fix: don't attempt to include super-long synth logs in a PR body

autosynth/change_pusher.py

@@ -177,13 +177,21 @@ def build_pr_body(synth_log: str, trailers: str = ""):
     build_log_text = ""
     kokoro_build_id = os.environ.get("KOKORO_BUILD_ID")
     if synth_log:
+        length_limit = 40000
+        if len(synth_log) > length_limit:
+            synth_log = "[LOG TRUNCATED]\n" + synth_log[-length_limit:]
         build_log_text = f"""
 <details><summary>Log from Synthtool</summary>
 
 ```
 {synth_log}
 ```
 </details>"""
+        if kokoro_build_id:
+            build_log_text += f"""
+
+Full log will be available here:
+https://source.cloud.google.com/results/invocations/{kokoro_build_id}/targets"""
     elif kokoro_build_id:
         build_log_text = f"""Synth log will be available here:
 https://source.cloud.google.com/results/invocations/{kokoro_build_id}/targets"""

tests/test_change_pusher.py

@@ -32,15 +32,26 @@ def test_build_pr_body_with_kokoro_build_id():
 
 def test_build_pr_body_with_synth_log_and_kokoro_build_id():
     with util.ModifiedEnvironment({"KOKORO_BUILD_ID": "42"}):
-        # The synth log should override the kokoro build id.
         synth_log = "A great pull request."
         pr_body = build_pr_body(synth_log)
         assert (
-            pr_body.find("https://source.cloud.google.com/results/invocations/42") == -1
+            pr_body.find("https://source.cloud.google.com/results/invocations/42") > -1
         )
         assert pr_body.find(synth_log) > -1
 
 
+def test_build_pr_body_with_very_long_synth_log():
+    with util.ModifiedEnvironment({"KOKORO_BUILD_ID": "42"}):
+        synth_log = "abcdefghi\n" * 10000
+        pr_body = build_pr_body(synth_log)
+        assert (
+            pr_body.find("https://source.cloud.google.com/results/invocations/42") > -1
+        )
+        assert pr_body.find("abcdefghi") > -1
+        assert pr_body.find("[LOG TRUNCATED]") > -1
+        assert len(pr_body) < 60000
+
+
 def test_build_pr_body_with_synth_trailers():
     synth_log = "synth log"
     pr_body = build_pr_body(synth_log, "a: b\nc: d")