfiles_subsystem: GenerateLockfilesSubsystem,
     python_setup: PythonSetup,
+    pex_subsystem: PexSubsystem,
 ) -> GenerateLockfileResult:
     pip_args_setup = await _setup_pip_args_and_constraints_file(req.resolve_name)
 
@@ -113,7 +115,6 @@ async def generate_lockfile(
             subcommand=("lock", "create"),
             extra_args=(
                 "--output=lock.json",
-                "--no-emit-warnings",
                 # See https://github.com/pantsbuild/pants/issues/12458. For now, we always
                 # generate universal locks because they have the best compatibility. We may
                 # want to let users change this, as `style=strict` is safer.
@@ -158,6 +159,8 @@ async def generate_lockfile(
         ),
     )
 
+    maybe_log_pex_stderr(result.stderr, pex_subsystem.verbosity)
+
     initial_lockfile_digest_contents = await Get(DigestContents, Digest, result.output_digest)
     metadata = PythonLockfileMetadata.new(
         valid_for_interpreter_constraints=req.interpreter_constraints,

src/python/pants/backend/python/goals/package_pex_binary.py

@@ -92,6 +92,8 @@ def generate_additional_args(self, pex_binary_defaults: PexBinaryDefaults) -> Tu
         args = []
         if self.emit_warnings.value_or_global_default(pex_binary_defaults) is False:
             args.append("--no-emit-warnings")
+        elif self.emit_warnings.value_or_global_default(pex_binary_defaults) is True:
+            args.append("--emit-warnings")
         if self.resolve_local_platforms.value_or_global_default(pex_binary_defaults) is True:
             args.append("--resolve-local-platforms")
         if self.ignore_errors.value is True:

src/python/pants/backend/python/goals/package_pex_binary_integration_test.py

@@ -135,6 +135,7 @@ def test_include_sources_avoids_files_targets_warning(
                 """\
                 python_sources(
                     name='sources',
+                    interpreter_constraints=["CPython==3.10.*"]
                 )
 
                 python_distribution(
@@ -147,12 +148,14 @@ def test_include_sources_avoids_files_targets_warning(
                         name='my-dist',
                         version='1.2.3',
                     ),
+                    interpreter_constraints=["CPython==3.10.*"]
                 )
 
                 pex_binary(
                     dependencies=[':wheel'],
                     entry_point="none",
                     include_sources=False,
+                    interpreter_constraints=["CPython==3.10.*"]
                 )
                 """
             ),

src/python/pants/backend/python/target_types.py

@@ -606,6 +606,7 @@ class PexEmitWarningsField(TriBoolField):
     def value_or_global_default(self, pex_binary_defaults: PexBinaryDefaults) -> bool:
         if self.value is None:
             return pex_binary_defaults.emit_warnings
+
         return self.value
 
 

src/python/pants/backend/python/util_rules/pex.py

@@ -31,7 +31,7 @@
 )
 from pants.backend.python.util_rules import pex_cli, pex_requirements
 from pants.backend.python.util_rules.interpreter_constraints import InterpreterConstraints
-from pants.backend.python.util_rules.pex_cli import PexCliProcess, PexPEX
+from pants.backend.python.util_rules.pex_cli import PexCliProcess, PexPEX, maybe_log_pex_stderr
 from pants.backend.python.util_rules.pex_environment import (
     CompletePexEnvironment,
     PexEnvironment,
@@ -403,10 +403,7 @@ async def find_interpreter(
     )
     path, fingerprint = result.stdout.decode().strip().splitlines()
 
-    if pex_subsystem.verbosity > 0:
-        log_output = result.stderr.decode()
-        if log_output:
-            logger.info("%s", log_output)
+    maybe_log_pex_stderr(result.stderr, pex_subsystem.verbosity)
 
     return PythonExecutable(path=path, fingerprint=fingerprint)
 
@@ -678,7 +675,6 @@ async def build_pex(
     argv = [
         "--output-file",
         request.output_filename,
-        "--no-emit-warnings",
         *request.additional_args,
     ]
 
@@ -758,10 +754,7 @@ async def build_pex(
         ),
     )
 
-    if pex_subsystem.verbosity > 0:
-        log_output = result.stderr.decode()
-        if log_output:
-            logger.info("%s", log_output)
+    maybe_log_pex_stderr(result.stderr, pex_subsystem.verbosity)
 
     digest = (
         await Get(

src/python/pants/backend/python/util_rules/pex_cli.py

@@ -4,6 +4,7 @@
 from __future__ import annotations
 
 import dataclasses
+import logging
 import os.path
 from dataclasses import dataclass
 from typing import Iterable, List, Mapping, Optional, Tuple
@@ -29,6 +30,8 @@
 from pants.util.logging import LogLevel
 from pants.util.meta import classproperty
 
+logger = logging.getLogger(__name__)
+
 
 class PexCli(TemplatedExternalTool):
     options_scope = "pex-cli"
@@ -156,6 +159,8 @@ async def setup_pex_cli_process(
 
     verbosity_args = [f"-{'v' * pex_subsystem.verbosity}"] if pex_subsystem.verbosity > 0 else []
 
+    warnings_args = [] if pex_subsystem.emit_warnings else ["--no-emit-warnings"]
+
     # NB: We should always pass `--python-path`, as that tells Pex where to look for interpreters
     # when `--python` isn't an absolute path.
     resolve_args = [
@@ -171,6 +176,7 @@ async def setup_pex_cli_process(
         *request.subcommand,
         *global_args,
         *verbosity_args,
+        *warnings_args,
         *pip_version_args,
         *resolve_args,
         # NB: This comes at the end because it may use `--` passthrough args, # which must come at
@@ -202,6 +208,15 @@ async def setup_pex_cli_process(
     )
 
 
+def maybe_log_pex_stderr(stderr: bytes, pex_verbosity: int) -> None:
+    """Forward Pex's stderr to a Pants logger if conditions are met."""
+    log_output = stderr.decode()
+    if log_output and "PEXWarning:" in log_output:
+        logger.warning("%s", log_output)
+    elif log_output and pex_verbosity > 0:
+        logger.info("%s", log_output)
+
+
 def rules():
     return [
         *collect_rules(),

src/python/pants/backend/python/util_rules/pex_environment.py

@@ -81,6 +81,14 @@ def iter_path_entries():
         ),
         advanced=True,
     )
+    emit_warnings = BoolOption(
+        default=True,
+        help=softwrap(
+            """
+            If warnings from Pex should be logged by Pants to the console.
+            """
+        ),
+    )
 
     @property
     def verbosity(self) -> int: