[API] Percent-encode spaces in Baggage::ToHeader

CHANGELOG.md

@@ -15,6 +15,10 @@ Increment the:
 
 ## [Unreleased]
 
+* [API] `Baggage::ToHeader` now percent-encodes spaces as `%20` instead of
+  `+`, as required by the W3C Baggage specification. Decoding is unchanged.
+  [#4765](https://github.com/open-telemetry/opentelemetry-cpp/pull/4765)
+
 * [API/SDK] Add `Enabled()` to synchronous metric instruments (ABI v2). It
   returns `false` when the Meter is disabled or every matching view uses the
   Drop aggregation

api/include/opentelemetry/baggage/baggage.h

@@ -320,10 +320,6 @@ class OPENTELEMETRY_EXPORT Baggage
       {
         ret.push_back(c);
       }
-      else if (c == ' ')
-      {
-        ret.push_back('+');
-      }
       else
       {
         ret.push_back('%');

api/test/baggage/baggage_test.cc

@@ -201,7 +201,8 @@ TEST(BaggageTest, ValidateExtractHeader)
        {"v1", "v2; metadata"}},  // key and value are trimmed
       {"1a-2f%40foo=bar%251,a%2A%2Ffoo-_%2Fbar=bar+4",
        {"1a-2f@foo", "a*/foo-_/bar"},
-       {"bar%1", "bar 4"}},                                       // decoding is done properly
+       {"bar%1", "bar 4"}},  // decoding is done properly, '+' still decodes as space
+      {"k=hello%20world", {"k"}, {"hello world"}},                // %20 decodes as space
       {"field=foo:bar", {"field"}, {"foo:bar"}},                  // colon in value
       {"mixed=a/b:c?d=e", {"mixed"}, {"a/b:c?d=e"}},              // mixed special characters
       {"k1=v1,invalidmember,k2=v2", {"k1", "k2"}, {"v1", "v2"}},  // invalid member is skipped
@@ -256,10 +257,10 @@ TEST(BaggageTest, ValidateInjectHeader)
                    {{"k3", "k2", "k1"}, {"", "v2", "v1"}, "k1=v1,k2=v2,k3="},  // empty value
                    {{"1a-2f@foo", "a*/foo-_/bar"},
                     {"bar%1", "bar 4"},
-                    "a%2A%2Ffoo-_%2Fbar=bar+4,1a-2f%40foo=bar%251"},  // encoding is done properly
+                    "a%2A%2Ffoo-_%2Fbar=bar%204,1a-2f%40foo=bar%251"},  // encoding is done properly
                    {{"foo 1"},
                     {"bar 1;  metadata ; flag=encoded%20value"},
-                    "foo+1=bar+1;  metadata ; flag=encoded%20value"}};
+                    "foo%201=bar%201;  metadata ; flag=encoded%20value"}};
 
   for (auto &testcase : testcases)
   {
@@ -272,6 +273,17 @@ TEST(BaggageTest, ValidateInjectHeader)
   }
 }
 
+TEST(BaggageTest, SpaceIsPercentEncoded)
+{
+  auto baggage = Baggage().Set("k", "hello world");
+  auto header  = baggage->ToHeader();
+  EXPECT_EQ(header, "k=hello%20world");
+
+  std::string value;
+  EXPECT_TRUE(Baggage::FromHeader(header)->GetValue("k", value));
+  EXPECT_EQ(value, "hello world");
+}
+
 TEST(BaggageTest, BaggageGet)
 {
   auto header  = header_with_custom_entries(Baggage::kMaxKeyValuePairs);