feat(log): set Abseil's level from a go-plugin level

README.md

@@ -54,6 +54,10 @@ go_plugin::log::InstallAbslBridge();   // LOG(WARNING) now reaches the host as a
 Abseil has no debug or trace severity of its own — they exist only as `VLOG` verbosities — so the bridge maps `VLOG(1)`
 to debug and `VLOG(2)` and above to trace, and it stops Abseil writing its own copy of each line to standard error.
 
+`go_plugin::log::SetAbslLevel(level)` sets the least severe level a plugin logs at, through Abseil as much as through
+`Write`: Abseil's minimum severity, and the `VLOG` verbosity that the same mapping gives debug and trace. It is for a
+plugin whose host moves its level while it runs.
+
 To add another backend, translate its records into `go_plugin::log::Submit` and add a target beside the Abseil one;
 nothing in the core changes.
 

include/go_plugin/log_absl.hpp

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "go_plugin/log.hpp"
+
 namespace go_plugin::log {
 
 struct AbslBridgeOptions {
@@ -29,4 +31,12 @@ struct AbslBridgeOptions {
  */
 void InstallAbslBridge(const AbslBridgeOptions& options = {});
 
+/**
+ * Sets the least severe level a plugin logs at, through Abseil as much as
+ * through Write: Abseil's minimum severity, and the VLOG verbosity that debug
+ * and trace map to under InstallAbslBridge's options. For a plugin whose host
+ * moves its level while it runs.
+ */
+void SetAbslLevel(Level min);
+
 }  // namespace go_plugin::log

src/log_absl.cpp

@@ -1,5 +1,6 @@
 #include "go_plugin/log_absl.hpp"
 
+#include <atomic>
 #include <mutex>
 #include <string>
 
@@ -15,6 +16,8 @@
 namespace go_plugin::log {
 namespace {
 
+std::atomic<int> g_trace_from_verbosity{AbslBridgeOptions{}.trace_from_verbosity};
+
 Level LevelFor(const absl::LogEntry& entry, const AbslBridgeOptions& options) {
     // A VLOG is filed at info severity; its verbosity is the only thing that
     // separates it from an ordinary info line.
@@ -64,6 +67,7 @@ class Bridge final : public absl::LogSink {
 void InstallAbslBridge(const AbslBridgeOptions& options) {
     static std::once_flag once;
     std::call_once(once, [&options] {
+        g_trace_from_verbosity.store(options.trace_from_verbosity, std::memory_order_relaxed);
         // Never destroyed: Abseil holds the pointer for the life of the process
         // and a LOG(FATAL) unwinds nothing.
         static Bridge bridge(options);
@@ -75,4 +79,21 @@ void InstallAbslBridge(const AbslBridgeOptions& options) {
     });
 }
 
+void SetAbslLevel(Level min) {
+    SetLevel(min);
+
+    const int trace_from = g_trace_from_verbosity.load(std::memory_order_relaxed);
+    int verbosity = 0;
+    absl::LogSeverityAtLeast severity = absl::LogSeverityAtLeast::kInfo;
+    switch (min) {
+    case Level::Trace: verbosity = trace_from; break;
+    case Level::Debug: verbosity = trace_from - 1; break;
+    case Level::Info: break;
+    case Level::Warn: severity = absl::LogSeverityAtLeast::kWarning; break;
+    case Level::Error: severity = absl::LogSeverityAtLeast::kError; break;
+    }
+    absl::SetGlobalVLogLevel(verbosity);
+    absl::SetMinLogLevel(severity);
+}
+
 }  // namespace go_plugin::log

tests/test_log_absl.cpp

@@ -72,3 +72,27 @@ TEST_F(AbslBridge, CarriesTheCaller) {
     ASSERT_EQ(Lines().size(), 1u);
     EXPECT_NE(Lines()[0].find(R"("caller":"test_log_absl.cpp:)"), std::string::npos) << Lines()[0];
 }
+
+// One level moves Abseil as a whole: its VLOG verbosities by the bridge's own
+// mapping of debug and trace, and its severities.
+TEST_F(AbslBridge, SetAbslLevelDecidesWhatAbseilLogs) {
+    go_plugin::log::SetAbslLevel(go_plugin::log::Level::Debug);
+    VLOG(1) << "debug line";
+    VLOG(2) << "trace line";
+    ASSERT_EQ(Lines().size(), 1u);
+    EXPECT_NE(Lines()[0].find("debug line"), std::string::npos);
+
+    Lines().clear();
+    go_plugin::log::SetAbslLevel(go_plugin::log::Level::Trace);
+    VLOG(2) << "trace line";
+    EXPECT_EQ(Lines().size(), 1u);
+
+    Lines().clear();
+    go_plugin::log::SetAbslLevel(go_plugin::log::Level::Warn);
+    VLOG(1) << "debug line";
+    LOG(INFO) << "info line";
+    LOG(WARNING) << "warning line";
+    ASSERT_EQ(Lines().size(), 1u);
+    EXPECT_NE(Lines()[0].find("warning line"), std::string::npos);
+    EXPECT_EQ(go_plugin::log::GetLevel(), go_plugin::log::Level::Warn);
+}