runtime: add comment capability

docs/configuration/overview/runtime.rst

@@ -70,6 +70,14 @@ that :option:`--service-cluster` has been set to ``my-cluster``. Envoy will firs
 If found, the value will override any value found in the primary lookup path. This allows the user
 to customize the runtime values for individual clusters on top of global defaults.
 
+Comments
+--------
+
+Lines starting with ``#`` as the first character are treated as comments.
+
+Comments can be used to provide context on an existing value. Comments are also useful in an
+otherwise empty file to keep a placeholder for deployment in a time of need.
+
 Updating runtime values via symbolic link swap
 ----------------------------------------------
 

source/common/runtime/runtime_impl.cc

@@ -224,7 +224,17 @@ void SnapshotImpl::walkDirectory(const std::string& path, const std::string& pre
       // theoretically lead to issues.
       ENVOY_LOG(debug, "reading file: {}", full_path);
       Entry entry;
-      entry.string_value_ = Filesystem::fileReadToEnd(full_path);
+
+      // Read the file and remove any comments. A comment is a line starting with a '#' character.
+      // Comments are useful for placeholder files with no value.
+      const std::vector<std::string> lines =
+          StringUtil::split(Filesystem::fileReadToEnd(full_path), "\n");
+      for (const std::string& line : lines) {
+        if (!line.empty() && line.at(0) == '#') {
+          continue;
+        }
+        entry.string_value_ += line + "\n";
+      }
       StringUtil::rtrim(entry.string_value_);
 
       // As a perf optimization, attempt to convert the string into an integer. If we don't

test/common/runtime/runtime_impl_test.cc

@@ -110,6 +110,11 @@ TEST_F(RuntimeImplTest, All) {
   EXPECT_EQ(2UL, loader->snapshot().getInteger("file3", 1));
   EXPECT_EQ(123UL, loader->snapshot().getInteger("file4", 1));
 
+  // Files with comments.
+  EXPECT_EQ(123UL, loader->snapshot().getInteger("file5", 1));
+  EXPECT_EQ("/home#about-us", loader->snapshot().get("file6"));
+  EXPECT_EQ("", loader->snapshot().get("file7"));
+
   // Feature enablement.
   EXPECT_CALL(generator, random()).WillOnce(Return(1));
   EXPECT_TRUE(loader->snapshot().featureEnabled("file3", 1));

test/common/runtime/test_data/root/envoy/file5

@@ -0,0 +1,2 @@
+# This is a comment in a file with an integer.
+123

test/common/runtime/test_data/root/envoy/file6

@@ -0,0 +1,2 @@
+# This is a comment in a file with a string.
+/home#about-us

test/common/runtime/test_data/root/envoy/file7

@@ -0,0 +1,2 @@
+# This is a comment in an empty file.
+# This file was intentionally left blank.