zwoop commented on code in PR #13018:
URL: https://github.com/apache/trafficserver/pull/13018#discussion_r3030427518
##########
doc/admin-guide/logging/formatting.en.rst:
##########
@@ -314,8 +314,54 @@ prior to the log field's name, as so::
Format = '%<{User-agent}cqh>'
The above would insert the User Agent string from the client request headers
-into your log entry (or a blank string if no such header was present, or it did
-not contain a value).
+into your log entry (or ``-`` if no such header was present).
+
+Header fields can also be chained with a fallback operator, ``??``, when you
want
+the log to use the first header that exists among n headers. For example::
+
+ Format = '%<{x-primary-id}cqh??{x-secondary-id}cqh??{x-tertiary-id}cqh>'
+
+|TS| evaluates the candidates from left to right and logs the first header that
+exists. If none of the headers exist, |TS| logs ``-`` by default. A header that
+exists but has an empty value is considered present, so |TS| logs the empty
+value instead of falling back. So in the example above, the value of
+x-primary-id of the client request is logged if it exists, otherwise the value
+of x-secondary-id is logged if it exists, otherwise ``-`` is logged if neither
+of the headers is present.
+
+The final log field in the chain can be a non-header log field whose value
+specifies the final fallback in the chian. This symbol is used only after all
Review Comment:
Typo, chian -> chain
##########
src/proxy/logging/LogFormat.cc:
##########
@@ -475,6 +501,41 @@ LogFormat::parse_symbol_string(const char *symbol_string,
LogFieldList *field_li
"')' in %s",
symbol);
}
+ } else if (LogFieldFallback::has_fallback(symbol)) {
+ Dbg(dbg_ctl_log_format, "Field fallback symbol: %s", symbol);
+ std::string parse_error;
+ auto parsed = LogFieldFallback::parse(symbol, parse_error);
+ if (parsed.has_value()) {
+ std::unique_ptr<LogField> fallback_field;
+ bool valid_fallback = true;
+
+ if (parsed->fallback_symbol.has_value()) {
+ std::string fallback_symbol = *parsed->fallback_symbol;
+ std::string field_symbol = fallback_symbol;
Review Comment:
Do we really need two copies here for the string?
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