scm: truncate long command output in logs (Bug 2036740)

src/lando/main/scm/git.py

@@ -24,6 +24,7 @@
 from lando.settings import LANDO_USER_EMAIL, LANDO_USER_NAME
 from lando.utils.const import URL_USERINFO_RE
 from lando.utils.github import GitHub
+from lando.utils.strings import truncate_output
 
 from .abstract_scm import AbstractSCM
 
@@ -620,13 +621,14 @@ def _git_run(cls, *args, cwd: str | None = None, rstrip: bool = True) -> str:
             )
 
         if out:
+            truncated_output = truncate_output(out)
             logger.info(
                 "output from git command #%s: %s",
                 correlation_id,
-                out,
+                truncated_output,
                 extra={
                     "command_id": correlation_id,
-                    "output": out,
+                    "output": truncated_output,
                     "path": cwd,
                 },
             )

src/lando/main/scm/hg.py

@@ -35,6 +35,7 @@
     TreeClosed,
 )
 from lando.main.scm.helpers import GitPatchHelper, HgPatchHelper, PatchHelper
+from lando.utils.strings import truncate_output
 
 logger = logging.getLogger(__name__)
 
@@ -697,7 +698,8 @@ def _run_hg(self, args: list[str]) -> bytes:
         out = out.getvalue()
         err = err.getvalue()
         if out:
-            out_string = (out.rstrip().decode(self.ENCODING, errors="replace"),)
+            decoded_output = out.rstrip().decode(self.ENCODING, errors="replace")
+            out_string = truncate_output(decoded_output)
             logger.info(
                 "output from hg command #%s: %s",
                 correlation_id,

src/lando/utils/strings.py

@@ -0,0 +1,19 @@
+LOG_OUTPUT_HEAD_LIMIT = 500
+LOG_OUTPUT_TAIL_LIMIT = 200
+
+
+def truncate_output(output: str) -> str:
+    """Trim long command output to its head and tail.
+
+    Keeps log volume bounded for commands like `hg export` or `git format-patch`
+    that emit an entire patch, while preserving the most diagnostically useful
+    portions (commit metadata at the start, summary or error trailer at the end).
+    """
+    total = len(output)
+    if total <= LOG_OUTPUT_HEAD_LIMIT + LOG_OUTPUT_TAIL_LIMIT:
+        return output
+
+    head = output[:LOG_OUTPUT_HEAD_LIMIT]
+    tail = output[-LOG_OUTPUT_TAIL_LIMIT:]
+    omitted = total - LOG_OUTPUT_HEAD_LIMIT - LOG_OUTPUT_TAIL_LIMIT
+    return f"{head}\n...[{omitted} bytes omitted]...\n{tail}"

src/lando/utils/tests/test_strings.py

@@ -0,0 +1,48 @@
+import pytest
+
+from lando.utils.strings import (
+    LOG_OUTPUT_HEAD_LIMIT,
+    LOG_OUTPUT_TAIL_LIMIT,
+    truncate_output,
+)
+
+
+@pytest.mark.parametrize(
+    "case_name,input_string",
+    [
+        ("empty", ""),
+        ("short", "hello world"),
+        (
+            "just_under_boundary",
+            "x" * (LOG_OUTPUT_HEAD_LIMIT + LOG_OUTPUT_TAIL_LIMIT - 1),
+        ),
+        ("at_boundary", "x" * (LOG_OUTPUT_HEAD_LIMIT + LOG_OUTPUT_TAIL_LIMIT)),
+    ],
+)
+def test_truncate_output_passes_short_input_through(case_name, input_string):
+    assert (
+        truncate_output(input_string) == input_string
+    ), f"`truncate_output` should leave `{case_name}` input unchanged."
+
+
+@pytest.mark.parametrize("middle_size", [1, 1000, 100_000])
+def test_truncate_output_long_keeps_head_and_tail(middle_size):
+    head = "H" * LOG_OUTPUT_HEAD_LIMIT
+    middle = "M" * middle_size
+    tail = "T" * LOG_OUTPUT_TAIL_LIMIT
+    long_output = head + middle + tail
+
+    result = truncate_output(long_output)
+
+    assert result.startswith(
+        head
+    ), "Truncated output should preserve the first `LOG_OUTPUT_HEAD_LIMIT` characters."
+    assert result.endswith(
+        tail
+    ), "Truncated output should preserve the last `LOG_OUTPUT_TAIL_LIMIT` characters."
+    assert (
+        f"[{middle_size} bytes omitted]" in result
+    ), "Truncated output should include a marker reporting the omitted byte count."
+    assert (
+        "M" not in result
+    ), "Truncated output should not contain any of the omitted middle section."