Fix nullptr crash in RecConfigOverrideFromEnvironment with runroot

When runroot is active and the PROXY_CONFIG_* environment variables for
path records (bin_path, local_state_dir, logfile_dir, plugin_dir) are
not set, RecConfigOverrideFromEnvironment() returned nullptr. This
violated its documented contract ("return either the overridden value,
or the original value") and caused a std::logic_error crash during
records.yaml parsing:

  basic_string: construction from null is not valid

The nullptr was assigned to std::string in SetRecordFromYAMLNode(),
which threw the exception. The bug was masked in autests because the
framework always sets these env vars.

Fix: return the original config value instead of nullptr when
RecConfigOverrideFromRunroot() matches. The record value is then
resolved by Layout::relative() at runtime, producing the correct path.

Add a test that unsets the 4 path env vars to exercise the runroot code
path, and validates record values via traffic_ctl config get.

Author: brbzull0

src/records/RecConfigParse.cc

@@ -123,7 +123,7 @@ RecConfigOverrideFromEnvironment(const char *name, const char *value)
   if (envval) {
     return envval;
   } else if (RecConfigOverrideFromRunroot(name)) {
-    return nullptr;
+    return value;
   }
 
   return value;

tests/gold_tests/records/records_runroot_null_override.test.py

@@ -0,0 +1,108 @@
+'''
+'''
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+Test.Summary = '''
+Verify that records.yaml parsing does not crash when runroot is active and
+path records are present without corresponding environment variables.
+
+Additionally, validate that:
+1. Env var overrides work correctly (record value comes from env).
+2. Runroot-managed path records use config file values when their env vars
+   are unset (the bug scenario).
+'''
+
+ts = Test.MakeATSProcess("ts")
+
+# Set an env var override for debug tags. This env var is NOT set by the
+# test framework, so it specifically tests the env override path in
+# RecConfigOverrideFromEnvironment().
+ts.Env['PROXY_CONFIG_DIAGS_DEBUG_TAGS'] = 'from_env_override'
+
+# Set a config value for tags that will be overridden by the env var above.
+ts.Disk.records_config.update('''
+    diags:
+      debug:
+        enabled: 0
+        tags: from_config_value
+    ''')
+
+# Add the runroot-managed path records. Values must match the sandbox layout.
+ts.Disk.records_config.append_to_document(
+    '''
+    bin_path: bin
+    local_state_dir: runtime
+    log:
+      logfile_dir: log
+    plugin:
+      plugin_dir: plugin
+''')
+
+# Unset the 4 path env vars to exercise the runroot code path in
+# RecConfigOverrideFromEnvironment(). In real deployments (e.g.
+# traffic_server --run-root=/path), these env vars are NOT set.
+# The test framework always sets them, which masks the bug.
+original_cmd = ts.Command
+ts.Command = (
+    "env"
+    " -u PROXY_CONFIG_BIN_PATH"
+    " -u PROXY_CONFIG_LOCAL_STATE_DIR"
+    " -u PROXY_CONFIG_LOG_LOGFILE_DIR"
+    " -u PROXY_CONFIG_PLUGIN_PLUGIN_DIR"
+    f" {original_cmd}")
+
+# Test 0: Start ATS
+tr = Test.AddTestRun("Start ATS and verify records parsing")
+tr.Processes.Default.Command = 'echo 1'
+tr.Processes.Default.ReturnCode = 0
+tr.Processes.Default.StartBefore(ts)
+tr.StillRunningAfter = ts
+
+# Verify no basic_string crash.
+ts.Disk.traffic_out.Content = Testers.ExcludesExpression(
+    "basic_string", "records.yaml parsing must not crash with 'basic_string: construction from null'")
+ts.Disk.traffic_out.Content += Testers.ContainsExpression(
+    "records parsing completed", "ATS should complete records parsing without errors")
+
+# Test 1: Verify env var override works for diags.debug.tags
+tr = Test.AddTestRun("Verify env var override for diags.debug.tags")
+tr.Processes.Default.Command = 'traffic_ctl config get proxy.config.diags.debug.tags'
+tr.Processes.Default.Env = ts.Env
+tr.Processes.Default.ReturnCode = 0
+tr.StillRunningAfter = ts
+tr.Processes.Default.Streams.stdout = Testers.ContainsExpression(
+    'proxy.config.diags.debug.tags: from_env_override', 'Record should have the env var override value, not the config file value')
+
+# Test 2: Verify runroot-managed path records have their config file values
+tr = Test.AddTestRun("Verify path records have config file values")
+tr.Processes.Default.Command = (
+    'traffic_ctl config get'
+    ' proxy.config.bin_path'
+    ' proxy.config.local_state_dir'
+    ' proxy.config.log.logfile_dir'
+    ' proxy.config.plugin.plugin_dir')
+tr.Processes.Default.Env = ts.Env
+tr.Processes.Default.ReturnCode = 0
+tr.StillRunningAfter = ts
+tr.Processes.Default.Streams.stdout += Testers.ContainsExpression(
+    'proxy.config.bin_path: bin', 'bin_path should have the config file value')
+tr.Processes.Default.Streams.stdout += Testers.ContainsExpression(
+    'proxy.config.local_state_dir: runtime', 'local_state_dir should have the config file value')
+tr.Processes.Default.Streams.stdout += Testers.ContainsExpression(
+    'proxy.config.log.logfile_dir: log', 'logfile_dir should have the config file value')
+tr.Processes.Default.Streams.stdout += Testers.ContainsExpression(
+    'proxy.config.plugin.plugin_dir: plugin', 'plugin_dir should have the config file value')