fix: prefix external command output for better log clarity

Most log output uses the package name as a prefix (via FromagerLogRecord)
so it is possible to tell which package is being processed when a message
is logged. However, external command output (like uv) lacked both:
1. Visual indication that it came from an external command
2. Package name on continuation lines for grep

This fix adds the package name to every line of external command output
and uses a visual prefix ('  > ') to distinguish it from fromager's own
messages. The package name is retrieved from the log context and manually
added to each continuation line, enabling grep to find all lines related
to a specific package (e.g., `grep 'numpy:'`).

Implementation:
- Use shlex.join() for command formatting
- Single logger call combining command info and output
- str.replace() to add prefixes to each line
- Package name manually added to continuation lines for greppability

Benefits:
- Every line has package name prefix for grep
- Visual clarity: '> ' prefix distinguishes external command output
- Parallel builds: Single log call prevents line interleaving
- Simple implementation with no code duplication

Example output:
  numpy: command failed with exit code 1: uv pip install numpy
  numpy:   > Resolving dependencies...
  numpy:   > Error: Could not find package

Fixes: #753

Co-authored-by: Claude <claude@anthropic.com>

Signed-off-by: Ioannis Angelakopoulos <iangelak@redhat.com>

Author: iangelak

src/fromager/external_commands.py

@@ -7,6 +7,8 @@
 import typing
 from io import TextIOWrapper
 
+from . import log
+
 logger = logging.getLogger(__name__)
 
 HERE = pathlib.Path(__file__).absolute().parent
@@ -98,8 +100,31 @@ def run(
             stdin=stdin,
         )
         output = completed.stdout.decode("utf-8") if completed.stdout else ""
+
+    # Get package name from context to add to each line for greppability
+    try:
+        req = log.requirement_ctxvar.get()
+        line_prefix = f"{req.name}: "
+    except LookupError:
+        line_prefix = ""
+
+    # Prefix output lines with visual marker and package name for greppability
+    formatted_output = None
+    if output:
+        output_prefix = line_prefix + "  > "
+        prefixed_output = "  > " + output.replace("\n", f"\n{output_prefix}")
+        formatted_output = f"{line_prefix}{prefixed_output}"
+
     if completed.returncode != 0:
-        logger.error("%s failed with %s", cmd, output)
+        # Log complete error message with command, exit code, and output
+        output_to_log = "\n" + formatted_output if formatted_output else ""
+        logger.error(
+            "command failed with exit code %d: %s%s",
+            completed.returncode,
+            shlex.join(cmd),
+            output_to_log,
+        )
+
         err_type = subprocess.CalledProcessError
         if network_isolation:
             # Look for a few common messages that mean there is a network
@@ -113,5 +138,9 @@ def run(
                 if substr in output:
                     err_type = NetworkIsolationError
         raise err_type(completed.returncode, cmd, output)
-    logger.debug("output: %s", output)
+
+    # Log command output with visual prefix to distinguish from fromager output
+    if formatted_output:
+        logger.debug(formatted_output)
+
     return output

tests/test_external_commands.py

@@ -29,7 +29,10 @@ def test_external_commands_log_file(tmp_path: pathlib.Path) -> None:
     assert "test\n" == file_contents
 
 
-@mock.patch("subprocess.run", return_value=mock.Mock(returncode=0))
+@mock.patch(
+    "subprocess.run",
+    return_value=mock.Mock(returncode=0, stdout=b"test output\n"),
+)
 @mock.patch(
     "fromager.external_commands.network_isolation_cmd",
     return_value=["/bin/unshare", "--net", "--map-current-user"],